DISEASES OF FIELD PEA (Pisum sativum L.)

IN THE PEACE RIVER REGION OF ALBERTA

by Paul Laflamme PAg.

B.Sc. Agriculture - University of Alberta - 1986

THESIS SUBMITTED IN PARTPAL FULFILEMENT OF

THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF PEST MANAGENEENT

in the Department

of

Biological Sciences

O Pau1 Laflamme 1998 SIMON FRASER UNIIVERSITY

July 1998

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Abstract A survey of diseases of field pea was done in the Peace River Region of Alberta during June to August of 1997. A total of 103 fields were surveyed duruig the seedling stage, at f l o w e ~ g and after pod set, and over 15,000 samples were obtained The incidence of root rot was determined by recording the number of plants with discolouration on the roots. Root rot severity was determined by rating the area of each root showing signs of discolouration, on a scale of O - 4. Root rot and foliar disease-associated fungi were identified by plating symptomatic tissue onto potato dextrose agar. Mean incidence and severity of root rot for the three sampling times was 88% and 1.2,88% and 1.5 and 95% and 2.3, respectively. Fusariurn spp. were the predominant fungi isolated fkom root tissues. Ascochyta spp. and Sclerotinia sclerotiorum were the fun@ isolated most fiequently from foliar tissues. Symptorns of mycosphaerella blight were found in ail fields by the third sampling time and symptoms of sclerotinia rot were observed in 54 fields by the third sampling time. Significant differences in root rot incidence and severity were recorded between different areas of the Peace with the Falhermeace River area having the lowest overall level of both root rot incidence and seventy. Growers were also surveyed on their agronomic practices. E s information was statistically analyzed for relationships between agronomic practices and root rot incidence and severity. When averaged across the 3 sampling times, the analysis revealed that many agronomic practices reported in this survey were correlated with small but significant diffaences in root rot incidence but only a few with significant differences in root rot severity. Fields that had been planted to canola in 1996 had significantly higher root rot incidence than other fields. Fields planted to legumes in 1995 had significantly higher root rot incidence and severity than other fields. Growers who used certified seed had a significantly higher root rot incidence and severity than growers who used common seed. Pea seed treated with metalaxyl had the highest root rot incidence while pea seed treated with thiram had the lowest. Use of peat based seed inoculant resulted in the lowest root rot incidence and no inoculant resulted in the highest. Use of fertilizer resulted in higher root rot incidence and severity than when no fertilizer was used. Banding of fertilizer resulted in the highest root rot incidence and severity while applying fertilirer with the seed resulted in the lowest. Minimum tillage resulted in the highest level of root rot incidence while zero tillage resulted in the lowest. Pea fields that were rolled immediately &er seeding had the hîghest root rot incidence while fields that were rolled before emergence or were not roIled had the lowest. Growers who sprayed their pea fields with herbicides had significantly higher root rot incidence and severity than growers who didn't spray Disease in previous pea crops and herbicide residues had no significant effect on root rot incidence. A disease management program for field peas grown in the Peace River region of Alberta is presented.

Acknowledgements

1 want to especiaily thank my wife Diana, for all her support, encouragement and patience while 1 was working on this project. Also my three children, Sara, Chantal and Jeremy for leaving their fnends and taking a year out of their Lives to move to Bumaby so that 1 could do the course work for this Masters degree. I hope it was as much of an adventure for them as it was for me. I also a-ant to thank Dr. James Rahe and Dr. Zamir Punja for al1 their help and advice in the editing of this thesis. A big thank-you also goes to my employer, Alberta Agriculture, Food & Rural Development who provided me with the opportunity, support and financial resources to undertake and complete this Masters degree. And hal ly , thanks to the Alberta Agriculture Research Institute and the Alberta Pulse Growers Commission for providuig funding for this project.

Table of Contents

Approval ............................. .. .......................................... ..................................................................... Abstract .......................................... .,.,

Ac know ledgements ... .. .. ... ........................................................................................ Table of Contents ........................................................................................................ List of Tables ............................................................................................................ List of Figures ........................................................................................................ Chapter 1 -Background ...............................................................................................

......................................................................... 1.1 History of Pea Production

1.2 History of Pea Production in Canada ....................................................... ................................................................................. 1.3 Climate of the Peace

................................................................................................ 1.4 Objectives . . Chapter 2 - Pea Disease Descriptions .........................................................................

................ 2.1 Soil-borne Diseases . Seedling Blight, Seed Rot and Root Rot

2.1 1 Fusarium Root Rot ......................,............................................ 2.12 Fusarium Wilt ............................................................................. 2.1 3 Pythium Root Rot ........................... .... ... .. .... 2.14 Rhizoctonia Root Rot .................................... .... .......................... 2.1 5 Aphanomyces Root Rot .............................................................. 2.16 Thielaviopsis Root Rot ............................................................... 2.17 Prevention & Control of Seedling Blight, Seed and Root Rot ....

2.2 Foliar Diseases ......................................................................................... 2.2 1 Ascochyta/Mycosphaerella Blight ..............................................

. . 2.22 Sclerotmia Rot ............................................................................. 2.23 Powdery Mildew ......................................................................... 2.24 Downy Mildew ...........................................................................

.................................................... 2.25 Gray MoId .......................... .. .................................................... ......................... 2.26 Anthracnose .,,

Page

Table of Contents

Page

2.2 Foliar Diseases (cont.)

.................................... 2.27 Alternaria Blight- .......................... .... 31

2.28 Septoria Blotch ............................................................................ 32

................................................................... 2.29 Cladosponum Blight 33

2.210BlackLea.f ........................................................................... 34

2.3 Bacteriai Diseases ..................................................-.........................-..... 35

2.3 1 Bacterial Blight .............................. ... ......................................... 35

2.32 Brown Spot ................... .... .................................................... 37

................ ...........................................................-- 2.33PinkSeed. .. 38

2.4 Vinises ...................................................................................................... 39

2.4 1 Pea Seed-borne Mosaic Virus .................................................... 40

2.42 Pea Enation Mosaic Virus ........................................................... 41

...................................... .............. 2.43 Bean (Pea) Leaf Roll Virus ... 42

2.44 Pea S treak Virus ................................................................... 43

2.45 Pea Stunt ................................................................................. 44

Chapter 3 - Field Survey for Pea Diseases .................................................................. 46

3.1 Materiais and Methods .............................................................................. 46

............................................. 3.1 1 Selection of Fields to be Sumeyed 46

3.12 Sampling Metho d. ....................................................................... 48

................................................ 3.13 Root Rot Pathogen Identification 52

3.14 Foliar Pathogen Identification ................................................ 52

3.15 Statistical Analysis of Survey Questionnaire ............................... 53

3.2 Results ........................................................................................................ 54

3.21 Incidence and Severity of Root Rot .......................................... 54

vii

Table of Contents

Page

3.2 Results (cont.)

3.22 Pathogen Identification ...................... ... .................................... 54

3.22 1 Root Rot Pathogens ....................................................... 55

3.222 F o l k Pathogens ............................................................ 55

3.23 Survey Questionnaire Results .................................................. 56

................ 3.24 Resuits of Statistical Analysis of S w e y Questionnaire

3 -24 1 Differences Between Areas .......................................... 3.242 Cropping History ................... .. ............................... 3.243 Disease in Previous Pea Crops .......................... ......... .... 3.244 Seed Source ................................................................... 3.245 Seed Treatment ............................................................. 3.246 Inoculant Formulation ................................................... 3.247 Fertilizer Use ................. ... ....................................... 3.248 Fertilizer Application Method ....................................... 3 -249 Tillage. ..........................................................................

........................................................................... 3.250 Rolling

3 -25 1 Herbicide Use ................................................................ 3 . 252 Herbicide Residues ........................... ... .........................

3.3 Discussion ................................................................................................. 3.3 1 Effect of Agronomie Practices on Root Rot Incidence & Severity

3.3 1 1 Differences Between Areas ........................................... 3 -3 12 Cropping Histo~y ........... :: ............................................

................................................................... 3.3 13 Seed Source

3 -3 14 Seed Treatment ............................................................. 3.3 15 Inoculant Formulation ...................................................

viii

Table of Contents

Page

3.3 1 Effect of Agronomie Practices on Root Rot Incidence & Seventy (cont.)

3.3 16 Fertilizer Use ......................... .... ............................ 81

....................................... 3.3 17 Fertilizer Application Method 82

3.3 18 Tillage ........................................................................... 82

3.3 19 Rolling ..................................................................... 83

3 -320 Herbicide Use .............................. ... ...................... 83

Chapter 4 - Pea Disease Management Program ................ .... ............................... 85

4-1 Cultural Practices ...................................................................................... 85

4.11 Conventional Tillage .......................... .. .................................... 86

4.12 Conservation Tillage ................ ... ............................................ 87

4.13 Other Cultural Practices ............................... ... ............................ 90

4.2 Host Resistance ........................... .. ............................................................ 91

4.3 Chemical ControIs .................................................................................... 94

4.4 Proposed Pea Disease Management Program ......................................... 96

Chapter 5 - Conclusions and Recommendations ........................................................ 101

................................................................................................................. Appendix 1 104

........................................................................................................... Literature Cited 106

List of Tables

Page

Table 1: Incidence and severity of root rot in the Peace River region in 1997 ......... 54

Table 2: Mean acreage, standard deviation and range of fields surveyed. ..............Y.. 56

Table 3: Cropping history of fields surveyed ......................... .. .............................. 58

Table 4.1 : Number, percent and Chi-square probabilities

for sources of variation by overall root rot incidence .................. ... ..... 71

Table 4.2: Number, percent and Chi-square probabilities

for sources of variation by overall root rot incidence ................................. 72

Table 5: Overall mean severiiy, coefficient of variance, F-value

and Pr > F for root rot sevenv by source of variance ................................. 73

List of Figures

Page

Figure 1 : Acreage of Field Pea in Alberta 1982-1997 ........................................... 4

Figure 2: Acreage of Field Pea Ui Peace Region of Alberta 1986- 1997 ........... .... 4

Figure 3 : Map of Survey Area and Location of Fields Surveyed .............................. 47

Figure 4: Field sampling method used to obtain pea plants for this study .................. 48

Figure 5: 1997 Growing Season Precipitation - Grande Prairie ................................. 50

............... ............ Figure 6: 1997 Growing Season Precipiîation . Falher ........ 50

..................................... Figure 7: 1997 Growing Season Precipitation . Peace River 50

.................................... Figure 8: 1997 Growing Season Precipitation . High Level 50

...................... Figure 9: 1997 Growing Season Precipitation . Overall Peace Region 50

Figure 10: 1997 Growing Season Mean Temperature - Grande Prairie ..................... 51

Figure 1 1 : 1 997 Growing Season Mean Temperature - Faber ................ .... .......... 51

Figure 12: 1997 Growing Season Mean Temperature - Peace River .......................... 51

Figure 13: 1997 Growing Season Mean Tempehue - High Level ........................... 51

Figure 14: 1997 Growing Season Mean Temperature - Overall Peace Region .......... 51

.................................. Figure 15:RootRotIncidenceinArea5 -Faher/PeaceRiver 78

Figure 1 5: Root Rot Severity in Area 5 - Falherff eace RWer ..................................... 78

Chapter 1 - Background

1.1 History of Pea Production

Peas (Pisurn sativum L.) have been cdtivated by humans for close to 10,000

years, almost as long as wheat (Trificurn aestiwm L.) and barley (Hordeum vulgaare L.).

Peas provide rnuch-needed protein to complement the starches of the cereals. The

appearance of a smooth seed coat is thought to be the most reliable indicator of

domestication. Wild peas generally have a rough or granular seed surface, while

domesticated peas are characterized by a smooth seed coat. Samples of carbonîsed

smooth coated pea seed have been found during archeologicai excavations in early

Neolithic villages of the Near East dating fiom 7000 to 6000 B.C. Peas made their first

appearance in Europe in the Neolithic period in various agricultural settlements such as

in Greece (5500 B.C.), in Bulgaria (4330 B.C.) and in Romania, Czechoslovakia, Austria,

Switzerland and the Iowa Rhine Valley (4400 - 4200 B.C.). Earliest finds of peas in

western Europe corne from the Bronze Age (Zohary & Hopf 1973).

Peas were originally calledpisos by the Greeks andpisum by the Romans. When

they first showed up in England they were called "peason", then 'peaseYy or "peasse" and

this was eventually shortened to "pea", the name in use today ( M m 1977).

Domesticated peas, Pisum satiwm L., have been traced back genetically to the

wild pea type P. humile, which is still found growing in the oak park forests of the Near

East (Zohary & Hopf 1973; Cousin 1997). Peas probably originated in Abyssinia and

Afghanistan with areas in the Meditemean region col onized later (Cousin 1 997).

Vavilov (1 95 1) in his search for the origin of plant species listed two "centres of

diversity" for peas. These were the centrai Asiatic centre and for larger seeded varieties.

the Mediterranean centre.

1.2 Ristory of Pea Production in Canada

Peas were first grown in Canada by French settlers who used them for making

"pea soup", and they were popular in the diets of pioneers who helped to settle the west.

Dry peas have been grown on a lunited acreage in western Canada since it was Grst

settled more than 100 years ago (Slinkard et al. 1994b).

Peas were a popular crop in Eastern Canada in the late 1800's. An average of

29 1,384 hectares were grown each year fiom 1883 - 1902. Production declined however

as other crops became more common. By 1970, only 25,09 1 hectares of peas were grown

in Canada with Manitoba accounting for 70 percent of this area (Anonpous 1997a).

Peas did not become popular again until the wheat glut of the mid 1970's. The demand

for field peas was increasing and western Canadian f m e r s were willing to diversi@

their operations to fil1 the need (Slinkard et al. 1994b). Western Canadian f m e r s

recognized that field peas had mmy desirable agronomie characteristics they were

looking for. They could be seeded and harvested with conventional farm machinery, they

fixed their own nitrogen and they fit well into crop rotations heIping to break many

disease cycles.

Alberta f m e r s embraced this new Cinderella crop. The fxst record of field peas

in Alberta is in 1 890, when 43 hectares were seeded (Anonymous 1 996a). By 19 10, there

were 12 1 hectares grom (Woychuk et al. 1972). Production fluctuated between 162 and

1 1,736 hectares until 1987 when a record 26,305 hectares were grown (Anonymous

1996a). Interest in field peas was sparked by bigh pBces for green peas because of a

drought in the Pacific Northwest of the USA (Slinkard et al. 1994b). Alberta had been

growing mainly green peas and reaped much of the benefit fkom this pnce increase.

Alberta has the highest pea yields in Canada, averaging 35 percent higher than either

Saskatchewan or Manitoba. In 1997, Saskatchewan had the highest acreage (607,000

hectares) folowed by Alberta (155,800 hectares) and then Manitoba (83,000 hectares)

(Anonymous 1997b).

Figures 1 and 2 illustrate how production has increased in Alberta since 1982 and

in the Peace River region since 1986. Reasons for the dramatic increase in acreage of

field peas are nurnerous. One of the main reasons is that markets for animai feed and

human consumption have increased both domestically and intemationally and that

growing field peas is now very profitable (Anonymous 1996a). The introduction of new

semi-leafless, erect and high yielding pea varieties has also helped to keep f m e r s

interested by making harvesting easier and increasing returns.

Year

Figure 1 : Acreage of Field Pea in Aiberta 1982 - 1997

1986 1990 1991 1 QB2 1993 1884 1995 1996 1897 Yaar

Figure 2: Acreage of Field Pea in the Peace River Region of Alberta 1986 - 1997

Source: Alberta Agriculture, Food & Rural Development - Statistics & Production Economics Branch 1998.

5

More than 70 percent of the peas grown in Canada are exported. The largest

buyers of Alberta peas are Spain (52,332 tonnes), Belgium (5 1,2 1 1 tomes), Cuba

(25,220 tonnes), India (22,682 tonnes), France (1 1,922 tonnes) and the Peoples Republic

of China (1 l,82 1 tonnes) (Pekalski 1997). Within Canada, the pork industry in Alberta

and Manitoba accounts for the largest domestic connimption of peas (Anonymous

lW6a).

1.3 Climate of the Peace River Region

The climate of the Peace River region is characterized by relatively cold winters

and moderately warm summers (Reeder & Odynsky 1965). Though annual precipitation

is low, it is relatively well distributed during the growing season. Moderate temperatures

during the growing seson also help to keep evapotranspiration low. Long s u m e r days

prornote rapid development of crops and rnost mature within the fiost fiee period (Carder

1965). The region can be divided into two distinct climatic areas; the Upper Peace and

the Lower Peace (Carder 1965; Carder & Siemens 1971). The Upper Peace (55 O to 57"

latitude) encompasses the bulk of the agricuftural area in the region. The lower Peace

(58 O to 60" latitude), so named because of the lower elevation is slightly warmer in

summer but much colder in winter. The Lower Peace is also generally drier but has less

wind so evapotranspiration is reduced. Longer daylength in the Lower Peace also

promotes more rapid growth of crops and compensates for the shorter growing season

(Carder & Siemens 1971). In general, July is the w m e s t month of the year followed by

August and then June (h4cKenzie & Hall 1976). The 30 year (1 967- 1997) average

growing season (May to September) precipitation ranges from 306.7 mm in Grande

Prairie, to 242.2 mm in Peace River, to 257.8 mm in High Level. The 30 year average

annual growing degree days greater than 5°C for the region is 1287.9 (Anonymous

1998~). Frost fiee periods (0°C) vaiy fkom 113 days at Grande Prairie in the Upper Peace

to 9 1 days at Fort Vemülion in the Lower Peace. (Anonymous 198 1). The agicultural

potential of the region is improved substantially by the long summer day length. It should

be noted that large variations in temperature and precipitation exist fkom area to area

within the region and fiom year to year (Carder 1965; Carder & Siemens 197 1;

McKenzie & Hall 1976).

