1989 Daniel McAlpine Memorial Lecture

Ecology, epidemiology and control of take-all, Rhizoctoniabare patch and cereal cyst nematode in wheat

Albert RoviraCSIRO Division of Soils, Glen Osmond, South Australia 5064

Introduction

I consider it an honour to have been asked topresent the Daniel McAlpine Memorial Lecture. Inmany ways, since I ventured into plant pathology inthe early 1970s after a career in soil and rhizospheremicrobiology, I have followed in the footsteps ofDaniel McAlpine. It was McAlpine (1902; 1904) whofirst isolated and identified the fungus Ophiobolusgraminis (now Gaeumannomyces graminis (Sacc.)Arx & Olivier var. tritici Walker) as the causativeagent for the root disease take-all of wheat in Aus­tral ia. He was so concerned with the severe lossestake-all was causing that he distributed a question­naire to farmers to get more information on thedisease with the aim of developing control meas­ures. Here, my interests were similar to those ofMcAlpine's -to improve our understanding of thedisease in the field so that farmers can overcomethe problem and increase grain yields.

Take-all causes large yield losses throughout theworld, and initially the aim of my research was tostudy the ecology and epidemiology of take-all in thefield in relation to rotation and tillage systems.However, the scope of the research expanded whenearly field experiments with soil fumigation andnematicides demonstrated the extent and magni­tude of the cereal cyst nematode (CCN) problemcaused by Heterodera avenae Woli. My interestsexpanded further when farmers' reports and our owntillage research highlighted Rhizoctonia bare patch,caused by Rhizoctonia solani KUhn, as a majorimpediment to the adoption of conservation tillagepractices.

Our soil fumigation studies showed that farmerswere suffering huge losses through root diseases,despite the fact that the diseases had been recog­nised in the field for 40 to 100 years and a great dealof scientific effort had gone into understanding them.

I decided that if we were to help farmers overcomethese problems and increase productivity, we shouldconcentrate on field experiments, the results ofwhich would be expressed not only in terms ofpathology, but also in terms of yield. In this way,farmers could assess the losses they were suffer­ing and decide whether it would be worth their whileadopting strategies to minimise these losses. Thishas been the philosophy behind my research whichhas been supported by Wheat Research Counciland CSIRO.

Australasian Plant Pathology Vol. 19 (4) 1990(Volume 19 (3) was issued on 2 November 1990)

I will present some of the highlights of theresearch which is the result of a team effort. I havebeen extremely fortunate that such dedicated andenthusiastic people have been associated with theresearch from its inception.

A great deal of work has been conducted in Aus­tralia on these three cereal root diseases and I referreaders to recent reviews on take-all (Rovira et a/.1991), Rhizoctonia bare patch (MacNish and Neate1991), and cereal cyst nematode (Brown 1987;1991).

Soli fumigation to determine the Importance ofsoilborne root diseases

Field trials in south-eastern Australia have shownyield responses of 15% to 450% following soilfumigation (Rovira and Ridge 1979). Responses atsix sites in four seasons are illustrated in Figure 1.

By conducting trials which include fertiliser treat­ments and selective biocides as well as fumigationit was possible at several sites to compartment­alise the responses and separate the effects ofnutrient supply, nematode control and fungal con­trol (Meagher et a/. 1978; Rovira and Simon 1985).Fumigation through areas identified as Rhizoctoniapatches in a preceding crop and also through sur­rounding areas of the good plant growth on calcare­ous sandy soils enabled Simon and Rovira (1985)to demonstrate yield response of 322% and 29%inside and outside the patches, respectively. Whenthis information was combined with aerial photo­graphs which showed that the patches made up18% of the crop, it was possible to estimate the yieldloss from Rhizoctonia bare patch.

Development of long-term field trials

In the mid-1970s ICI and Monsanto introduced'knock-down' herbicides which enabled crops to bedirect drilled into uncultivated soil. At this stage littlehad been published, either locally or overseas, onthe effects of tillage on cereal root diseases.Because cereal root diseases appear to be moreserious in southern Australia than in Europe and theUnited States, I decided to expand the scope of theresearch to include different tillage systems in therotation trials at three sites typical of the major cerealgrowing areas of south-eastern Australia.

101

Figure 2 Relationship between the incidence of take-allonroots andgrain yield ofwheat sown withcultivation (CC)andbydirectdrill (DD)following different rotations atAvon,South Australia, in 1979.

it was the major problem. Soil and climate atCalomba are similar to those at Avon. Extensiveresearch on CCN was also conducted throughoutSouth Australia (Rovira et al. 1981; King et al. 1982).