Because field peas have been grown on a large scale for only a short time in the

Peace region, very little work has been done on pea diseases. Hamison & Laflamme

(1996) did a field pea root rot survey of 38 fields in the Peace region in 1995. They f o n d

that root rot was present in al1 fields surveyed. The main disease organisrns identified

were Fusarium species. Uther diseases observed were sclerotinia rot (Sclerotinia

sclerotiomm (Lib. de Bary), ascochyta blight (Ascochyta spp.), downy mildew

(Peronospora viciae (BRk.) de Bq), powdery mildew (Evsiphe pisi Syd.) and gray

moId (Bavtis cinerea Pers. ex Fr.).

With field pea acreage increasing in the Peace River region, more research was

needed to identiQ the presesence and severity of pea diseases in the region. This

information dong with a better understanding of field pea diseases would allow acreage

to increase wi& minimal risk h m disease. Many crops now grown in the region require

a minimum 4-year rotation to prevent problems with diseases. Canola, for example, a

crop which is considered to be the highest returnùig cash crop in the region should only

be grown once every 4 years to prevent diseases such as sclerotinia stem rot and virulent

blackleg from becoming a problem (Harrison 199 1). Rotations could be planned with the

howledge that disease cycles are being broken and crops are benefiting from the

inclusion of peas in the rotation.

1.4 Objectives

The objectives of t h i s study were to:

Conduct a field survey to assess the incidence and severity of root rot and to

identiQ root and foliar diseases in peas grown in the Peace River region of

Alberta,

Conduct a production s w e y of pea growers in the Peace River region of Alberta

to determine if a correlation exists between any production practices and root rot

incidence and severity.

Use the results of the survey and a literature review of field pea diseases to

propose and develop a field pea disease management program for field pea

growers in the Peace River region of Alberta.

Chapter 2 - Pea Disease Descriptions

2.1 Soil-borne Diseases - Seedling Blight, Seed Rot & Root Rot

This disease complex is caused by several soil-borne fungi. Pythium ultimurn

Trow and other Pythium species, Fusannum soluni (Mart-) Sacc. Esp. phi (Jones) Snyd. &

H~IIs., F. oxysponim Schlecht. emend Snyd & Ham. fhp. pisi (Hall) Snyd. & Hans. and

Rhizoctonia solani Kahn are aU involved in this complex in Alberta. Aphanomyces

euteiches Drechs. f.sp. pisi Pfend. & Hag. and ntielaviopsis basicoln Berk. & Br.,

although reported as part of this disease complex in other pea growing areas of the world

are not presently lmown to occur in Alberta (Howard RJ. - Personal Communication

1998).

These f h g i cm attack individuaIly or collectively anytime fkom seed germination

to rnaturity of the pea plant. Together they cause a complex of seed rot, damping off,

seedling blight, root and foot rot diseases, the symptorns of which overlap and therefore

are difficult to differentiate in the field. Soil-borne diseases are considered the main

limiting factor in increasing and stabilizing pea yields in North Amenca (Kraft 199 1).

Each of these mot diseases will be descnied individually. Because these pathogens are

rareIy found individually in the field (Swamon et al. 1984; Hwang and Chang l989),

however, control recommendations will be given for the disease complex as a whole.

2.11 Fusarium Root Rot

Fusarium soIani (Mart.) Sacc. f. sp. pisi (Jones) Snyd. and Hans. is a serious

C 9

pathogen of peas in many parts of Canada and the US. Yield losses of 26 - 57% have

been reported in both dryland and irrigated areas (Basu et al. 1 976; Kraft 1 984; Hwang et

al. 1995a). Fu~arium spp. are the most common root rot pathogens isolated fkom peas in

Alberta (Surnar & Howard 1979; Hwang & Chang 1989).

Symptoms:

S ymptoms first appear on the hypocotyls, epicotyls and cotyledonary attachent

area. Reddish brown streaks initially develop on the primary and secondary roots and

eventudly coalesce to fom a dark reddish-brown colour on the primary root up to the

soil line. Graying, yellowing, necrosis of lower foliage, and stuntirxg can al1 occur if

infection is severe (Kr& & Kaiser 1993). Foliar symptorns often appear after periods of

w m temperatures and heavy r;tinfdl. Fusarium root rot is favoured by poor crop

rotations, high soil temperatures (22' - 30" C), moist soils, acidic soils (pH 5.1 - 6.2) and

low fertility (Kraft 1984; Tu 1994). Soi1 compaction by f m machinery has also been

shown to increase the incidence and severity of F u s a root rot (Tu 1994).

Disease Cycle:

Initial infection usually occurs through the stomata on the epicotyl and hypocotyl

and the pathogen spreads into the root system. Infection can also occur through direct

peneîmtion of the cuticular surface of the epicotyl. The primary suMval structures are

chlamydospores. Chlamydospore germination is triggered by the release of exudates as

pea seeds germinate (Kraft 1984).

2.12 Fusarium Wilt

Fusarium wilt of peas is caused by Fwurzùrn oxysponrm Schl. f. sp. pisi (Hall)

Snyd. & Hans. Several races have been identified with races 1,2,5 & 6 the most

important in North Amaica Race 2 is also hown as near-wilt. Races 1 and 2 are more

of a problem in eastern North America and races 5 and 6 in western North America

(Haglund 1984). Tu (1992) reported that although fus& wilt was not the moa

common disease of peas in southwestern Ontario, it was the most serious because of its

severity.

Symptoms:

Symptoms for race 1,5 & 6 are very sirnilar. Race 2 symptoms have some

variation. In mots when cut longitudinally, there is a yellowish, orange colour in the

vascular tissue, which can extend into the basal area of the stem. For race 2, the

discolouration is more pronounced and is dark orange to red in colour and extends to the

crown of the plant. M e r symptoms common to al1 races include dowward curling of

the leaves and stipules, thickening of the basal intemode and brittle leaves and stem.

Yellowing of the leaves progresses &om the base of the plant to the top of the foliage.

Plants often die due to loss of the root system (Haglund 1984; Tu I987a).

Disease Cycle:

Chlamydospores are the primary çunival structures of this fungus and can remain

infective in soils for 10 or more years. F. oxyspomm Esp. pisi can also be seed-borne.

Soi1 temperatures of 20 ' to 2 1 OC are considered optimal for infection by race 1,5 and 6

and 25 OC for infection by race 2. Chiamydospores germinate and infect pea plants

through the fibrous roots. Infection then progresses into the vascdar system. Race 1, 5

and 6 are most often found in small circular patches in fields. Race 2 is usually found in

plants scattered throughout the field (Haglund 1984). Resistant cultivars are available for

the different races (Cousin 1997).

2.13 Pythium Root Rot

Seed rot and seedling damping-off caused by Pythium species are considered a

major Iimiting factor in pea production in Alberta (Hwang et al. 1997) e t h i u m species

are common in Alberta soils. Hwang and Chang (1 98 9) found an average of 600

propagules (fimgal colonies) of q.thizrm species per gram of air dried soi1 in a survey of

soils in northeastern Alberta P. irregulare and P. ultimum have been the two species

most fiequently isolated fiom field pea in Alberta (Hwang S.F. - Persona1

Communication 1998).

Symptorns:

Seeds are rotted and when removed f?om the soil, emerge with a layer of soi1

around them. This layer of soil is full of whitish fimgal hyphae. Ernerging radicles or

plumules, if produced, may be soi? and watery and cotyledons may or may not be rotted

(Haman 1984a). Germinating seeds are only susceptible for 48 - 72 hours. Once the

radicle emerges korn the seed coat, the seed is no longer susceptible to infection. New

developing tissue, however, remains susceptible. Infection can occur at the tips of feeder

12

roots where juvenile tissue can be attacked and destroyed. This can lead to root pninuig

and/or reduction in root length. Depending on the severity of the infection, seedlings may

become stunted and chlorotic and collapse as the root base decays and hims tan to light

brown in colour (Hwang et al. 1997). This results in patchy stands in the field due to

missing seedlings. Infected plants tend to lack vigour and often yield poorly.

Disease Cycle:

Pylhium spp. are found in most soils as dormant sporangia or oospores. These

germinate very quickly in the presence of a sîimulus such as exudates fkom seeds. Seed

coat infection c m occur in as little as 6 hours and infection of cotyledons within 40 hours

(Hamm 1984a). Once infection occurs, large numben of sporangia and oospores are

produced. Infection is very dependent on seed quality. Cracked seeds le& more exudates

and are more likely to attract Pythium and be infected than intact seed. Severd studies

have shown that q>thiurn spp. are more of a problem when soil temperatures are cool,

soils are poorly drained and soil moisture is high (Benedict 1969; Kraft & Roberts 1969;

Hwang et al. 1997). Cool soil temperatures slow the rate of seed germination and make

them susceptible to infection over a longer period of thne (Leach 1947).

2.14 Rhizoctonia Root Rot

Rhizoctonia solani Kühn is endemic in most pea producing areas of Alberta and

can cause serious damage under the nght enviromenta1 conditions (Hwaog & Chang

1989). The AG2 and the AG4 groups have been the most fiequently isolated fiom field

pea in Alberta (Hwang S.F.-Personai Communication 1998).

Symptoms:

R . soloni prefers well-aerated soil near the surface and most often infects the

hypocotyl, epicotyl and seed (Kraft & Kaiser 1993). Symptoms on seedings k t appear

as water-soaked Lesions that eventually tum reddish brown. Death of the growing point

may occur as it emerges fiom the ground which can lead to multiple shoots emerging and

dying. Symptoms on older plants appear as reddish-brown sunken lesions on the epicotyl.

This can cause girding and often leads to stunted plants. Seedlings generaily become Iess

susceptible as they get older. Infection most often occurs close to the soil surface where

oxygen levels are high and this is where symptoms are most often found (Kraft &

Harman 1984).

Disease Cycle:

R. solani cm survive in the soil for many years as a saprophyte and only

decreases in number in the presence of non-host crops. Infection occ-irs when

genninating hyphae growing through the soil make contact with a susceptible host. An

infection cushion is fonned on the surface of the host and an uifection peg penemtes the

epidermal surface. Growth continues inside the host both inter and uitracellularly.

Infection normally occurs early in the life of the seedling and seedlings become less

susceptible with age. Overwintering occm as mycelium (Kraft & Harman 1984; Hwang

& Chang 1989). High soi1 temperatures (24'-30°C) are known to cause higher rates of

infection in pea plants ( K . & Kaiser 1993; Xi et al. 1995).

2.15 Aphanomyces Root Rot

World-wide, aphanomyces root rot, caused by Aphanomyces euteiches, Drechs.

f.sp. pisi Pfend. & Hag. is considered one of the most serious root rot diseases of peas

(Jacobsen & Hopen 198 1; Muehlchen et al. 1990; Parke et al. 199 1) because of its

resistance at attempts to find culmral, chernical and genetic controls (Pfender 1984). Ln

North America, it is a much more serious problem in the Great M e s and northeastem

States of the United States than in the western pea growing areas (Pfender 1984).

Aphanomyces root rot has been reported on peas in Manitoba (Grau et al. 1991) and

Ontario (Tu 1992). The presence o f aphanomyces root rot has yet to be confirmed in

Alberta (Howard R.J. - Persona1 Communication 1998).

Symptoms:

Symptoms can develop in as little as 7-14 days when inoculum levels are high

(Pfender & Hagedorn 1983; Kraft & Kaiser 1993). S ymptoms f i s t appear on roots as

straw-coloured lesions that gradually spread causing a sofi, watery rot close to the soil

line that is fairly distinctive. Darnage to the root cortex results in slmghing off of the

cortex when infected pea plants are pulled from the soil. This damage also Ieads to

yellowing from the bottom up and stunting of the plant as it is unable to extract nutrients

and moisture from the soil (Pfender 1984)-

Disease Cycle:

Aphanomyces root rot c m infect peas at any growth stage but occurs more

cornrnonly at thz seedling stage (King & Parke 1993; Kraft & Kaiser 1993). Infection on

pea roots occurs when oospores germinate via genn tubes. Mycelium then grows

15

throughout the root tissue causing degradation of the root cortex. Oogonia and antheridia

are produced which leads to the formation of mature oospores in 7 - 14 days. These are

dispersed in the soi1 as the roots decornpose and are present for reinfection when host

plants are next grown (Pfender 1984). Oospores are very persistent in the soil.

Recommendations for reducing inocdum to safe Ievels range fiom 6-8 years (Temp &

Hagedorn 1967), to 9 years (Pfender & Hagedorn 1983), to over 10 years (Pfender 1984)

without peas.

2.16 Thielaviopsis Root Rot

Thieimriopsis h i c o l a Berk & Br. is considered a senous root rot pathogen of

peas in some pea growing areas of the United States. Although reported in Ontario, it is

not yet considered a problem in Canada (Tu l987a).

Symptoms and Disease Cycle:

Thielaviopsis root rot is aiso known as black root rot because of the necrosis of

the cortex it causes on both tap and lateral roots. Chlamydospores are produced which

germinate in the soil to infect new plantings of peas. T. basicola often occurs with F.

solani and together they cause a necrosis and blackening of the entire mot system.

Occasionally, small, black lesions also occur on the lower leaves of infected plants. High

soil temperatures (28 OC) favour this disease (Lloyd &- Lockwood 1963) but severe losses

have been reported at 20°C. Heavier soils also seem to be prefened by this pathogen

(Blume & Harman 1979; Haman 1984b).

16

2.17 Prevention and Control of Seedling Blight, Seed Rot and Root Rot:

Since al1 of these root rot pathogens are rareiy found individually in a fie14

control measufes should be aimed at controhg the complex of pathogens. Since

aphanomyces and thielaviopsis root rots are not known to occur in Alberta at present,

these control recommendations will focus mainly on the other root rot pathogens known

to occur in Alberta.

The most effective control strategy for these soil-borne diseases is a 4-5 year crop

rotation away fiom peas or other susceptible hosts such as dfalfa and clovers.

The use of seed with high vigour is another very important way to avoid problerns with

these fun@ (Kraft & Kaiser 1993). High vigour seed genninates and emerges quickly,

thus decreasing the time the pea plant is susceptible to infection. Vigour tests for peas are

available from selected seed testing labs and should be used to select pea seed with high

vigour.

Good soi1 fertility will also shorten the time that the pea plant is in a susceptible

growth stage. Peas inoculated with the correct strain of rhizobia will produce most of the

nitrogen they require but adequate amounts of phosphorus, potassium and sulphur are

needed to ensure that peas get off to a good start. Peas need to have adequate nutrients to

produce healthy plants that are able to resist attack by various soil-borne and foliar

pathogens.

Liming of acid soik is another way to reduce root rot. Tu (198%) showed that

soi1 pHs between 6.0 and 6.4 Ied to the lowest level of root rot. Soi1 pH lower or higher

than this range had higher levels of root rot.

Reducing soi1 compaction has also been shown to decrease the incidence of root

rot (Ex 1994). This can be accomplished by subsoiling to break up the compacted layer

of soi1 below the depth of cultivation. Compaction can dso be reduced by the adoption of

minimum and zero-tillage which involves fewer heavy machinery passes on the field.

Thulim ( T a 75 WP) and captan (Captan FL) are registered for control of seed

decay, damping off, root rot, and seedling blight. Metylaxyl (Apron FL) is registered for

control of seed rot and seedluig blight specific to Pythium species and can be mixed with

thiram to give a broader control spectnun (Anonymous 1998a). These hgicides,

however, offer no protection once the plant is past the seedlhg stage.

A biocontrol agent, ACM94 1, for root rot is presently being tested in western

Canada and is showing controi levels similar to thKam (Xue 1996).

Herbicides such as (4-chloro-2-methylphenoxy) acetic acid (MCPA), 4-(4-chloro-2-

methylphenoxy) butanoic acid (MCPB) or a mix of MCPA and MCPB (Tropotox Plus)

should be avoided as it has been shown that they cause stress on the plants and

consequently increase root rot severity (Tu & Hamil1 1986; Tu 1992). Many effective

herbicides are presently registered for use on field peas that do not cause stress on the

plants (Anonymous l998a).

2.2 Foliar Diseases

This disease complex is caused by three different species of fungi, each of which

produces a clearly recognizabl e symptom: MycosphaereZZu pinodes (Berk. & B lox .)

Vestergr. (the pdect stage of Ascochyta pinodes) which causes blight; A. pisi Lib. which

causes leaf and pod spot and Phoma rnedicaginis var. pinodella (Jones) which causes foot

rot (Wdlen 1974; Lawyer 1984a; Kraft 1991). M.pin0de.s is the most common on field

pea in western Canada (Wallen et al. 1967; Berkenkamp & Kirkham 199 1; Rashid et al.

1994; Xue & Burnett 1994; Xue et al. 19%) and the disease it causes is ohen called

ascochyta blîght (Warkentin et al. 1 996) or mycosphaerella blight (Anonymous 1 997a).

Average yield losses due to mycosphaerella blight on field pea in Westem Canada

are estimated to be 10 percent. Yield losses of up to 80 percent have been reported in

Westem Canada when a heavy infection occurs in mid-June (Anonymous 1997a) and up

to 50 percent on processing peas in eastem Canada (honymous 1996b; Martens et al.

1984).

The three species are grouped together as a cornplex because they often occur

together and distinguishing between them in the field is difficult. Al1 three pathogens are

common throughout the temperate pea growing areas of the world, such as Australia,

New Zealand, North America and Europe (Krafi 199 1). M. pinodes has even been

reported fkom subtropical regions in Central and South Arnerica, Afiica and Haiti

(Lawyer 1984a).

Symgtoms:

These fùngi attack roots, leaves, stems, flowers and pods. S ymptoms of infection

by A. pisi are different âom those of M. pinodes or P. m e d i ~ a ~ n i s var. pinodella.

Lesions caused by A. pisi are partially sunken, tan in colour and surrounded by a well-

defmed dark brown margin. Circular lesions are found on pods and leaves while lesions

are more elongate on stems (Skolko et al. 1954; Lawyer 1984a). Numerous pycnidia are

usually found in the lesions. Lesions of A. pisi are rarely found on any plant parts below

the soi1 line. (Lawyer 1984a)

It is much more difficult to Merentiate between lesions caused by M. pinodes

and P. m e d i ~ a ~ n i s var. pinodella without a detailed laboratory examination. However,

the assumption can usually be made that if the lesions on leaves, stems and pods are

widespread and severe, M. pinodes is the cause whereas if the lesions are severe on

subtemean parts, P. rnedicaginis var. pinodellu is the cause. M. pinodes produces small,

brown to purplish, irregular flecks without definite margins that initially appear on pods,

leaves, stems and the cotyledonary area (Skolko et al. 1954). These lesions enlarge if

weather conditions are fàvoufab1e. Optimal temperatures for disase development by M.

pinodes are between 15 O and 18 O C (Wallen et al. 1967). Dark-brown to black pycnidia

are eventually produced and as the lesions enlarge, a distinct concentnc tan and brown

ring pattem is formed This is ofien more pronounced on leaves and pods than on other

plant parts. Lesions developing on stems tend to form long, wide purple to bluish-black

streaks that eventually codesce and may completely girdIe the stems, pedicels or tendrils

(hwyer 1984a). These streaks are more common near the nodes and on the lower

20

portion of the stem (Skoko et al. 1954). Pod infection can lead to seed iafection if it

occurs early enough. Seed infection may show no visible symptoms if infection is light;

if severe, vaqhg degrees of shrinkage and discolouration can develop (Anonymous

19960). Under drier conditions, the concentric ring pattem of the symptoms is less

pronounced and may show up only as a uniform yellowing of lower leaves. If the

blossom becomes infeste4 girdiing of the sepal often occurs leading to pod drop or

distortion. Bretag et al. (1995) showed that in southem Australia, yield was closely

correlated to disease severity. For most pea varieties evaluated in their study, there was a

5 to 6 percent yield reduction for wery 10 percent of the stem area affected by ascochyta

blight.

Disease CycIe:

These pathogens can be seed, stubble or soil-borne. Seed-borne infection by M.

pinodes is considered to be the primary source of inoculum in virgin pea fields (Xue et

al. 1996). M. pinodes also competes well with other soi1 micoflora and survives as

thickened mycelia (sclerotia), chlamydospores and pycnidia on pea straw hgments and

in the soil. When moisture and temperature conditions are favourable, residual pycnidia

mature, new pycnidia develop and spores are released to infect new plants.

Pycnidiospores are produced throughout the growing season during periods of moist

weather and cm be carried by rain splash to healthy plants. Ascospores are also produced

which can be carrïed by whd for a kilometre or more (Martens et al. 1984). P.

medicnginis var. pinodellu zlso produces chlamydospores and pycnidia and is also quite

persistent in the soil. A. pbi. on the other han& competes vay poorly with other

21

microflora in the soi1 and overwinters very pooriy. The main source of infection fiom A.

pisi is fkom seed-borne spores. Symptoms of M. pinodes and P. medicagink var.

pinodeh appear in 2 to 4 days. A. pisi symptoms appear in 6 to 8 days. Under optimal

moisture conditions the pathogen cm spread rapidly in a field.

Prevention & Control:

Pea seed should be examined for Ascochyta presence by an accredited seed Iab

and only pathogen-fkee seed should be used. If disease fYee seed cannot be found, seed

with as low a level of Rscochyta as possible should be used and the seed treated. Seed

treatrnent will help but only if seed is used on virgin pea land (Wallen et al. 1967).

Thiram, a mix of thiram and iprodione (Rovral) or a mix of metdaxyl (Apron FL),

carbathiin and thiabendazole (Crown) has been shown to give excellent control of seed-

borne M. pinodes on peas (Rhashid et al. 1996). Only thiram, however, is registered in

Canada for use on peas (Anonymous 1998a).

Chlorothalonil (Bravo 500) has recently been registered for use on field peas in

Western Canada to control mycosphaerella blight. Chlorothalonil is applied in the early

btoom stage and can be repeated up to three times at 10 to 14 day intervals if conditions

for disease development are favourable (warm and humid) (Anonymous 1998a).

BenomyI (Benlate) has also been shown to provide good control of this disease

(Warkentin et al. 1996) but is not registered for use on field peas in Alberta at this time

(Anonymous l998a).

Crop rotations done do not provide effective control of this disease cornplex

since ascospores can travel long distances by wind (K& 1991). Care should be taken to

22

avoid seeding next to any previous year's pea fields.

Plowing or discing to bury all crop residue as soon as possible &er harvest will

also help to prevent the fungus f b r n being dispersed by wind and min.

2.22 Sclerotinia Rot

Sclerotinia rot in peas, caused by the fungus ScZerotinin sclerotionrm (Lib.) de

Bary, has been reported fiom Canada, the United States, Morocco, Bermuda, New

Zealand, Brazil, Argentins, the Netherlands and Scotland (Huang & Kokko 1992). It is

often cailed white mold or pod rot in peas. This disease is common in canola fields in

Western Canada and has recently started showing up in pea fields. niere is defmitely

concern about sclerotinia rot when peas and canola are grown in the same rotation.

Davies (199 1) reported a higher incidence of sclerotinia stem rot in canola when peas

were included in the rotation. If conditions are favourable and infection is early,

sclerotinia rot can cause serious yield losses in peas.

Symptoms:

The later stages of plant development, from f i owering to maturity, is when the

disease occm most often. Dense canopies and high humidity are favourable to outbreaks

of sclerotinia. The fmt sign of infection is the appearance of a light brown to brown

discolouration on the stem, leaves or pods. With pod rot, this occurs at the base of the

style with a small water-soaked lesion appearing on green tissue at the end of the pod

(Huang & Kokko 1992). With stem rot, the symptoms are more common in the node

23

area. Whitish mycelial mats develop over the affected areas that over time cause the

tissue undemeath to turn soft and decay (Lawyer 1984b). Black, hard bodies known as

sclerotia form in the mat and within the stem and pods. AfZected plants often appear

wilted and ripen premahlrely due to rotting of the stems. Lodging is common in afEected

areas.

Disease Cycle:

S. sclerotiorum will grow over a wide range of temperatures eom O O to 28 O C.

Optimal temperatures, however are between 20 O and 25 O C . Lawyer (1 984b) descnbes

the mycelium as hyaline, septate and branched. The mycelium forms sclerotia that vaiy

in size and shape depending on environmental conditions and age. Sclerotia consist of a

layer of pigmented external cells, three to four layes thick, m a h g them hard and

resistant to decomposition. When humidity is hi& and moisture is abundant such as

within a thick canopy, tan, fleshy, golf-tee shaped apothecia emerge f?om the sclerotia.

Numerous ascospores develop within the apothecia and are released into the air to infect

nearby plants. Ascospores cannot infect pea plants directly and need a source of

senescent fissue such as stamens or flower petah to germinate (Huang & Kokko 1992).

Following germination, hyphae f o m that spread and fonn numerous infection cushions

that penetrate the plant by mechanical pressure (Lawyer l984b; Huang & Kokko 1992).

The fungus overwinters as sclerotia either in plant debris or in the soil. infection

cm occur by one of two methais. Direct infection by mycelia c m occur fiom sclerotia

located in or on the soil surface or fiom ascospores released from apothecia Airborne

ascospores are the most common method of infection in peas (Huang & Kokko 1992).

Ascospores cm be released over a considerable period of t h e if conditions are

favourable and can be dispersed by air currents for several kilometres to infect

neighbouring fields. The critical period of infection, however, is during the flowering

stage (Hamson 199 1). Under moist conditions, ascospores land on senescent stamens,

filaments or petals that become colonized by threadlike running hyphae. These hyphae

spread fkom the senescent tissue to healthy tissue and fomi numerous infection cushions.

Peneîration occurs at the bottom layer of hyphal celk of each infection cushion. Pollen

grains are another source of nutnents for the hyphae to colonize. Lesions appear 4 to 5

days afier infection and within 8 days, dark to p y i s h brown lesions ranging nom 1 to 5

mm cm be seen on the infected tissue (Huang & Kokko 1992). When the peas are

harvested, sclerotia are either harvested with the seed or fall to the ground with the vines.

Only sclerotia in the top 6 cm of soil c m produce apothecia. Sclerotia buried deeper

remain donnant and c m survive for 5 to 7 years in the soil untiI favourable conditions

exist for apothecia formation (Lawyer l984b; Harrison 199 1).

Prevention and Control:

A 4 to 5-year crop rotation out of susceptible crops is recommended to prevent

possible infection. Sclerotinia has a hown host range of 360 plant species and

susceptible crops incliide canola ( h s i c a mpus & B. rupa), d o w e r s , beans, alfalfa,

clover and potatoes. Common weed hosts include stinkweed (Thlaspi anense L.), hemp

nettle (Galeopsis tetruhit L.), thistles (Cirsium anense L. & Sonchus arvensis L.),

shepard's-purse (Capsella bursa-pasto* K.] Medic.) and wild mustard (Brassica kaber

[D.C.] Wheeler var. pinnat3da [Stokes] Wheeler) (Harrison 1 99 1). Moldboard plowing

25

for deep burial of sclerotia can prevent the sclerotia fkorn germinating. Planting semi-

leafless varieties will help to create a more open canopy that is less likely to have the

high rnoishire needed for infection to occur. Fungicides such as benomyl (Benlate) and

iprodione (Rovrai) are used in canola to control the disease (Harison 1 99 1 ). However,

neither is registered in Canada for use on field pea at this tune (Anonymous 1998a).

Biological control of sclerotinia rot in peas may be an option in the not-too-

distant future. A field trial is presentiy being done at the Lethbndge Research Station to

study the efficacy of an application of antagonistic bactena to peas to control sclerotinia

rot (Huang et al. 1993).

2.23 Powdery Mildew

This disease is caused by Blumeriapisi f.sp. pisi DC. and occurs worldwide

wherever peas are grown, generally in late mahiring crops, and is often considered of

little economic importance @ixm 1978; Zimmer 1984). It shows up as clouds of dust

when the peas are swathed or combined. Yield and quality of the pea seed itself are

usually not affecteci. In yean when the disease occurs early, however, it can cause

serious yield and quality losses.

Symptoms:

Symptoms first develop as off-coloured spots on the upper surface of the lowest

and oldest laves, then rapiciiy spread to cover the entire surface of leaves, stems and

pods with a fine, powdery, bluish-white mildewy growth. Undemeath the mildew, the

26

tissue often tums purplish in colour. As plants age, tiny pinhead shed fniiting bodies

(cleistothecia) develop. These are brownish in colour at kt, eventually tuming black at

maturity. Infection can also reçult in withering of foliage. If infection occurs on the pods,

penetration of the pod may occw and cause the seed to turn grayish-brown in colour. If

infection is severe, it can result in hollow se&. Other effects of severe infection are

reductions in plant weight, weight of pea seed, number of peas per pod and pods per

plant, as wdl as plant height (Dixon 1978; Reiling 1984a). Nodulation and nitrogenase

activity are also known to be reduced by Blumeriapisi f.sp. pisi. The earlier the infection

occurs, the greater is the reduction (Singh & Mishra 1992).

Disease Cycle:

BZumeriapisi fsp. pisi is an obligate parasite which obtains nutrients fkom the

plant through haustoria in epidermd cells (Agrios 1988). The cleistothecia oveNvinter in

plant debris and on other hosts. The disease can also be seed-borne (Dixon 1978). As

temperatures increase in the spring, the cleistothecia release ascospores that are c h e d

by wind cwents to healthy susceptible plants. Within 2 hours, under dry conditions

conidia gemiinate to produce a single germ tube which forms an appresso~um. Infection

pegs develop fiom the appressorium, pierce the ce11 wall and form a haustorium within

the cell. The first hypha to develop grows fiom the surface of the conidium followed by

second and third hyphae. Subsequent hypbae grow from the appressorium (Smith et al.

1996). Appressoria develop at regular intervals dong the hyphae. Conidiophores also

develop on hyphae as they spread over the leaf surface (Falloon et al. 1989).

Powdery mildew is seldom a problem until late in the season when conditions of

high humidity without rainfall are more common favouring development of the disease.

S u m e r rahs damage the conidia causing them to burst instead of genninate.

Cleistothecia are not affected by min but usually develop in the fa11 and contribute to the

spread of the pathogen. In late summer or early fa& when dews occur without rain the

disease cm spread across a field of peas very quickly (Martens et al. 1984). Fomuiately,

most peas have rnatured suficiently by this tirne that the pathogen causes little if any

yield or quality losses.

Prevention and Control:

Seeding early (Zimmer 1984), use of earlier maturing varieties, crop rotation and

burial of crop residues al1 help to reduce uifection by powdery mildew.

Kumulus DF, an 80% sulfur product has recently been registered to control powdery

mildew on peas in western Canada (Anonymous 1998b). Varieties have also been

developed that are resistant to powdery mildew (Warkentin et al. 1995; Cousin 1997).

2.24 Downy M d e w

This disease caused by Peronospora viciae @&) Casp., is favoured by cool,

wet growing conditions and occurs most ofien on early seeded crops.

Symptoms:

Symptoms c m be expresseci either systemically, tocally (on leaves, tendrils and

flowers) or as pod infections. Systemic uifections produce the most severe effects on the

pea plants resulting in stunting, distortion and the proliferation of white, cottony growth

28

on the plant. Profuse sporulation of the fungus occurs on the surface of the plant. Locd

infections appear as fiuffy, white to bluish, cottony patches on the undersides of leafiets.

The upper side of the kaflets becomes yellow and dies. Plants ofien die before fiowerinp

or if pods are produced, they are flatteneci, yellow, distorted and rarely set seed

Pod infection f h t appears as blotches which become brown with green areas inside

them. This causes small, brown, sunken spots to occur on the seeds which if swere can

lead to seed abortion. A cottony growth cm be visible inside the pod that contains

numerous oospores. Plants often develop resistance as they age and new infections may

only appear as çpotting on the leaves (Dixon 198 1; Reiling I984b).

Disease Cycle:

The pathogen overwinters primarily as oospores in the soil and on plant debris.

Longevity of oospores in the soil can be as long as 15 years. Seed-borne infection cm

also occur and often causes seed to fail to gemiinate. Seedings can become infected

systemically or locally from soil-bome oospores. Sporangiophores originate fiom the

mycelium, which produce sporangia and are the main source of secondary infection on

stems, pods and Ieaves. They genninate to produce a germ tube that penetrates through

the stomata and produces a haustorium. The pathogen progresses intercellularly. Spore

dispersal and infection occur mainly through &dl. Cool, moist weather favours the

diseaçe. Spore survival decreases with inmeashg temperatures and decreasuig humidity

(Reiling 1 W b ) .

Prevention and Control:

A 2 to 3-year crop rotation will control this disease very well. Deep burial of crop

29

residue can help prevent spread of the pathogen to a subsequent crop. Use of disease-fiee

seed is also very important to prevent seed-borne infection. Resistant varieties are

presently behg developed Metalaxyl (Apron FL) has been shown to provide some

protection against soi1 and seed-borne infections (Reiling 1 984b)-

2.25 Gray Mold

This disease is caused by BohytiF cinerea Pers. :Fr. and occurç under humid

conditions in areas of North Amenca, New Zealand, Japan and Europe (Lawyer 1 984~).

Gray mold can cause a rot on pods, stems and leaves on pea plants or on fiesh peas and

pods in m i t or storage.

Symptoms:

Funy, gray lesions are the h t symptoms that appear on lower areas of the stem.

Under humid conditions, the fungus spreads until the entire lower foliage appears a

fuuy, gray colour. The fûzziness and gray colour corne fiom the abundance of conidia

produced on the leaves and stems. As the disease progresses, leaves become shrivelled

and dry and defoliation of the lower leaves occurs. Pod infections cause the most

damage. Clinging blossoms provide a humid enviromnent fiom which the tip of the

young pod can be infected. Small, oval, water-soaked lesions develop and spread up the

pod. These lesions are tan at f k t but him grayish with age and ofien develop sclerotia

which appear as small black specks.

Peas in transit or storage develop water soaked, grayish green lesions that quickly

30

become covered in a grayish, white mycelial mat. The infection spreads quickly and

causes pods and the seed h i d e to rot (Lawyer 1984~).

Disease Cycle:

B. cinereu overwinters as smail, thin, black sclerotia Mycelium and spores can

also serve as overwintering stages but are not as resilient as the sclerotia Infection occurs

quickly under optimal conditions of 16 " to 2 1 OC and 100% humidity. Dispersal of

conidia c m occur through rain splash, by wind or by irrigation water (Lawyer 1984~).

Prevention and Control:

The use of potassium fertilizer in potassium deficient mils has been show to

reduce the severïty of gray mold (tawyer 1984~). Systemic fùngicides such as benomyl

will provide some control but no chernicals are presently registered for the control of

gray mold on peas in Alberta (Anonymous 1998a).

2.26 Anthracnose

Anthracnose of p a s is caused by ColZet~~chumpisi Pat. It is considered to be of

minor importance since infections are usually small and isolated (Hagedorn 1 %Ma).

Symptoms:

Lesions due to anthracnose can occur on foliage, stems and pods. Pod lesions are

circular, sunken, with reddish to brown margins and reddish coloured centres. Close

examination of pod lesions o h reveal hi t ing bodies called acervuii containing orange-

pink spores masses. Leaf and stipule lesions are oval with brown margins and grayish

3 1

centres. Stem lesions are elongate and appear copper-coloured when moist and grayish

when dry. Severely diseased plants have an overall reddish-brown appearance (Jones &

Vaughan 192 1 ; Hagedorn l984a)-

Disease Cycle:

C. pisi can be seed or soil-borne. The pathogen overwinters in plant debris and

this is the most common source of inocdum. Rain splash spreads conidia to the surface

of leaves and stems where they germinate to form a germ tube that produces an

appressonum from which an infection peg penetrates the pea plant. Warm temperatwes,

high humidity and recurrent rains al l favour development of this disease (Hagedorn

l984a). Aathracnose is commonly found occming with Ascochyla spp. and behaves

more like a secondary pathogen colonizing lesions causeà by the Ascochyta spp. (Ou &

Walker 1945).

Prevention and Control:

Use of disease fkee seed and crop rotation with non-leguminous plants will

prevent problems with this disease.

2.27 Aiternaria Blight

Altemaria alternata (Fr.:Fr.) Keissl. is the causal organism of this disease.

Although symptoms c m be damaging to a pea plant, this diseaçe is not considered to be

of economic importance because of its infiequent and isolated occmence (Susuri et al.

1982).

Symptorns:

Foiiar lesions due to A. altemata are oval with a concentric ring pattern inside,

tannish-brown in the centre and lighter towards the rnargins. Up to half or more of the

leaves and stipules c m be covered by lesions when an outbreak occurs. Pod lesions are

smaller, raised and brownish in appearance. Slight stunting of pea plants can also occur.

Prolonged high humidity (3 or more days) and warm temperatures (16 O - 24°C) favour

development of altemaria blight (Susuri et al. 1982; Lawyer l984d).

Prevention and Control:

Rotation with non-cmciferous crops reduces air-borne inoculum nom plant debris

and is the best means of prevention (Martens et al. 1984). No resistant varïeties of peas

are known.

2.28 Septoria Blotch

Septona blotch, caused by Septoriapisi Westend., occun mainly on senescent

foliage, pods and stems of peas. As such, it generally causes little quality or yield loss

and is not considered of economic importance (Hagedorn 1984b).

Symptoms:

Lesions are blotchy in appearance with no distinct margins and develop rnainly on

the lower, older leaves, pods and stems of the pea pIant. Early formed blotches are

yellowish-green and eventually darken and enlarge and are hegular in size and shape.

Entire leaves or stipules may be covered by blotches which coalesce. Black pycnidia

33

develop profusely over the surface of the affected areas (Zaumeyer 194 1 ; Hagedorn

i984b).

Disease Cycle:

S pLsi can be either seed or soil-borne, although seed transmission is not

considered important. The pycnidia on crop debns rel ease pycni diospores which are

spread by water splash to infect nearby plants. Temperatures of 21 O- 27 O C and high

humidity favour the development of this pathogen (Hagedorn 1984b).

Prevention and ControI:

The best control measure to prevent problems with this disease is a rotation to

non-leguminous crops to allow crop debris to decompose.

2.29 Cladosporium Blight

This is a disease most commody found in areas of high humidity. It is caused by

CZudosporium cladosporiodes (Fresen.) De Vries fkp. pisicoh (Snyd.) de Vries and is

often referred to as "scab" (Lawyer 1984e).

Symptoms:

Inward curling of leaves as they unfold is one of the first symptoms of infection

by C. pisicolum. Leaflesions can be irregular or circular in shape, are gray at fust and

evennially tum a tan to brown colour. Lesion margins-are thin and dark brown in colour.

Shredding of leaves can occur as necrotic areas fdl out or tear fkom wind. Brownish-

black lesions develop on most o h plant parts except for the pods and cadkers often

34

develop as these lesions age and crack Infected pods are often poclmiarked and have a

bumpy feel due to dark irregular pimples occuning on the pods. Seed infection can result

if pods are penetrated. Severely infected seeds develop black, circula spots with well-

defhed borders. A Iight infection may show up as light scattered black spots on the seed

(Lawyer 1984e).

Disease Cycle:

C. pisicoZum c m be either seed or soil-borne. Infection occurs under conditions of

high humidity and temperatures of 16" - 21 OC requiring three to seven days. n i e disease

is only known to occur on Pisum species and is more prevalent on young plants that are

actively growing (Lawyer 1 984e).

Prevention and ControI:

Use of disease-fkee seed, a two-year crop rotation and avoidhg low spots prone to

fog for planting are the main control recornmendations.

2.210 Black Leaf

Black leaf of peas is caused by Fusicladiurnpisicola Linford and is not

considered to be of economic importance because susceptible varieties are no longer

grown (Laver 19840.

Symptoms and Disease Cycle:

Small, white spots on the underside of leaves are the fkst symptoms, and become

sunken as they enlarge, to fom a concentric ring pattern progressing fiom tan coloured in

35

in the centre to gray to black at the edges. Lesions eventually darken as conidia form on

the underside of the leaf and eiongated Iesions may streak the Ieaves starting with the

lower leaves and progressing upwards. Blackening is due to closely packed conidia on

the leaf surface. Bottom leaves may eventually turn totally black, dry out and become

shredded in appearance (Lawyer 1984f).

Prevention and Controk

Use of resistant varïeties is the best control strategy.

2.3 Bacterial Diseases

2.3 1 Bacterial Blight

Bacterial blight caused by Pseudomonas syringae pv. pisi (Sackett) Young, Dye

& Wilkie is a senous disease of peas throughout the pea growing regions of the world

(Clarke 1990; Roberts et al. 1995; Hollaway et al. 1996).

Symptoms:

Shiny, water-soaked lesions initially appear near the nodes and stipules. These

spread to the stems, peduncles and tendrils and become darker in colour. The underside

of leaves and stipules develop water-soaked spots that appear dark green to brown on the

upper surface. With age, the spots deveIop an angular shape with dark margins and a

light coloured, papery appearance in the centre. Lesions appear translucent when held up

to the light (Lawyer 1984g). Lesions on pods are also water-soaked at kt, and him

darker as they mature and become sunken. The suture area is often a site of infection and

36

infected seeds may or may not show injury symptoms. Watery, dark spots sometimes

appear on the seed but more commody no visible symptoms appear on the seed. The

earlier the infection, the more senous is the damage to the pea crop. Seedlings attacked

by P. m g a e pv. pisi rnay not survive.

Disease Cycle:

P. syrirzgae pv. p l i ovefwinters predominantly as a seed-borne pathogen (Skoric

1927; Roberts 1992; Roberts 1993; Hollaway et al. 1996). The seed both extemaily and

intemally is covered by a dry, white f h of bacteria that is invisible to the naked eye

(Skoric 1927; Lawyer 19848). Seed can rernain infected for up to three years. Infection

occurs as the plumule contacts the infected seed coat during germination. The 3 lowest

stipules are the most cornmon infection sites. Soi1 moisture plays a role in the infection

process. Optimal temperatures for P. syringae pv. pisi growth are 26" to 28 OC with a

minimum of 3 OC (Lawyer 1984g). The higher the soi1 moisture, the higher is the rate of

infection (Skoric 1927; Wark 1954; Roberts 1992; Hollaway et al. 1996). Injus, nom

hail, fiost, wind, animals or machinery predisposes the plants to infection. Secondary

spread occws by rain splash, wind, machineiy, animals and irrigation (Lawyer 1984g).

Four races of P. syn'ngae pv. pisi are presentiy known and cultivars resistant to each race

are available.

Prevention and Control:

Since the pathogen is predominantly seed-borne, the use of disease-fke seed is

the primary method for avoiding the disease. Planthg resistant cultivars wiU also help to

reduce the impact of this disease. Seed production should be limited to more arid areas

and irrigation should be avoided.

Machinery should be disinfecteci between fields and spraying of pesticides should

be done by air instead of using ground applicators to avoid mechanical injury.

2.32 Brown Spot

This bacterial disease of peas is caused by fieudomonas syrfngae pv. syringae

van Hall, and aithough not as important as bacterial blight, can cause senous damage

under optimal environmental conditions (Lawyer 19848).

Symptoms:

In the field, symptoms of brown spot are difficult to distinguish from those of

bacterial blight. Under laboratory conditions, however, it is easier to tell the two diseases

apart especiaily at the seedling stage. Initially, the symptoms are small, water-soaked

lesions on leaves, petioles and stems which eventually tum a tan colour and take on a

bunit appearance. Stem lesions are sunken and tend to elongate upwards and ofien cause

stems, petioles and growing points to become distortai Infected leaves eventually dry up

and fdl off. Ki@ humidity is needed for the disease to become a senous problem

(Lawyer 1984g).

Disease Cycle:

P. syringae pv. syn'ngae c m be seed or soil-borne. It is generally only found on

the surface of the seed coat and as such does not survive for as long a period on the seed

as P. syringae pv. pisi. This pathogen does, however, have greater sunivability in the

38

soi1 than P. synngae pv. pisi. Optimum temperature for this pathogen is 24°C.

Othenvise, the disease cycle is much the same as bacterial blight (Lawyer 19848).

Prevention and Control:

Control strategies for brown spot are simila. to those for bacterial blight The use

of disease-fkee seed is the best way to prevent the disease fiom o c c M g . With brown

spot, however, s t o ~ g the seed for one year prior to planhg should be enough to

virhially elhinate the pathogen. Other known hosts of this pathogen (ie. dry beans

(Phaseolur vulgaris)) should also be avoided the year before or after peas (Lawyer

l984g).

2.33 Pink Seed

This bacterial disease is caused by Erwinia rhupontici (Miller) Burkholder and

was found for the first time on peas in southem Alberta in 1988. It now occurs fkequently

in southem Alberta and is often isolated f?om seed cleaning facilities (Huang et al. 1990;

Huang 1991).

Symptoms:

Tan to dark-brown lesions develop only on pods. Seed tums a pink colour and is

often shrivelled. Pink seed is more common in imgated pea fields than in dryland fields.

High humidity and mechanical injury fiom irrigation may d o w the bacteria to penetrate

hough wounds to infect the pea plants (Huang et al. 1990).

39

Disease Cycle:

Bactena enter pea pods through wounds caused by insects or mechanical injury

and infect the seed inside the pods. Quaiity of the seed is afEected more than the yield

(Huang et al. 1990).

Control:

Control methods have yet to be developed for this bacterial disease. Use of

disease-free seed should help to reduce the incidence of this disease. Cleaning between

seed lots in seed cleaning fadities may also help to reduce spread of the pathogen.

2.4 Viruses

There are more than 50 viruses known to infect peas worldwide (Hampton 1984).

Of these, only a few occur in Canada and only rarely is damage serious in western

Canada due to the severe winters and short growing season (Slinkard et al. 1994a).

There has never been a serious Wal disease outbreak on peas reported in Alberta

(Howard RJ. - Personal Communication 1998). As such, only a brief description will be

given of some of the main virus problems occurring in the United States, particularly in

the Pacific Northwest, an area that is geogmphically close to Alberta and where field pea

has been grown for many years.

The main vinises considered to be of economic importance in the USA are pea

seed-borne mosaic virus, pea enation mosaic vins, bean (pea) leaf roll vins, pea streak

virus and red clover mosaic virus ~ u s i n g pea stunt.

40

Other vinises reported on peas in the United States and Cana& are bean yellow mosaic

virus, alfalfa mosaic virus, cucuinber mosaic virus, watermelon mosaic vim 2, lettuce

mosaic virus, turnip mosaic virus, plantago mosaic virus, white clover mosaic virus,

clover yellow mosaic virus, clover yellow vein virus, cowpea aphid-borne mosaic virus,

desmodium yellow motîle virus, peanut rnottle Wus, peanut shint virus, soybean mild

mosaic virus, soybean mosaic vinis, broadbean wilt virus, clover blotch virus, pea dwarf

mosaic virus, pea early-browning virus, red clover mottle vhs, red clover necrotic

mosaic virus, and tomato black ring vinis (Hampton 1984). Tomato spotted wilt virus has

been reported on peas grom in a greenhouse in Manitoba (Zimmer et al. 1992).

2.41 Pea Seed-borne Mosaic Virus (PsbMV)

PsbMV was first detected in Canada in 1974 when breedulg lines of peas fiom

Morden and Saskatoon grown in winter increase plots in Califomîa caused an outbreak of

the disease in USDA lines also grown at the same location (Chico & Zimmer 1978;

Hamilton 1997). It is the only seed-borne virus occunfng on peas that is considered of

economic importance in North Amenca.

Symptoms:

Symptoms include stunting and downward curling of leaflets on seedlings shortly

after emergence. Leafmosaic, rosetting, shortening of intemodes and stunting may or

rnay not be present. Pod set may be affected in some cultivars or distorted pods may

develop with seeds that have split seed coats. Delayed maturity may also occur. Some

41

varieties may be infected and show no visible symptoms. (Stevenson & Hagedorn 1970;

Hampton 1984). Chico & Zimmer (1978) showed that yields of the varieties Trapper and

Centuy were reduced by 1 1 and 36 percent, respectively, when infected with PsbMV.

Disease Cycle:

PsbMV is a member of the potyvvus group that is seed-borne and transmissible in

a stylet-borne marner by aphids (Gonzales & Hagedorn 1971). Infected seed has been the

primary source of dissemination of this virus worldwide. In the field, the pea aphid

(Acyrthosiphonpisum L.) is the main vector of the virus spread from infected to healthy

plants, although other aphids are known to transmit PsbMV as well.

Prevention and Control:

The virus is not known to persist in infected plant hosts but survives fiom year to

year in infected seed of pea, lentil and broad bean (Hampton 1984). Resistant varieties

and use of Wus-6ee seed are the main control strategies for this virus. Control of aphid

populations has not proven a reliable control method (Zimmer & Lamb 1993).

2.42 Pea Enation Mosaic Virus (PEMV)

This is considered to be the most important virus disease of peas in the United

States, and is widespread in most U.S. pea producing regions (Hampton 1984).

Symptoms:

If infection fiom PEMV occurs early, plant distortion and death may occur.

Symptoms of later infections include chlorotic fiecks, stunting, upward rolling of leaves

42

and pod distortion. Foiiage may also develop elongate, chlorotic, tranducent lesions.

Seed size and qudity are ofien reduced. Tissue proiiferation may occur dong leaf veins

and on pods especially on some of the older susceptible varieties (McWhorter & Cook

1958; Hampton 1984).

Disease Cycle:

PEMV overwinters in leguminous crops or weeds. Alfalfa (Medicago sativa) was

until recently considered to be the primary reservoir of PEMV, but now has been shown

to be a non-host of this virus (Larsen et al. 1 996). Aphids, primarily the pea aphid

(Acythosiphon pisum L.), transmit the virus in a persistent mariner (feeding on infected

plants, incubation for 8- 1 2 hours and transmission on subsequent feedings) to host crops.

Transmissibility by the aphids is usuaily Iost after several feeding episodes. The virus c m

also be transmitted by mechanical means (Hagedorn et al. 1964). Cicer. Lathyrus. Lens,

Lupinus. MeIihu, Phaseoh, Pisum, TnfoZiurn and Vicia are known leguminous hosts

of PEMV (Hampton 1984).

Prevention and Control:

The primary method of control of PEMV is the use of tolerant varieties.

2.43 Bean (Pea) Leaf Roll Virus (BLRV)

Symptorns:

Severe stunting and death are symptoms of eariy infection by BLRV. Later

infections can cause chlorosis of only the plant tips or of the whole plants. Stuntuig may

43

also occur with Iate infections. Tolerant varieties may only show slight stunting or may

be symptomless. BLRV is often more severe if other v-es are already ùifecting the

plant.

Disease Cycle:

BLRV is a rnember of the luteovinis group and is transmissible by aphid vectors

in a persistent manner. Seed-borne transmission is not lmown to occur. BLRV

overwinters mainly in perenniai legumes such as alfdfa and clovers and is spread to pea

plants by migrating aphids. The pea aphid (Acyrthosiphon pisum L.) is the most common

vector for this virus. Once infected, the pea aphid can remain viniliferous for the

remainder of its He. After peas are harvested, aphids migrate back to nearby perennial

legumes to overwinter (Hampton 1984).

Prevention and Control:

Use of tolerant vmieties is the best control method presently available. Reducing

aphid populations has provided some control but is not always successful.

2.44 Pea Streak Virus @'SV)

Symptoms:

Sudden death of pea plants results fkom an eariy infection by PSV. Brown or

purple stem and leafspots or streaks are characteristic of a late infection. M y pod

symptoms may occw if inféction occurs drrruig the early bloom stage. Pods dwelop

necrotic spots or sunken pock marks (Hampton 1984).

Disease Cycle:

PSV is a memeber of the carlavirus group that is transmitted in a stylet-borne

rnanner by aphids, rnainly the pea aphid (Acyrthosiphon pimm L.). Ovemintering occurs

primarily in alfalfa, and PSV is transmitted when pea aphids migrate to pea crops in the

s p ~ g and summer. Persisteme of this vims in pea aphids is quite short and is often

dissipated after only four or five feedings. This disease is most severe when pea fields are

planted next to dfalfa fields (Hampton & Weber 1983a; Hampton 1984).

Brevention and ControI:

Use of tolerant varieties will help to keep this disease under control. Control of

aphids in pea fields and nearby alfdfa fields wili reduce the incidence of this virus but

not completely eliminate it. Planting pea fields away from alfalfa fields d l also help tu

reduce the incidence of PSV.

2.45 Pea Stunt

Pea shint is caused by the red clover vein mosaic virus (RCVMV). Common in

red clover and other clovers, this vins together with the pea enation mosaic virus can

cause a desiructive viral complex in pea fields (Hagedorn & Hanson 195 1; Hampton

1984).

Symptoms:

Susceptiile cultivars are often killed before tbey bloom if infected by R C W

early. Snuiting and terminai rosetting are symptoms of later infections. Vein chlorosis or

necrosis may or may not occur. Low temperatures often have a masking eEect on

symptoms of pea shint (Hagedorn & Walker 1949; Hampton 1984).

Disease Cycle:

RCVMV is a member of the carlovirus group which is transmitted in a stylet-

borne manner by aphids, primarily the pea aphid (Acyrthosiphon pisum L). The virus

overwinters rnainly in perennial red clover fields and is çpread when aphids migrate in

the spring and early summer. When peas are harvested, aphids migrate back to clover

fields compieting the disease cycle (Hampton & Weber 1983b; Hampton 1984).

Prevention and Controi:

The use of tolerant varieties is the best means of controlling this viral disease.

Chapter 3 - Field Survey for Pea Diseases

This s w e y was carried out during the 1997 growing season. The puipose of the

survey was to assess root rot incidence and severity and to identie root and foliar

diseases of field peas occumng in the Peace River region of Alberta.

3.1 Materials and Methods

3.1 1 Selection of fields to be suweyed

The sampling region was divided into 6 geographic areas which were separated

by ratura1 existing features such as rivers or hills, and agronomie retail centres where

farmers normally sold their crops and purchased seed, fertilizers, machinery parts, etc.

(Figure 3). A proportional weighting was given to the number of growers in each

geographic area using names fiom a mailing list supplied by the Alberta Pulse Growers

Commission. Area 1 had 12 fields surveyed and encompassed the Municipal District

(M.D.) of Northem Lights and the M.D. of MacKenzie. Area 2 had the highest number of

growers with 33 fields surveyed and included the M.D. of Peace, M.D. of Fairview, and

M.D. of Clear Hills. Area 3 had 1 1 fields surveyed and comprised the M.D. of Spirit

River, M.D. of Birch Hills and M.D. of Saddle Hills. Area 4 had the second highest

number of growers with 29 fields surveyed and included the County of Grande Prairie

and the western part of the M.D. of Greenview. Area 5 had eight fields surveyed and

encompassed the M.D. of S m o b River and M.D. of East Peace. Area 6 had 10 fields

surveyed and included the M.D. of Big Lakes and the eastern part of the M.D. of

Greenview.

! .. . . r..

I

i I I I

Figure

1

---_ ---

. - 7

- . .

48

Growers in each area were selected without bias kom the list, contacted in early

June by phone and asked if they had seeded peas in the spring of 1997. If they had, the

objectives of the disease survey were explained to them and they were asked if they

wanted to participate in the survey. Growers could include as many pea fields as they

wanted. A total of 103 fields were sampled from 74 growers. The survey was conducted

f b m mid-June to late August.

3.12 Sampling Method

Sampling of pea plants was done without bias and a minimum of 50 plants were

collected from each field. The plants were collected by first walking 50 paces into a field.

digging up 10 plants in a row, and continuing on for 5 locations along the arms of a' W'

pattern (Figure 4).

Figure 4: Field sampling method used to obtain pea plants for this study

Enter field here Exit here

50 poccs

5

53 puer

49

Sampling was conducted at three different times during the growing season - at

the seedluig stage (late June to early July), during flowering (rnid-July to late Juiy) and

afier pod set (mid to iate August) in each field to detect both early and Iate diseases and

to observe the progress of various diseases during the summer. In total, fiom the 3

sarnpling &es, approximately 15,450 plants were to be collected (103 fields * 10 plants

* 5 locations * 3 dates = 15,450 plants).

Growers were contacted again during the summer and requested to complete a

questionnaire on crop rotation, cultural practices and pest control used in each field

surveyed. A copy of the questionnaire is included in Appendix 1. This was dune to see if

there was a correlation between management practices and root rot incidence and

severity .

Daily growing season precipitation and temperatures were collected fiom various

weather stations amund the Peace River region. Figures 5 - 9 show growing season

precipitation and Figures 10 - 14 show growing season mean temperatures for selected

locations in the region. T m - y e a r averages are included as a cornparison.

Figure 5: 1997 Growing Season Precipitation -Grande Prairie Figuru 6: 1997 Gmwing Season PmcipitnUon - Falher

June July Month

June July Aug Month

30 yr. avg.

Figure 7: 1997 Growing Season Prccipitation - Peace River Figura 8: 1997 Growing Season Precipitation - High Level

200 - 1 6 0 4 i ;

May June July Aug Month

May June Juiy Aug Month

1997 30 yr. avg. 0 1997 30 yr. avg.

Figure 9: 1997 Growing Seoson Prccipitation - Overall Peace Region

120 - 118.6

May June July Aug Month

Source: Aiberta Agriculture, Food & Rural Development - Conservation & Development Branch

Fbum to: 1997 Growing Sealon Mern Tarnpsrotum - Grande Prairie Figure 11: 1397 Growing Saaron Mean Tempamhim - Folhsr

20 - 20 -

May June July A 4 Month

Figure 12: 1997 Growing season Mean Temperature - Peace River

May June July Month

May June July A W Monîh

1997 30 yr. avg.

Figure 13: 1997 Growing Season Mean Temperature - High Level

20 1

May June July *ug Month

0 1997 30 yr. avg.

Figure 14: 1667 Growing Searon Mewn Tarnpeniure - Overall Peace Rapion

20 -

May June July Aug Month

Source: Alberta Agriculture, Food & Rural Development - Conservation & Development Branch

3.13 Root Rot Pathogen Identification

Sampled plants were brought to the Iaboratory within 48 hr after collection and

stored in a cooler at 5OC for a maximum of 7 days. Roots were washed under tap water

and severity ratings were assigned based on a scale of O to 4, where O = healthy root with

no discolouration, 1 = 1 - 10% root discoIouration, 2 = I l - 25% root discolouration, 3 =

26 - 50% root discolouration, and 4 = 51 - 100% root discoIouration (Howard et al. 1995;

Hwang et al. 1995b; Hwang et al. 1997).

From the 50 plants collected per field, five plants with signs of root discolouration

were then selected without bias for pathogen identification.

Segments of diseased roots were cut, surface-sterilized for 90 seconds in 70%

alcohol and plated onto water agar (WA). Records were made of whether discoIouration

occurred at the top, rniddle or bottom of the main root. Plates were stored at room

temperature (2 1 OC) for 1 to 3 days until rnycelial growth appeared. Mycelia were

transferred to potato dextrose agar (PDA) and stored at room temperature for 10 to 14

days until identification. Identification was done by cornparhg plates to plates with

known pathogen cultures. Known pathogen cultures were obtained from Dr. S.F. Hwang,

plant pathologist with the AIberta Research Council, Vegreville, Alberta No attempt was

made to identifi the pathogens with a microscope and reference key.

3.14 Foiiar Pathogen Identification

While assessing for root rot, records were kept of any spots, lesions or

discolouration present on the foliage, stems, pods or tendrils. These plants were set aside

and five were selected without bias for identification. Sections of diseased areas were

53

surface-stenlized for 30 to 60 seconds in 70 % alcohol and plated ont0 WA. Lacation and

a brief description of spots or lesions was noted. Plates were stored for 1 to 3 days at

room temperature (21°C) until mycelial growth appeared. This was then transferred to

PDA and stored at room temperature for 10 to 14 days until identification. Identification

of foliar pathogens was dso done by comparing plates to plates with known pathogen

cultures.

3.15 Statistical Anal ysis of Suwey Questionnaire

Results from the questionnaire were used to determine if any correlations existed

between agronomie practices used by growers and the incidence and severity of root rot.

Results were statistically analysed using SAS software from the SAS hstitute, Inc.

(1990). Root rot incidence was statistically analysed using a Chi-square test for non-

pararnetric data. The Chi-square test was chosen for analysing incidence for two reasons.

One is that since incidence is a recording of whether root rot is present or absent, the Chi-

square test is ideal for testing for significance. The second reason is that the analysis is

also more accurate when a large number of samples are analysed. This study dealt with

over 15,000 plants that were visually checked for presence or absence of root rot.

For root rot severity, a General Linear ModeIs (GLM) Procedure was used to

check for significant differences. If a significant difference was found, a Student-

Newman-Keuls (SM) test was used to determine Ieast significant differences between

means. Square root transformation of the original root rot severity data was also

computed to improve normality of the raw data and is reported here. Ln dl cases, it

lowered the coefficient of variance and showed similar or greater significance.

3.2 Results

3.21 Incidence and Severity of Root Rot

Pea plants with root rot were found in d of the fields s w e y e d Mean incidence

and seventy of disease increased in al1 areas as the summer progressed. The mean

incidence and s e v e r i ~ of root rot for the three sarnpling times were 88% and 1.2,88%

and 1.5, and 95% and 2.3, respectively (Table 1). The total number of fields vmied £kom

sampling time to sampling time because of worked under crops, road construction and

harvested fields.

Table 1: Incidence and severity of root rot in the Peace River region in 1997

Root rot severity ratings: O = healthy with no root discolouration 1 = 1-10% root discolouration 2 = 1 1-25% mot discolouration 3 = 26-50% root discolouration 4 = 5 1- 100% root discolouration

&ezt

1

2

3

3

5

6 I

Sample 1 Sample 2

n of

fields

12

33

1 1

29

8

10

IO3

X of

fields

13

33

1 1

23

7

IO

96

Sample 3

Severity 0 -4 $of

fields

12

33

1 1

26

7

I O

99

Incidence %

Mean

1.1

1.1

1.0

13

1.0

13

Mean

83

83

89

94

93

95

Range

0.6-1.9

0.4-3.0

0.61.4

0.623

03-12

1.0-1.8

incidence %

Range

55-100

36-100

61-100

62-100

81-100

85-100

12

Mean

78

91

92

95

51

Incidence % Severity 0- 4

88

Range

55-100

56-100

87-100

86-100

13-100

Meai

98

90

100

98

85

99

Mean

12

1.6

1.5

1.8

0.8

1.6

S e v e r i ~ 0 - 4

98 91-100

88

Range

87-100

28-100

100-100

76-100

69-100

94-100

Range

0.7-1.8

0.8-2.9

1.0-2.1

0.84.0

0.2-2.5

1.1-2.2

Mean

2.6

1.8

2.4

2.6

1.9

2.7

95 15

Range

1.8-3.5

0.4-3.8

2.1-2.9

1.44.0

13-33

1.8-3.6

2 -3

3.22 Pathogen Identification

3.221 Root Rot Pathogens

The main root rot pathogen identified in this survey was Fusarium spp. which

occurred in 99 of the 103 fields surveyed. Of the four remaining fields, one had

Rhizoctonia spp., one had Bowîis spp. and the other two were unidentified No other

pathogens were positively identified. Eight other fields also had Rhizoctonia spp. and six

had Bovtis cinerea dong with Fusankm spp.

3.222 Foliar Pathogens

Only 1 6 of the 1 03 fields in the first sampling tirne showed any foliar symptoms.

Seventy of the 96 fields in the second sampling time had foliar symptoms and dl the

fields in the third sampling time had some foliar symptoms. Foliar pathogens identified

were Ascochyta spp ., Srlerotinin sclerotiorum, Fusarium spp ., and Botrytis cinerea.

The two main diseases identified fiom the symptoms observed on plants were

mycosphaerella blight and sclerotinia rot. Symptoms of mycosphaerella blight were

present on plants fiom al1 the fields by the third sampling t h e . Symptoms of sclerotinia

rot were observed on plants nom 54 of the 99 fields in the third sampling time.

Five samples of isolated Fusariurn spp. were selected without bias and sent to Dr.

Keith Seifert, (Eastern CereaI and Oilseed Research Centre of Agriculture and Agri-Food

Canada, Ottawa, Ont.) for identification to species. Three were recovered from root

lesions and two fiom f o l k lesions. Two of the root isolates and both of the f o l k isolates

were identified as F. avenaceum (Fr.) Sacc. The other root isolate was identified as F.

sambucinum Fueckel.

3.23 Suwey Questionnaire Results

1. How many acres of field peas are you growing in 1997?

Acreage of field peas grown varied widely across the region with the mean acreage at

93.0 hectares (standard deviation of 67.1 hectares). The overall range was fiom 4.9

hectares to 323.8 hectares. Table 2 gives the mean acreage, standard deviation and range

for each area surveyed. The 74 growers surveyed grew 6,922 hectares of field peas in

1997. Based on the 1997 acreage of 28,329 hectares of field peas in the Peace River

region (Boje W. - Personal Communication 1998), this represents approximately 24

percent of the pea acreage in the region.

Table 2: Mean acreage, standard deviation and range of fields suweyed

Area

1 l

2

3

4

5

6

Overall

Number of fields

12

Number of growers

11

20

IO

20

4

9

74

Range (hectares)

44.5 - 234.7

Mean Acreage ' (hectares)

107.1

8.1 - 242.8

20.2 - 129.5

16.2 - 234.7

80.9 - 323.8

4.9 - 229.9

4.9 - 323.8

Standard Deviation (hectares)

58-4

57.8

40.3

64.6

109.8

67.7

67.1

33

11

29

8

10

1 03

77.8

68.2

105.8

194.3

63.6

93.0

2. How many years have you been growiing field peas?

1st time 1 year 0 2 years 3 or more years

Seventy-two of the 74 growers (97.3 %) sweyed had 3 or more years experience in

growing field peas. Of the remaining two, one was a first time pea grower (1.35 %) and

the other had 2 years experience (1 -35 %).

3. What has been the cropping history in this field?

The crop immediately preceeding peas was wheat in 47.6 % of pea fields surveyed in

1997, followed by barley (2 1.4 %) and canola (20.4%). Canola was the most common

crop grown in these fields in 1995 and 1994 (43 -7 % and 29.7% of fields surveyed,

respectively). Wheat was the most common crop grown in 1993 and 1992, in 34.5% and

38.5% of fields surveyed, respectively. Only two of the 103 fields surveyed were seeded

ont0 pea stubble. Table 3 shows the number of fields of each crop type over the 5 years

before the swey. Only 1 996 and 1 995 have cropping history for al1 fields in the survey.

Some growers had not kept records and could not remember back 5 years. Others had not

f m e d the land for the full 5 years and did not h o w previous cropping history.

4. Have you had any diseases in your peas in previous years? If yes, did you have it

identified?

Most growers (72%) had not noticed any disease in their pea crops in previous years. Of

the 28% of growers that had noticed disease in their pea fields in previous years, 15% did

not know what disease it was and did not have it identified, 6% reported ascochyta blight,

4% reported root rot, 2% reported sclerotinia rot, and 1% reported downy mildew.

Table 3: Cropping history of pea fields surveyed

5. Did you use certified or better seed? If yes, what variety of peas are seeded in this

field?

Certified or better seed was the most prevalent seed source with 55.9% of growers having

used it. The remahhg 44.1% of growers used their own seed or purchased it fiom a

neighbour (common seed). Of the certified or better seed used, Carneval was the most

common variety, with 18.6% of growers planting it, followed by Espace with 1 1.8%,

Majoret with 8.8%, and Carrera with 4.9%. 0 t h varieties used were Caderos, Highlight,

Mustang, Orb, Eiffel, Danto, 1141, Ascona, Profi, and Grande.

%

43.7

0.0

26-2

5.8

1.9

0.0

2.9

2.9

10.7

0.0

2.9

2.9

fieIds

1995

45

O

27

6

2

O

3

3

11

O

3

3

1 03

Crop Number of

1996 %

canola

flax

wheat

barley

oats

canqseed

creeping red fescue

bromegrass

Peas

alfa1 fa

hay/pasture

summerfaliow

# of fields

1994

30

O

28

12

3

O

2

3

IO

O

7

6

101

21

O

49

22

2

1

% --------- 29.7

O .O

27.7

11.9

3 .O

0.0

2.0

3 .O

9.9

0.0

6.9

5.9

20.4

O

47.6

2 1.4

1.9

1 .O

1993

16

1

29

7

2

O

5

1

13

2

5

3

84

L

2

2

O

1

2

1 03

1 .O

1.9

1.9

O

1 .O

1.9

%

13.8

0.0

38.5

16.9

0.0

0.0

9.2

1 -5

0.2

3.1

9.2

1.5

t

%

19.0

1 -2

34.5

8.3

2.4

0.0

6.0

1 -2

15.5

2.4

6.0

3.6

1992

9

O

25

11

O

O

6

1

4

2

6

1

65

59

6. Did you use seed treatment? If yes, please iodicate which one you used.

Thiram 7 5 W D Captan FL 0 Apron FL Other

Most growers (69.7%) did not use seed treatment. Of the 30.3% who used it, 17.1% used

thiram, 6.6% used metalaxyl, 5.3% used a combination of diiram and metalaxyl, and

1.3% indicated they had used another type of seed treatment. None of the growers used

captan.

7. Did you use inoculant? If yes, what type was used?

peat powder liquid granular

Inoculant was used in 102 of the 103 fields surveyed. Granular soi1 inoculant was the

most common inoculant used by 48.7% of growers. Peat powder seed inoculant was next

at 40.8% and liquid seed inoculant was used by only 10.5% of growers.

8. Did you apply any fertilizer? If yes, please indicate type, rate and method of

application.

Most growers (74.7%) appiied fertilizer with their pea seed. The most common fertilizer

used was 12-5 1-0, with 46.7% of growers having used it. Other fenilizers used were

various blends of nitrogen, phosphoms, potassium and sulphur. Five fields had anhydrous

ammonia (82-0-0) applied prior to seeding of the peas. Rates varied as much as did types

of fertilizer blends.

The most common method of application, used by 67.9% of growers, was with the seed,

followed by banding at 30.4% and broadcasting at 1.7%.

9. Were your peas zero-tilled?

yes If yes, how many years has this field been zero-tilled?

1st t h e U 1 year 2 years 3 or more years

0 no If no, how many tillage passes were made prior to seeding?

one tillage p a s 0 two tiiiage passes three or more Nage passes

Approximately one-third (30.3%) of the fields surveyed were zero-tilled. Forty percent of

fields were minimum-tilled having received only one tillage pass prior to seeding of the

peas. The remaining 29.7% of fields were conventionally tilled receiving two or more

passes before seeding.

Of the zero-tilled fields, 37.9% were zero-tilled for the first tirne, 10.4% had been zero-

tilled the previous year, 27.6% had been zero-tilled for the last

been zer&lled for at Ieast three years.

10. Did you use a land rouer after seeding? If yes, when did

immediately after seecàing 0 before emergence of peas 0

two years, and 24.1 % had

you roii the land?

after emergence of peas

Most growers (82.9%) did not roll their land after seeding peas. Of the 17.1% of growers

who rolled their fields, 6 1.5% rolled after emergence of the peas, 23.1 % rolled before

emergence of the peas, and 15.4% rolled immediately after seeding.

11. Have you used any herbicides on this pea field? If yes, please indicate type and

rate used.

Ten growers (13.5%) did not use any herbicides on at l e s t one of their fields. Of the

remahing 64 growers (86.5%) that used herbicides, the most common herbicides were a

combination of the broadleaf herbicide imazethapyr (Pursuit) and the gras herbicide

sethoxydim Poast). This mixture was applied to 42.7% of the fields in the survey. The

next most commonly used herbicide was imazamox and imazethapyr (Odyssey) sprayed

on 12.6% of fields, followed by imazethapyr alone applied to 9.7% of fields. Imazethapyr

was a very commonly used herbicide on fields in this survey. It was used alone or in

combination with other herbicides on 56.3% of fields surveyed. Other herbicides used

were Select (clethodim), Sencor (metnbuzin), Venture (fluazifop-p-rnethyl), MCPA

amine, MCPA Na, Assure (quizalofop-ethyl), Hoegrass (diclofop-metyl), Troporox Plus

(MCPB + MCPA) and Roundup (glyphosate).

12. What herbicides were used in this field in the last two years? (1996 and 1995)

Thirty difTerent herbicides were used in the 2 years pior to this survey. Of these, only

four, Lontrel (clopyralid), Prevail (tralkoxydim + clopylid + MCPA ester), Muster

(etharnetsulfixon-methyl) and Ally (metsulfùron methyl) could pose residual problerns

that might affect peas. In 1996, 11 fields had one or more of these herbicides applied to

them. In 1995,2 1 fields had one or more of these herbicides applied to them.

3.24 Results of Statisticd Analysis

3.241 Differences Between Areas

The first test was for differences in root rot incidence and severity between areas.

Areas were divided as follows: (Figure 3)

Area 1 : Manning/Forî Vendion Area 4: DeboltGrande Prairie Area 2: BemydFairview Area 5: FalherPeace River Area 3 : Spirit RiverEaglesham Area 6: Valleyview/High Prairie

62

Incidence

In the f h t sampling time, there was an association between root rot incidence and

the area in which peas were grown, ie., incidence of root rot varied between different

areas. The highest incidence of root rot was in area 6 (Valleyview/High Prairie) at 94.9%

and the lowest was in area 2 (Berwyn/Fairview) at 82.3%. In the second sampling tirne,

an association between root rot incidence and the area in which the peas were grown was

also found. The highest incidence of root rot was in area 6 (Valleyview/High Prairie) at

97.9% and the lowest was in area 5 (FalherReace River) at 51.4%. In the third sampIing

tirne, there was also an association between root rot incidence and area. The highest

incidence of root rot was in area 3 (Spirit RiverEaglesharn) at 100% and the lowest was

in area 5 (Falhermeace River) at 85.2%. When root rot incidence was averaged over the

three sampling times, an association was also found between root rot incidence and area.

The highest incidence of root rot was in area 6 (Valleyview/High Prairie) at 97.4% and

the Iowest was in area 5 (Falher/Peace River) at 76.9%. The number, the percent and the

Chi-square probability for area variation by root rot incidence can be found in Table 4.1.

Severity

No differences were detected in root rot severity between the six areas in the fnst

sampling tirne. h the second sampling time, root rot severity in area 5 (FalherPeace

River) was significantly lower than in areas 1,2,3,4 and 6. Ln the third sampling tirne,

root rot severity in area 5 (FalherfPeace River) was significantly less than in areas 3 , 4

and 6 but not area 2 (Berwyn/FaWiew). Root rot severity in area 2 (Berwyn/r;airview)

was significantly lower than in areas 3 , 4 and 6 but not area 5 (FalherPeace River).

63

When averaged across the 3 sampling M e s , root rot severity in area 5 (FalherPeace

River) was significantly less fhan in areas 1,2,3,4 and 6. Significant differences in

overall root rot severity between different areas are reported in Table 5.

3.242 Cropping History

Previous cropping history is known to 8 e c t the incidence and severity of root rot

in peas (Kraft 1984). h this analysis, crops were combined into three groups for testing:

1. Cereals: wheat, barley, oats, canaryseed, bromegrass and creeping red fescue (for seed) 2. Oilseeds: canola 3. Legumes: peas and hay/pasture

Summerfdlow fields were omitted since no crop was grown on these fields in the

previous year. Only 1996 and 1995 cropping hîstory was used because the cropping

history of dl the fields was not available in the other years.

Incidence

In d l three sampling times and when averaged overall, there was an association

between root rot incidence in the 1997 pea crop and the type of crop grown in both 1996

and 1995. Fields that had canola in 1996 had the highest incidence of root rot (9 1.7%)

while fields that had cereals had the lowest (90.1%). Fields that had legumes in them in

1995 had the highest incidence of root rot (95.7%) while fields that had oilseeds had the

lowest (89.1 %). The number, the percent and the Chi-square probability for cropping

history variation by root rot incidence can be found in Table 4.1.

Severity

There was no significant difference in root rot severity of the 1997 pea crop between the

three groups of crops grown in 1996 in al1 three sampling times and when averaged

64

overall. For 1995 crops, the fmt and second sarnpling times showed no difference in root

rot severity between the three groups of crops, however, in the third sampling tirne, root

rot severity was significantly higher if legumes had been grown than if oilseeds had been

grown. There were no significant differences between legumes and cereals.

Overall, there was a significant difference in root rot severïty. If legumes were grown in

the field in 1995, root rot severity was çignificantly higher in the 1997 pea crop than if

cereals or oilseeds were grown. Significant differences in overdl root rot severity

between previous crops are reported in Table 5.

3.243 Disease in Previous Pea Crops

Incidence

Only the second sampling time showed an association between root rot incidence

and growers who had noticed disease in prior pea crops. Growers who had not noticed

disease in their pea fields in prior years had a higher level of root rot incidence (90.2%)

than growers who had noticed disease in their pea fields in previous years (82.9%).

Severity

There was no signifïcant difference in root rot severity between growers who had

noticed disease in their previous pea crops and growers who had not.

3.244 Seed Source

If the growers used their own seed or bought uncertified seed fkom neighbours,

this was known as common seed. If growers purchased certified or better seed, then a

varietal name could be attached to the pea seed. Certified seed was used by 55.9% of

growers and common seed was used 44.1 %.

65

Incidence

There was an association between root rot incidence and seed source in al1 three

sampling times and when averaged overall ie., there were significant differences in root

rot incidence between certified and common seed. Growers who used certified seed had a

higher incidence of root rot (92.8%) than growers who used comrnon seed (87.4%). The

number, the percent and the Chi-square probability for cropping history variation by root

rot incidence can be found in Table 4.1.

Severiiy

In the f ~ s t and thïrd sampling times, there was no difference in root rot severity

between common and certified seed.

In the second sampling time and when averaged overall, however, root rot severity was

significantly higher when certified seed was used rather than comrnon seed. Significant

differences in overall root rot severity between seed sources are reported in Table 5.

3.245 Seed Treatrnent

Differences between metalaxyl, thiram and no seed treatment were tested in this

analysis. No growers used captan and only one grower had used another unknown seed

treatment.

Incidence

There was an association between root rot incidence and use of seed treatment in

the second and third sampling times and when averaged overall but not in the e s t

sampling time. Metalaxyl treated seed had the highest incidence of root rot (93.5%) and

thiram treated seed had the lowest (88.7%). The number, the percent and the Chi-square

66

probability for seed treatment variation by root rot incidence can be found in Table 4.1.

Severiîy

The only significant difference to show up in root rot severity was in the second

sampling t h e . Metalaxyl treated seed had significantly higher root rot severity than

thiram treated seed and untreated seed There was no difference between thiram treated

seed and untreated seed. When averaged across al1 three sampling times, no significant

difference in root rot severity was found.

3.246 InocuIant Formuiation

Incidence

There was an association between the three types of inoculants used and

incidence of mot rot in al1 three sarnpling times and when averaged overall. The highest

incidence of root rot was when no inoculant was used (96.7%) and the lowest was when a

peat based seed inoculant was used (88.7%). The number, the percent and the Chi-square

probability for inoculant formulation variation by root rot incidence can be found in

Table 4.1.

Severity

No significant differences were observed between root rot severity and the type of

inoculant used.

3.247 Fexailizer Use

There were 26 different kinds of fertilizer blends used by the various growers. To

simp1iQ the analysis, groupings were done.

The three groups were:

1. Nitrogen + Phosphorus (5 1 fields) 2. Nitrogen +/or Phosphorus +/or f otassium +/or Su1fb.r (27 fields) 3. No fertilizer used (25 fields)

Incidence

There was an association between root rot incidence and fertiiizer use in al1 three

sampling times and when averaged overall. The highest incidence of root rot occurred

when a blend of nitrogen a d o r phosphorus and/or potassium anaor sulfur was used

(92.3%) and the Iowest incidence occurred where no fertilizer was used (87.6%). The

number, the percent and the Chi-square probability for fertilizer variation by root rot

incidence cm be found in Table 4.2.

Severiîy

In the frst and third sampling times, root rot severity was significantly higher

when fertifizer was used than when no fertilizer was used. There were no diflerences

between the iwo types of fertilizer used. In the second sampling time and when averaged

across the three sampling times, there were no sigiificant differences in root rot seventy

between the three groups.

3.248 F e r m e r Application Method

Incidence

There was an association between root rot incidence and fertilizer application

method in al1 three sampling times and when averaged overali. The highest incidence of

root rot occurred when fertilizer was banded (94.5%) and the lowest b e l occurred when

fertilizer was placed with the seed (90.1%). Al1 three methods of application had higher

68

incidence of root rot than when no fertiher was used. The number, the percent and the

Chi-square probability for variation of fedlizer application method by root rot incidence

c m be found in Table 4.2.

Severity

For the f ist sarnpling Mie , root rot severity was significantly higher when

fertilizer was either banded, broadcast or when it was applied with the seed compared to

when it was not used at dl. In the second sampling time, there were no significant

differences between fertilizer that was banded, broadcast or applied with the seed. In the

third sampling time, root rot severity was significantly higher when fertilizer was

broadcast or banded than when it was applied with the seed or not used at dl. When

avemged overall, root rot severity was significantly higher when fertilizer was broadcast

or banded than when it was not used at dl . There was no difference between fertilizer

that was broadcast or banded and fertilizer that was applied with the seed- There was also

no difierence between fertilizer that was applied with the seed and when no fertilizer was

used. Significant differences in overall root rot severity between fertilizer application

methods are reported in Table 5.

3.249 Tiiiage

Incidence

In the first and third sampling tirnes, and when averaged overall, there was an

association between root rot incidence and type of tillage. No association showed up in

the second sampling time. Fields that were minimum-tilled had the highest incidence of

root rot (90.9%) while fields that were zero-tilled had the lowest (80.6%). The number,

69

the percent and the Chi-square probability for tillage variation by mot rot incidence cm

be found in Table 4.2.

Severity

No significant differences in root rot severity and type of tillage showed up in any

of the sampling tirnes or when averaged overall.

3.250 Rolling

Incidence

In al1 three sampling times and when averaged overall, an association between

root rot incidence and rolling was found. Fields that were rolled immediately after

seeding had the highest incidence of root rot (97.4%) while fields that were rolled before

emergence or weren't rolled had the lowest level(89.5%). The nwnber, the percent and

the Chi-square probability for rolling variation by root rot incidence can be found in

Table 4.2.

Severity

No significant differences were observed between root rot severity and rolling.

3.251 Herbicide Use

An andysis of whether herbicide use had an effect on root rot intensity and

severity in the 1 997 pea crop revealed some interesting results.

Incidence

In the second and third sampling times and when averaged overall, an association

between root rot incidence and whether herbicides had been used or not was found. No

association was found in the first sampling tirne. Use of herbicides caused a higher

70

incidence of root rot (9 1.3%) than when no herbicides were used (8 1.9%). The number,

the percent and the Chi-square probability for herbicide use variation by root rot

incidence can be found in Table 4.2.

Severity

The first sampling time revealed no significant differences in root rot severity

whether herbicides were used or not; however, in the second and third sampling times

and when averaged overall, root rot severity was significantly higher when herbicides

were used than when no herbicides were used. Significant differences in root rot severity

between herbicide use and non-use are reported in Table 5.

3.252 Herbicide Residues

Incidence

Only the second sampling time revealed an association between root rot incidence

and use of residual herbicides in 1996 that could damage peas. Fields that had been

sprayed with a residual herbicide treatment in 1996 had a higher incidence of root rot

(94.1%) in 1997 than fields that had not been sprayed with a residual herbicide (87.8%).

Severity

No significant differences were fomd between root rot seventy and use of

residud herbicides.

TabIe 4.1: Num ,er, percent and Chi-square probabiiities for sources of variation by overall mot rot incidence

- -

~ o o t Rot ~ r z - ~ l Root Rot Absent

Source

Area

96 Crop

Number of YO Plants

43 2.6

200 4.6

Groupings Num ber of Plants

2

1

5

Oilseeds

Cereals

95 Crop

Variety

Seed

Treaîment

Inoculant

--

Cereals

Oilseeds

Certified - ppp

Common

Thiram

Liquid

Peat

Table 4.2: Number, percent and Chi-square psobabilities for sources of

Source

Fertiiïuer

Application

Method

Tiage

Rolling

97 Herbicides

variation by overd root rot incidence

I

After 479 97.4 13 seeding

r i

Groupings

NPKS

NP

None

Banded

Broadcast

With seed

None

After 1 1892 / 95.2 1 emergence

- -

Before 1 668 1 89.5 1 78 1 10.5 emergence 1 I I I

Root Rot Present RootRot Absent

Number of Plants

3934

7350

3544

3166

305

7813

3544

Number of Plants

328

734

502

185

20

857

502

YO

9 2 3

90.9

87.6

94.5

93.9

90.1

87.6

YO

7.7

9.1

12.4

5 5

6.2

9.9

12.4

None

Herbicides

None

11789

13677

1151

10.5

8.7

18.1

I

89.5

91.3

81.9

1378

13 10

254

Table 5: Overall mean severity, coefficient of variance, F-value and Pr > F for root rot severity by source of variance

Groupings Mean Coefficient Severity of

Variance

Source

Area

95 Crop

Legumes

Variety Common -

Certified

Application

Method With seed

97 Herbicide No herbicide

Herbicide

a,b Means followed by the same letter are not statistically different at the P = 0.05 level of probability

3.3 Discussion

The mean root rot incidence and seventy for the îhree sampling times ie., 88%

and 1 .2,8 8% and 1.5, and 95% and 2.3, respectively, are fairly high given what has been

reported previously in the Peace River region and in other parts of Alberta. Harrison and

Laflamme (1 W6), in a root rot survey of Peace River region pea fields in 1995, found a

mean disease incidence and severity of 64% and 1.2, respectively. Hwang et al. (1997) in

a root rot survey of north-centrai Alberta pea fields found a mean root rot incidence and

severity of 34% and 0.5, respectively. Howard er al. (1995) in a root rot survey of pea

fields in southern Alberta reported a disease incidence and severity of 6 1 % and 1.2,

respectively.

One reason for the differences may be the t h e the s w e y s were conducted.

Hwang et al. (1997) and Howard et al. (1995) conducted their surveys only once in June,

which is quite early in the season. The root rot figures of 88% and 1.2 in the first

sampling time are very similar to what is reported in the rest of the province. The

incidence and severity in this s w e y were found to hcrease as the season progressed.

Consequentiy, it can be assumed that the later in the season that a root rot assessrnent is

done the higher will be the incidence and severity.

A second reason for these differences in results codd be due to the higher than

normal precipitation recorded in 1997. As can be seen in Figures 5 - 9, most of the Peace

region expenenced precipitation well above the 30-year average. This combined with

cool temperatures in May (Figure 14) (1997 average of 9A°C vs. 30-year average of

75

10.0 OC) which reduced evapotranspiration may have provided an environment which

favoured root rot pathogens.

There was some concern about the use of plastic bags to store plants and the

amount of time needed to transport the samples back to the lab. Distance was the main

factor for the amount of time it took to get the samples back to the lab. For example, the

distance fkom the lab in Fairview where the samples were stored to Fort VermiIion is 4 15

kilometres. It was impossible to drive to this area, collect pea plants fiom the 12 fields in

this area and r e m to FaWiew in one 24-hour period. Most of the samples were in the

cooler on the same day they were collected and none of the samples was more than 48

hours old before being put into the cooler. To decrease the rïsk that samples were not

being colonized by secondas. pathogens, plants were visually checked for any signs of

secondary colonization. Only one set of samples fkom the third sampling time in area 6

(ValIeyview/High Prairie) showed signs of secondary colonization. These plants had

been collected on a showery day and were quite wet when put into the bags. They were

discarded and fiesh samples were collected to replace them.

The identification of Fusan'um spp. as the main soi1 root rot pathogen is

supported by similar findings in other parts of Alberta (Sumar & Howard 1979; Hwang

& Chang 1989; Howard et al. 1995). F. avenaceum was identified as the species isolated

on the plates. This species is very common in Alberta soils and is often isolated fkom the

roots of peas, lentils, wheat, barley, canola and many other crops grown in Alberta. It is

also known to be very fast growing on PDA culture (Howard R. J. - Persona1

Communication 1998; Hwang S.F. Personal Communication 1998). This could be the

76

reason why it was the Furarium species to be identified as the samples sent for

identification were fiom plates that had been in culture for one to two months. F.

uvenacmm may have grown over everything eIse in the plate.

No Pythium spp. were identified in this survey. This rnay be due to the time of

year when the survey was conducted. Fythium spp. are known to occur early in the

season when soils are relatively cool and moist @ m a n 1984a; Hwang et al. 1997). As

soi1 warms up, Fusarhm spp. become more common and may overwhelm the Pythium

spp. Had the first sampling tune occurred soon after emergence of the peas, Pythiurn spp.

might have been isolated.

Foliar pathogens proved to be very difficult to identiw fiom culîure. Only four

plates were positively identified as having Ascochyta spp. even though visual symptoms

of the disease were present on most of the plants. It could be that the pathogens were

inactive because of dry conditions at the time of collection. Another reason may have

been the use of PDA to isolate the pathogens. The use of a selective medium for

Ascochyta spp. might have been a more usefbl method to isolate diis species in culture.

Syrnptoms of sclerotinia rot were also present on numerous plants fiom the third

sampling time but only nine plates confhed the presence of Sclerotinia sclerotiorurn.

Again, it could be that the pathogens on many of the plants were inactive because of dry

conditions at the time of collection.

3.31 Effect of Agronomic Practices on Root Rot Incidence and Severity

3 3 11 Differences Between Areas

Because of the size of the Peace River region, it can be expected that some

77

differences in moisture, soil types and soil pH will be present between areas. This could

account for some of the differences seen in root rot incidence and seventy between areas.

When averaged across al1 three sampling times, root rot incidence was highest in area 6

(Valleyviewfigh Prairie) and lowest in area 5 (Falher/ Peace River). Root rot severity

was significantly lower in area 5 (FalherReace River) than in al1 the other areas.

Area 5 had the Iowest fevel of both incidence and severity. This is an area that has not

followed the trend of the rest of the Peace River region in increasing pea acreage.

Growers in area 5 are wary of growing peas because of poor drainage due to heavy clay

soils and a generally fi at topography. Crops often suffer from excess moisture especially

after a heavy summer rainstorm. The infrequency of peas in cropping rotations in this area

may be one reason why root rot incidence and seventy were lower in this area than in

O ther areas.

It is difficult to explain why root rot incidence and severity in area 5 declined from the

first sampling time to the second sampling time and then went back up again in the third

sampling time. Figures 15 and 16 show this trend. As c m be seen from Table 1, trends for

root rot incidence and severity in areas 2, 3 , 4 and 6 were to increase with each sampling

time. Area 1 had a slight decrease in incidence in the second sampling time but had a high

incidence by the third sampling time. When viewed as an overall region, this same trend

continued. The drop in area 5 of both root rot incidence and severity in the second

sampling time and the lower incidence level in the third sampling time than in the f i t

sampiing time remains an anomaly that is dificult to explain given the nature of this

projet.

1 2 3 Sarnpling Time

Figure 15: Root rot incidence in Area 5 - FalherPeace River

1 2 3 Sarnpling Tirne

Figure 16: Root rot seventy in Area 5 - FalherPeace River

79

3.312 Cropping History

Peas grown in 1997 on fields that had oilseeds planted in 1996 had slightly higher

incidence of root rot (91.7%) then legurnes (90.4%) and cereais (90.1%). Peas grown in

1997 in fields that had legumes in them in 1995 also had higher incidence of root rot

(95.7%) then cereals (90.5%) and oilseeds (88.1 %). The type of crop g r o m in 1 996 did

not have an effect on root rot severity on peas grown in 1997; however the type of crop

grown in 1995 àid have a significant effect. Legumes in the rotation are known to be a

cause of higher levels of root rot (Salt & Delaney 1984; Tu & Findlay 1986) and a 4 to 5-

year crop rotation between legume crops should be practised to ailow pathogen levels to

decrease to where they are no longer causing economic damage (Kraft 1984). Results are

not as clear for peas grown on fields that had legumes in them in 1996. The incidence

levels were much closer together and significance was not as hi& (0.019) as with fields

that had legumes in 1995 (0.00 1). Severity showed no significant differences for fields

that had legumes in 1996. This is interesting since one would expect the most recent crop

to have the largest effect on the current crop. However, when the data are examined more

closely, there were only three fields of legumes in 1996 (two pea fields and one hay

field). ïhis represents oniy 3 percent of the fields in the survey and may not have been

enough to provide a significant difference in root rot severiq.

The situation is different for crops grown in 1995. The larger number of legume

fields (1 1 pea fields and 3 hay/pasture fields) represents 14 % of fields and this time

significant differences showed up. If legumes (peas, hay/pashire) were in the field in

1995, then root rot severity was significantly higher in the 1997 pea crop.

80

3.313 Seed Source

The fact that use of certified or better pea seed resuited in higher incidence and

severity of root rot than use of common seed came as a bit of a surprise. Since none of

the root rot pathogens are seed-borne, one would assume that seed source should not be a

cause of higher root rot- One possible cause of this could be damage occurring to pea

seed due to the greater amount of handling that occurs with certified seed as cornpared to

cornmon seed. With certified seed, seed is hanrested, cleaned, brought back to a bin on

the farm then reloaded to go to the retailer, then sold and brought to the buyer's bin. It is

then reloaded to go into the seeder. Common seed is harvested, cleaned and then put in a

bin until it is needed for seeding. Unless the certified seed is handled very gently with

belt augers (versus screw augers), the extra handikg may result in more cracks

developing on the seed coat. This could result in more exudates leaking fiom the seed

whick wouId attract root rot pathogens and increase both incidence and severity of root

rot (Hamian 1984a; Hwang et al. 1997). Given the nature of this project, the effects of

seed source on root rot cannot be adequately explained.

3.314 Seed Treatment

Metalaxyl treated seed had the highest level of root rot incidence when averaged

across the three sampling times. It also had a significantly higher level of root rot severity

than other treatments in the second sampling tirne. The reason for this may be that since

the main root rot organism identified in this survey was Furankm spp., and metalaxyl is

specific for control of Pythium spp. (Anonymous 1998a), Fusarium spp. were still able to

attack the pea plants resulting in higher incidence of root rot. This does not, however,

81

explain why metdaxyl treated seed had higher incidence of root rot dian untreated seed.

More research would be needed to further expiain this association.

3.315 Inoculant FormuIation

Use of peat based seed inoculant resulted in the Iowest incidence of root rot

(88.7%), followed by granular soi1 inoculant (9 1.4%) and liquid seed inoculant (92.4%).

These differences cannot be adequately explained given the nature of this project. Use of

inoculant Iowered the incidence of root rot in this study. Plants taken from fields that had

inoculant applied had lower incidence of root rot than plants from fieIds where no

inoculant was used (96.7%). However, only one field out of the 103 fields in the survey

had no inoculant applied. Further research wouId be needed to c o n f m this association.

3.316 FertiIizer Use

In this study, use of fertilizer resulted in higher levels of root rot incidence than

when no fertilizer was used. Use of a blend of one or more nutrients had the highest level

of root rot (92.3%), followed by nitrogen/phosphorus (90.9%) and then no fertilizer

(87.6%) which had the lowest level of root rot incidence. Fertilizer use also resulted in

higher levels of root rot severity in the first and third sampling times.

A reason for this may be that use of fertilizer usually results in increased root and

plant growth. This increased growth also results in higher levels of root exudates in the

rhizosphere around the roots which may attract more root pathogens to the roots and lead

to higher Ievels of root rot.

Tu and Findlay (1986) mention in their study that use of high levels of fertilizers often

leads to increased levels of root rot. Srihuttagum and Sivasithamparam (1991) in their

82

study on the effects of fertilizers on root rot in peas, found that al1 treaûnents containing

fertilizers, with die exception of phosphorus/potassium, nitrogen or nitrogen/phosphorus

resulted in higher levels of root rot severity than the no fertilizer treatment. They also

mention that root rot caused by some pathogens such as Fusariurn oxysponun and

Rhizoctonia sulani are reduced with addition of certain fertilizers while other pathogens

such as Pythium vexans appear to increase in root rot severity with the addition of sorne

fertilizers. In their snidy, when dl three pathogens were combined, fertilizer failed to

reduce severity. Since a cornplex of pathogens including Fusarium spp., Rhizoctonia

solani and Pythium spp. are thought to cause root rot in Alberta (Swanson et al. 1984;

Hwang & Chang 1989; Hwang et al. 1995b), the addition of fertilizer may actually be

increasing both the incidence and severity of root rot.

3.317 Fertiiizer Application Method

Banding of fertilizer resulted in the highest incidence of root rot (94.5%) and

placing the fertilizer with the seed resulted in the Iowest incidence (90.1 %). There were

no significant diEerences in severity of root rot between banding, broadcasting or seed

placed fertilizer. The effects of fertilizer placement on root rot may be related very closely

with the use of fertilizer. Regardless of whether the fertilizer was banded, broadcast or

seed placed, root rot incidence and seventy were always higher than when no fertilizer

was used (87.6%). Given the nature of this study, the difference between application

methods cannot be adequately explained.

3.318 Tillage

The lower level of root rot incidence reported here under zero-tillage as compareci

83

to conventional tillage is contrary to other research. There were no differences in root rot

incidence of peas at matunty between zero, minimum and conventional tillage treatments

in some research done in Saskatchewan (Bailey et al. 1992; Bailey et al. 1997). The

moisture difference between the Peace River region of Alberta and central Saskatchewan

where previous studies were done might be one possible explmation for the differences

in results.

This study showed no diflerence in root rot severity between tillage treatments. This

agrees with research done by Gossan et al. (1996), who found no consistent effects of

root rot severity in peas over the 4 years of their study. Turkington et al. (1997), however,

in a 4-year study found that average root rot severity was significantly higher under

conventional tillage cornpared to zero-tillage.

3.319 Rolling

The effects of rolling of a field on root rot incidence and severity in peas has not

previously been reported in the literature. Incidence may be affected more than severity as

no significant differences in severity were seen. The higher incidence of root rot reported

on peas rolled imrnediately after seeding could be due to the eKects of soil compaction

from the weight of the land rolIer on soils that have a higher moisture content. Tu (1987a)

States that peas grown in compacted soils have increased incidence and severity of root

rot. Soi1 moisture is usually higher at seeding time than later on when peas have emerged.

This higher soil moisture at this time could increase the effect of soil compaction.

3,320 Herbicide Use

Herbicide use resuIted in higher levels of both root rot incidence and severity than

84

where no herbicides were used. Use of herbicides always increases the stress put on pea

plants even if there are no visual symptorns. Tu (1992) showed that the use of phenoxy

herbicides such as MCPA and MCPB that cause stress to the peas aIso increased root rot

severity. Generally, however, the amount of stress inflicted by the new chemistry of

herbicides used on peas is minimal and the effect on yield is much less than would occur

if weeds were not controlled. Research on timing of weed removal in peas has s h o w that

10 to 15 bushels per acre is lost for every week that weeds are not removed or controLled

after crop emergence (Harker K-N. - Personal Communication 1998).

Chapter 4 - Pea Disease Management Program

Disease in peas is the result of an interaction between a pathogen, the pea plant

and conditions favourable to development of a disease. A good disease management

program involves manipulating or modifjhg one or more of these parameters to stop the

interaction fi-om occ&g. For example, the use of a resistant variety of peas will not

dlow the interaction between the pathogen and the host pea plant to occur even if

environmental conditions are favourable. This will effectively "control" the disease.

Tbere are many other ways to achieve this "control". This chapter will illusirate the best

ways to manipulate or modi@ the host peas, the pathogens and the environment to

prevent, stop or minimize diseases of pea in the Peace River region of Alberta.

4.1 Cultural Practices

One of the most common ways to control disease is to make changes to the

environment in which the peas are growing. Modi-g cultural practices is the easiest

way to do this.

The type of modifications done will Vary according to the type of farming

operation used by the grower. Conventional tülage and conservation tillage are the two

main types of farming operations in use in the Peace River region of Alberta at present.

Conventional tillage is still the most common type of famiing system in use, with 62 %

of cultivated hectares being farmed using this method. Conservation tillage, however, has

grown in popularity over the last decade and is now used on 38 % of cultivated hectares

in the Peace River region of Alberta (Anonymous 1997~). Each of these farming methods

will be dealt with separately in this section.

4.11 Conventional TiiIage

Conventional tillage is the traditional method of farming in which fields are tilled

once or twice in the fall after harvest and two to three more tirnes in the spring pnor to

seeding. Nitrogen fertilizer and pre-emergent herbicides are often applied during one of

the tillage operations in the fall. Summerfdlow, the practise of keeping land tilled during

the growing season, is still often included in cropping rotations. This method of famiing

has been widely practised in the Peace River region since it was pioneered shortly after

the turn of the century but like the rest of Alberta, it is slowly being replaced by

conservation tillage because of soil erosion problems, cost of fuel and rnachinery and

time constraints as fewer and fewer growers f m larger and larger tracts of land.

Tillage is one of the most effective ways to reduce and control many soil-borne

plant pathogens. Tillage improves aeration of the soil, speeds up drykg, incorporates

crop residue, and reduces the leveI of some soil pathogens by decomposition. By

incorporating crop residue into the soil, tillage also prevents the spread of spores by wind

or min splash and thus reduces the spread of many foliar diseases. 'The more often a soi1

is tilled, the more enhanced are these effects (Howard 198 1).

Summerfallow involves repeated tillage through the growing season to prevent

weed growth and conserve moisture. Other benefits include faster decomposition of crop

residue and consequently an increased reduction in the number of soil pathogens.

Plowing in the fa11 after harvest will bury any pathogen inoculum that is on crop

debris. Moldboard plowing reduced populations of Rhizoctonia s o h i in the top 5 - 7.5

87

cm of soil (Rothrock 1992). Deep plowing to bury sclerotia to a depth of at least 10 cm is

recornmended for control of sclerotinia rot. However, any additional tÎllage within a

season d l retuni sclerotia back to the soil surface before they have had t h e to

decompose and negate the benefit of plowing for control (Bailey 1996). Plowing or deep

tillage is a common control recommendation for diseases such as ascochyîa blight,

powdery mildew, downy mildew and sclerotinia rot (Howard 198 1 ; Hagedorn 1984).

Tillage also causes the soi1 to w m up faster and dry out more quickly (Rothrock

1992; Green et al 1994). Higher soil temperatures are beneficial for quicker germination

and emergence of pea seed, and reduce the chance of infection by soil pathogens such as

Pythium spp. (Hwang et al. 1997). Lower soil moisture also decreases the risk of

infection by soil paîhogens such as Pythium spp., Rhizoctonin solani and Fusarium spp.

al1 of which are favoured by high soi1 moisture. The incidence of infection by Pythiurn

ultimurn in peas was less when soi1 moisture was low (Rothrock 1992) and

chlamydospore germination of Fusariurn solani Esp. pisi in the rhizosphere of pea seed

was lower when soi1 water is reduced (Cook and Flentje 1967).

Deep tillage or sub-soiling to break up compacted Iayers in hardpan soils is also

beneficial in reducing the incidence and seventy of Fusarium spp. (Tu 1994). When

tillage is removed from a fa-g operation, one of the best cultural practices for control

of diseases is lost.

4.12 Conservation Tiüage

Conservation tillage is any tillage system that leaves at least 30 % of the soil

surface covered by crop residue after seeding (Rothrock 1992). There are two types of

88

conservation tillage systems in use at the present time in western Canada. One system,

known as high disturbance or minimum-tillage, usually involves a tillage pass pnor to

seeding in order to band fertilizer or apply soil incorporated pre-emergent herbicides.

This system generally uses sweeps on an air seeder which causes some mixing of soil and

crop residue at the time of seeding. It does, however, leave more than 30 % of crop

residue on the soil surface. Some aeration and warming of the soil occurs wiîh this

system.

The other system, Eaiown as low disturbance or zero-tillage, is based on disturbing

the soil as little as possible during seeding. It generally uses a coulter or knife system on

the seeder and leaves most of the soil and crop residue undisturbed.

High disturbance tillage accounts for 29.4 % of the cultivated hectares in the

Peace region while low disturbance accounts for only 8.6 ?h of cultivated hectares

(Anonymous 1 997c).

Peas are well suited to conservation tillage. Yields are equal or fiigher than when

grown under conventional tillage. Yields of peas grown under zero-tillage at Fort

Vermilion, AB and Indian Head, SIC were 17% and 9% higher, respectively, dian peas

grown using conventional tillage. Peas grown under zero-tillage had 4% higher L O00

seed weight than when grown under conventional tillage at both Fort Vermilion and

Indian Head (Clayton et al. 1993). Nitrogen fixation was 3 1% higher in peas grown

under zero-tillage as compared to peas grown under conventional tillage (Matus et al.

1997).

Peas are often the h t crop that fmers will grow when making the switch to a

89

conservation tillage system. Given this information, it is not surprishg to find that 70 %

of fields surveyed in this project were f m e d under reduced tillage (40 % had minimum-

ti1lage and 30 % had zero-tillage).

Crop residue on the soil surface provides a favourable environment for both soil

and residue borne pathogens. It is generally believed that crop residue decomposes more

slowly when lefi on the soil surface than when incorporated into the soil (Bailey et al.

1997). This has an impact on sumival of some pathogens. Twenty-four percent more

sclerotia of Sclerotinia sclerotiom, for example, have been shown to survive on the soil

surface than when buried in the soil (Bailey 1996).

Of the few studies that have been done on pea diseases under conservation tillage,

most have not shown disease to be any more of a problem under reduced tillage than

conventional tillage. Bailey et al. (1997) found that although severity of hiycosphaerella

pinodes was lowa under conventional tillage than reduced tillage, there was no effect on

the final yield of peas. They also found that although root rot in peas caused by Fusarizcm

spp. was sometunes lower under conventional tillage, &ere was little difference at

maturity between conventional tillage, minimum-tillage and zero-tillage. Similarly,

Gossan et al. (1996) f o n d no consistent effects of tillage on severity of root rot

(Ftcsarium spp .) or foliar blight (M. pinodes) in peas over the course of their 4-year

study. Turkington et al. (1 997), on the other hand, found that root rot severity of peas was

significantiy higher under conventional mage than under zero-tillage in their 4-year

study at Fort Vermilion in the Peace River region of Alberta.

Although previously rnentioned that sderotia of S. sclerotiomm have higher

90

survivd rates when Ieft on the soi1 surface, other studies have shown that survival of

sclerotia decreases with increasing soil moisture (Teo et al. 1989; Nasser et al. 1995).

Since soil moisture is always higher under conservation mage (Lafond et al. 1992;

Rothrock 1992; Arshad & Gill 1996; Bailey 1 W6), this should Iead to increased

degradation of sclerotia and lower survivability. Bailey (1996) States that any heavy

residue from a non-host crop will increase soil moisture and microbial activity which will

Iead to decreased survivability of sclerotia. She concluded that there should be no greater

risk of sclerotinia rot with minimum-tillage or zero-tillage than with conventionai tillage.

As farmers have switched their seeding equipment over to conservation tillage,

many have also increased the row spacing from 15 cm to 23 cm or 30 cm. This was done

to overcome dificulties in seeding through heavy crop residues in the wetter parts of the

prairies (Lafond et al. 1997). Not much research bas been done on row spacing and its

effect on disease. Wider row spacing should help to reduce the levels of some diseases

such as sclerotinia rot by increasing the airflow through the canopy and thereby

decreasing humidity. Lafond et al. (1997) in a sîudy on row spacings concluded that

wider row spacing sometimes decreased disease levels. He also stated, however, that

more research was needed in order to be crop and area specific.

4.13 Other CuItural Practices

Other cultural practices known to affect disease include changes in seeding dates

and seeding depth. Given the short growing season in the Peace region, making much of a

change in seeding dates is difficult. Peas are usually the fust crop to be seeded in the

spnng and research has shown that delaying seeding has a negative e f h t on yield. Both

91

the Alberta and Saskatchewan Pulse Growers Manuals recommend early seeding of peas

to achieve the best yields and quality (Anonymous 1993; Auonymous 1997a). Hwang et

al. (1997) showed that yield of peas decreased significantly with successive seeding dates

in 2 out of 3 years of their research. Early seeding is also recommended to prevent

powdery miIdew fiom causing significant yield and quality losses (Zimrner 1984)- It wiIl

also allow peas to reach a more mature stage before soil temperatures become wann

enough for fusarium wilt to become active.

Seeding depth will have an influence on the amont of time it takes for the pea

seed to germinate and emerge fiom the ground. Germuiating seeds are susceptible to

Pythiurn spp. for only 48 - 72 hours afier planting under good conditions (Harrnan

1984a). n i e deeper the seed is placed in the ground, the cooler the soi1 temperature and

the longer it will take for the seed to gemiinate and emerge fkom the gound (Green et al.

1994). This increases the amount of time that the pea seed is susceptible to infection

(Leach 1947). Current recommendations are to seed peas at a depth of 2.5 cm below

moisture (Clayton G.W. - Personal Communication 1997). For example, if moisture is

2.5 cm below the soi1 surface (top 2.5 cm of soil is dry), peas should be seeded to a depth

of 5.0 cm. This allows for the warmest soi1 temperature and sufficient moisture for rapid

germination and emergence fkom the soil.

4.2 Host Resistance

There are two methods to tramfer resistance to disease in peas. One is wirh

traditional plant breeding methods and the second is with genetic engineering.

92

Large collections of pea germplasm are held at many cenees around the world: Italy,

ICARDA, Syria, Poland, UK, Sweden, USA, Germany and India. Between 1500 and

4000 accessions are held in the majority of these centres (Davies 1993a). Due mainly to

the efforts of Swedish geneticists, more than 2000 mutations of peas are recognized and

of these over 400 developmentai and morphological mutants have been descrïbed (Marx

1977; Davies 1993a).

Traditional plant breeding methods using both wild pea types and mutants have

conferred resistance to many pathogens and diseases (Davies 1993a). Resistance to

Ascschytapisi has been successfully bred into many varieties, but breeding for

resistance to MycosphaereZZa pinodes has been unsuccessful (Cousins 1997). Only very

low Ievels ofresistance have been found in the germplasm evaluated for resistance to M.

pinodes. None of the varieties presently available in western Canada has any resistance to

M. pinodes. Over 30 Pisum genotypes with partial resistance have been identified in a

study at Morden, MB. The use of these genotypes in a breeding program will allow the

pyramiding of genes to give better and more long lasting resistance to M. pinodes (Xue et

al. 1997). Breeders expect the pyramiding of many genes, each with a small effect, to be

a long and labourious task (Johnston & Kutcher 1998). One unique attempt has been the

electrofusion of cells fiom field pea and Lathym spp. which possess high levels of

resistance to the pathogen (Blade 1998). Other traits to be successfully bred into peas

include resistance to downy mildew (caused by Peronospora viciae), powdery mildew

(caused by BIumeriapis& fus- wilt (caused by Fusarium oxysporum f.sp. pisi),

bacteriai blight (caused by Pseudornonas syringae pv. pisi), pea mosaic virus, pea enation

93

mosaic virus, pea feafroll virus, pea seed-borne mosaic virus and pea early browning

virus (Cousin 1997).

Genetic engineering is still in its infancy in field peas. Several research groups are

working on genetically mod img peas. In Canada, the Plant Biotechnology Institute in

Saskatoon and the University of Alberta in Edmonton both have researchers working on

introducing foreign genes into peas (Polowick 1996; Ozga & Reinecke 1998). The

Biotechnology Unit of CSIRO in Canberra, Australia, however, has been the most

successful so far. They have developed transgenic pea lines with resistance to herbicides,

insects, vinises and one line with increased methionine content. Field testing has

occuned in Australia and New Zealand and commercial development should occur in the

next 3 to 4 years (Blade 1998).

Virus resistance is one area that could benefit kom genetic engineering. The

introduction of genes for viral coat protein that simulates cross protection, satellite RNA

which modifies the seventy of symptoms of some viruses, antisense RNA to block RNA

and DNA vinises, ribozymes (short DNA sequences) which can cleave RNA molecules,

and viral replicase sequences are al1 examples of genes that have been successfblly

introduced into other plants (Davies 1993b) to provide resistance to viruses. Al1 of these

could potentially be used to confer Wal resistance in peas.

Opportunities for improvement by using traditional breecling rnethods still exist

for fungai pathogens (Davies 1993b); however, examples exist where plants have been

transformed using foreign genes to provide disease resistance. Chitinase genes fiom

beans (PhaseoZus vulgaris) and the bacterium Senatia marcescens have been introduced

94

into tobacco and canola and a gene for a riboryme-inactivahg protein fkom barley

inserted in tobacco have dl been shown to reduce the severity of Qmping off due to

Rhizoctonia solani. Since beans are also a legume, the likelihood that the chitinase genes

could be introduced into peas are very good. Pathogenesis-related proteins and lectins

could also be traosfered to convey resistance (Davies 1993b). In the near future,

traditional breeding methods may be the best way to improve resistance to fimgal

pathogens in peas. Nevertheless, it is important to remember that although there is plenty

of genetic stock available within the pea family, the time may corne when we will deplete

the useh1 genes fkom peas and foreign genes will be needed to help provide resistance to

fungal pathogens.

4.3 Chernical Controls

The number of fungicides registered for use on peas in western Canada is limited.

At the current time, there are only three fungicides for use as seed treatments and two as

foliar sprays. Thiram (Thhm 75WP) and captan (Captan FL) are registered as seed

treatments for control of seed decay, seedling blight, damping off and root rot and

metalaxyl (Apron FL) is registered for control of seed rot and seedling blight caused by

Pythiurn spp. Thiram and metalaxyl c m be mixed together to give a broader spectnim of

protection (Anonymous 1 998a).

Chlorothalonil (Bravo 500) is a protectant fungicide for the control of

mycosphaerella blight. It is applied at the first sign of disease spp toms and can be

repeated up to three times at 10-14 day intervals if weather conditions are conducive to

95

development of the disease (Anonymous 1 998a).

Sulphur (Kumulus DF) is registered for the control of powdery mildew. It can be

applied at the f k t appearance of the disease and repeated at 7 to 10 day intervals if

needed (Anonymous 1998b).

4.4 Proposed Pea Disease Management Program

1. Crop Rotation

Crop rotation is still one of the best means of avoiding problems with disease. It

lengthens the t h e between susceptible crops, allowing the pathogen population

to decline to levels that prevent significant yield losses. Crop rotation is even

more important under conservation tillage than under conventional tillage since

the benefits of tillage for disease control are lost. Growing peas only once every 4

to 5-years will reduce inoculum levels of rnost pathogens to levels where they

will no longer cause econornic damage. Davies (199 1) reported a higher incidence

of sclerotinia stem rot in canoIa when peas were included in the rotation.

Therefore, having 1 or 2 years of non-host crops between canola and peas is

important. A crop rotation consisting of cereal-oilseed-cereal-peas would be an

ideal rotation to rninimize disease problems in the crops grown in the Peace River

region.

2. Plowing

Deep plowing (10 - 15 cm) in the fa11 will bury inoculum of many pathogens and

is recommended if diseases such as mycosphaerella blight, sclerotinia rot,

p o w d q rnildew and downy mildew are present in the pea crop. Any additional

tillage, however, will return sclerotia of sclerotinia rot to the surface and negate

the effects of plowing. Unfortunately, the option of plowing is only available to

growers who are not practising conservation tillage.

3. Use of High Vigour Disease-Free Seed

Pea seed should have hi& germination, high vigour and be disease-fkee. Seed

grown in drier parts of the province is preferable since it is usually free of

pathogens. The benefits of using high vigour seed are numerous. Seeds germinate

more quickly and are better able to withstand stressfùl conditions such as cool,

wet soils. Pea seed can be vigour tested while being checked for germination at a

number of seed testing labs in Alberta.

Elec~cal conductivity is used to mesure vigour in peas and is expressed in

micro-siemens (JAS). A score of 0-20 ,US means that peas are very vigorous and

are suitable for early seeding into cool soils. A score of 20-30 / IS Uidicates

medium vigour. These peas may not perfom as well under adverse conditions but

should still be suitable for later seeding into warm soils. A score over 30 ,US

indicates low to very low vigour and use of these peas for seed is not

recommended (Foster-Stubbs 1997).

4. Seed Treatment

Seed treatment will prevent the introduction of seed-borne pathogens fkom

diseases such as mycosphaerella blight. It will also provide protection against soi1

pathogens such as Pythium spp., Fusarium spp. and Rhizoctonia soluni as the

seed geminates and emerges &om the ground. Hwang et al. (1 997) showed that

significantly higher yields were obtained when pea seed was treated than when

left untreated.

A number of fungicides are registered for use on peas for root diseases. Thiram

98

and captan will provide protection against seed decay, root rot, h p i n g off and

seedling blight, while metalaxyl gives protection against seed rots and seedling

blights caused by Pythium spp. Thiram and metalaxyl can be mixed to give a

broader s p e c m of protection (Anonymous 1 998a).

5. Seeding Depth

Peas should be seeded at a depth of 2.5 cm below moisture. This allows for the

wannest soil temperature and sufficient moisture for quick germination and rapid

emergence.

6. Lime if Seeding on Acid Soils

Since a large portion of the Peace River region has acid soils (Anonymous

1996~). liming to increase the pH to between 6.0 and 6.4 should help to decrease

the severiîy of root rot (Tu 1987b). Liming also has many other benefits such as

increasing yield (Arshad & Gill 1996), creating a more favourable environment

for soil microbiological activity, increasing the availability of several nutrients

and improving the physical properties of some soils (Anonymous 1996~).

7. Subsoil if Seeding on Hardpan Soils

Hardpan soils, consisting of a compacted layer of soi1 found at the depth of

tillage, are created by soil compaction fiom numerous heavy rnachinery passes

(Greevers and Boehm 1994) and are found in many Race River region fields.

Subsoiling to break up the hardpan will help to reduce both root rot incidence and

severity (Tu 1987b). Yield may also be positively afZected. Coy (1 985) in a

subsoiling study near Grande Prairie, AB showed that yields of wheat were

always higher in subsoiled plots than in non-subsoiled plots.

8. Use Foliar Funacides if Needed

Chlorothalonil (Bravo 500) has recently been registered for control of

mycosphaerella blight in peas. Pea plants should be checked regularly at earIy

flowering for signs of mycosphaerella blight and if symptoms appear, an

application of chlorothalonil can be made. If weather conditions are favourable

for development of the disease (warm and humid), applications can be repeated at

10 - 14 day intervals. A total of 3 applications per season c m be made if

necessary (Anonymous 2 998a).

Although powdery mildew was not reported in this survey, it has previously been

reported in the Peace River region (Hanison & Lafiamme 1996). If an early

infection of powdery mildew is noticed, sulphur (Kumulus DF) can be sprayed to

prevent senous damage to the pea crop. It is applied when the disease is e s t

noticed and can be repeated at 7 -10 &y intervaIs if needed. If the infection

occurs late, a fungicide is not required since no economic damage will occur to

the peas.

9. Avoid Seeding Next to Last Year's Pea Fields

Since symptoms of mycosphaerella blight were found in al1 pea fields surveyed in

1997, it can be assumed that this disease is widespread in the Peace River region.

Therefore, since ascospores c m be blown by wind for a considerable distance,

peas should not be seeded next to a field that had peas in it in the previous year.

1 O0

10. Seed Peas as Early as Possible

Peas shouid be the first crop in the ground in the Peace River region. Planting

peas as early as possible wiIl prevent serious damage Eom powdery mildew and

fusarium wilt, both of which are favoured by high temperahires. It will also

increase the possibility of obtaining the highest yields and quality.

Chapter 5 - Conclusions and Recommendations

Root rot, mycosphaerella blight and sclerotinia rot are the main diseases of field

peas in the Peace River region of Alberta. Other diseases may be present but are not yet

causing significant losses to growers in the region. It is, however, important to keep these

other diseases in rnind when planning a successful disease control program.

Kraft (1 99 1) States that roo t rot is the principle limiting factor to increasing or

stabilizing yields of peas in North America. The results of this study would agree with

this statement. Root rot is the disease with the potential to have the most serious effect on

viability of pea growing for this region. Although 1997 was a year of above normal

precipitation, and this may have been a cause for unusudly high incidence and severity of

root rot, there is research to show that root rot severity may even be worse in dry years

due to darnage to the root system (Tu 1994). Seed treatment will help to minimize the

effects of root rot but only provides protection for a shoa time during germination and

emergence of the peas. Other factors such as a 4 to 5-year rotation between pea crops, use

of high vigour seed, subsoiling, liming, and avoiding stresshl phenoxy herbicides such as

MCPA and MCPB will d l aid in reducing the effects of root rot.

Mycosphaerella blight also has the potential to become a serious disease of peas in

this region. The disease is very moisture dependent and may not be a problem in dner

years. However, the Peace River region is better hown for having excess moisture rather

than being deficient in moisture. A newly registered fungicide, chlorothdonil (Bravo

5ûû), is now available for controlling mycosphaerella blight in peas. This should help to

102

keep the disease fiom causing serious losses in high moisture years. Much research is

also being done in western Canada to breed resistance to this disease in pea varieties but

resistant varieties are not yet available. For the present t h e , rotation, seed treatment and

the use of f o l k fungicides will be very important means of keeping this disease in check.

Sclerotinia rot is not considered a serious disease in peas since it does not cause

significant yield losses (Johnston & Kutcher 1998). It is, however, a concern to any

grower who has canoIa in his rotation since it adds inocdum to the soil. Because there

are no fimgicides currently registered for the control of sclerotinia rot in peas, and since

the incidence of sclerotinia stem rot in canola is always higher when peas are in the

rotation, producers who grow both peas and canola may have to add a k g i c i d e

application for sclerotinia control in canola to prevent sclerotinia stem rot fiom becoming

a yield reducing disease in canola.

Awareness of these pea disease management strategies d l allow peas to

continue being a high yielding, viable crop for this region.

Given the resources available, I consider this survey to have been successful in

achieving the objectives of this project. The identification of pathogens was not as

successful as it could have been but other parts of the project gave some very interesting

results.

In the hope that any future surveys of the Peace Riverregion are more successful,

1 make the following recommendations:

1. Sample only one area at a thne. The Peace River region is very large and driving

the distances involved is costly and time connimhg. It would have been better to

103

have picked one area in the region and done the sampling, identification and

culturing more quicWy and at much less cost. There would have been less plants

to handle and more t h e to devote to the cultures and identification. The only

other option is to have more people helping if sampling of the whole region is to

be done.

2. Use selective media for a number of known pathogens. The visual identification

of symptoms wili give a clue as to what selective media should be used.

3. Do as much of the visuai identification of symptoms as possible in the field.

Again, because of the size of the Peace River region and time constraints this is

difficult to do unless a lot of people are available to help. n i e grower should also

be invited dong since most of them are very interested in what is happening to

theû pea crop. This could also involve more time since the f m e r will be asking

a lot of questions. From an extension point of view, however, this would be a very

valuable exercise. Another good reason to pick only one area instead of the whole

region.

Appendix 1

1997 Peace Region Field Pea Disease Survey

Name of Cooperator:

Ad dress:

Phone 3:

LegaI Land Location of Field:

1. How many acres of field peas are you growing in I997? acres.

2. How many years have you been growùig fieId peas? O 1st time C l 1 year Q 2 years Cl 3 or more years

3. What has been the cropping history of this field?

4. Have you had any diseases in your peas in previous years? I f yes, did you have it identified?

D no O yes Disease (s):

5. Did you use cenified or beîter seed? I f yes, what variety of peas is seeded in this field?

no O yes Variety: -

6. Did you use seed treatment? I f yes, please indicate which one you used. O no D Y = Cl Thiram 75 WP

O Captan FL O Apron FL O Other

7. Did you use inoculant? If yes, what Spe was used? il no O yes fl peat powder

17 liquid 17 granular

8. Did you apply any fertilizer? If yes, please indicate type, rate and meîhod of application? i l no 0 YS

Type:

Rate:

Method of Application: Eî broadcast CI banded iJ with the seed

9. Were yow peas zero-tilled? 17 yes If yes, how many years has this field been zero-tilled?

O 1st time El 1 year 172years O 3 or more years

O no If no, how many tillage passes were made pnor to seeding? Cl one tillage pass !l two tillage passes 0 three or more tillage passes

10. Did you use a land roller after seeding? If yes, when did you roll the land? Cl no O yes Ci immediately after seeding

Cf before ernergence of peas O after emergence of peas

1 1. Have you used any herbicides on this pea field? If yes, please indicate type and rate used. Cl no I l yes

Herbicide [sj used: Rate used:

12. What herbicides were used in tbis field in the last two years?

1996: Herbicide (s) used:

1995: Herbicide (s) used:

Rate used:

Rate used:

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