II

'C:F,

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Results

Take·all

Effects of rotation and cultivation The results fromboth Avon and Kapunda demonstrated the impor­tance of grasses in building up the levels of thetake-all fungus in soil, the damage to roots by thetake-all fungus and the consequent reduced yieldsof the following wheat crops.

The incidence of take-all in roots in late Augustat Avon in 1979 accounted for 53% and 67% of thevariation in grain yield of wheat sown by direct drill­ing and following cultivation, respectively (Figure 2).Direct drilled wheat after a pea crop or medicpasture, each sown by direct drilling, had a higherincidence of take-all and a lower grain yield thanwheat sown with cultivation following peas andmedic which had also been sown into cultivated soil.These results reflect the fact that, at that time, cul­tivation was necessary to control grasses in legumecrops and sown pastures (Figure 3) and hence, inthe mallee environment, continuous direct drillingwas not a practical proposition. In the mid 1980s,the development of selective herbicides, whichremove grasses from grain legume crops and past­ures, improved the prospects for direct drilling. Asthe trial developed at Avon and grass-control herbi­cides were used, comparable yields were obtainedwith direct drilling and cultivation, indicating thatcontrol of take-all by the appropriate rotations isnecessary before adopting direct drilling.

Results from the Kapunda site have confirmed this

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One site was at Avon near Balaklava 100 km northof Adelaide with a typical mallee environment ofcalcareous sandy soils (Classification Gc1, solon­ised brown soil, Northcote et al. 1975) and anaverage annual rainfall of 350 mm and a droughtfrequency of one year in three. This trial consistedof five 2-year rotations, viz. continuous wheat,grasslmedic pasture-wheat, medic pasture-wheat,peas-wheat, and oats-wheat; two tillage systems,conventional cultivation and direct drilling with theSIRODRILL (Venn et al. 1982), were applied to thefirst four rotations while the oats-wheat rotation wassown following cultivation but not by direct drill. Sixreplicate plots, each 200 m long, of each treatmentwere set up. Two identical phases of the trial wereset up one year apart so that each year one of thephases was in wheat while the other was in thealternative rotation. The first phase with the alter­nate rotations was set up in 1978, and the secondin 1979, in each case following a wheat crop froma long-term medic pasture-wheat rotation.

The second site was at Kapunda 100 km north­east of Adelaide with a red-brown earth soil (Classi­fication Dr2.33, red duplex soil, Northcote et a/.1975), an average annual rainfall of 496 mm and adrought frequency of one year in ten. There werethree 2-year rotations, viz. continuous wheat, lupins­wheat, and grass/subterranean clover pasture­wheat, with three tillage systems, viz. conventionalcultivation following the district practice of three cul­tivations to 5-7 cm with a dart point cultivator,reduced-tillage with one cultivation, and direct drill­ing initially with the SIRODRILL and after 1986 witha narrow sowing point. As at Avon, two identicaltrials were set up a year part to provide the twophases. The paddock had been under grass-cloverpasture for 9 years before Phase I was sown towheat using the three tillage systems in 1983; PhaseII was started with wheat using the three tillagesystems in 1984.

Neither Avon nor Kapunda sites had a significantlevel of CCN, so detailed research on this diseasewas conducted on a third farm at Calomba where

Figure 1 Effect of soil fumigation on wheat yields atdifferent locations in South Australlae and VictoriaS indifferent years.

102 Australasian Plant Pathology Vol. 19 (4) 1990

in our plots yields went from 30% to 80% of thepotential with high and low take-all, respectively. Theaverage yield for the district did not change sig­nificantly between the two years indicating thescope for increased yields if farmers adopted grassmanagement practices.

Use of artificial inoculum to screen for resistance totake-all One problem of screening cultivars forresistance to take-all in the field is the variation indisease levels in most fields. The development ofartificial inoculum of take-all with sterile ryegrass ormillet seeds by Simon et a/. (1987) enabled us toscreen 3500 cultivars from the Australian Wheat Col­lection for resistance to take-all. In this research,done over four seasons, we used either single rowsor microplots and found a considerable range inresistance or tolerance between cultivars (Simonand Rovira 1985).

Link between rainfall, incidence of take-all and grainyield Results over eight years at Avon demon­strated an average annual yield response in wheatfrom controlling take-all of 42% (viz. average yieldswent from 1,2 tlha to 1.7 t/ha).

Our research has shown that yield losses werestrongly related to disease incidence on wheat rootsand rainfall in September (r2 = 0,91) but were poorlyrelated to disease incidence alone. Adequate rain­fall in spring was necessary to allow the develop­ment of the take-all fungus on and inside roots andto reduce yield. Survival of inoculum of the patho­gen until the following growing season, as indicatedby the percentage of wheat plants infected with take­all, was also influenced by spring rainfall. A regres­sion model was developed to predict the incidenceof take-all in a wheat crop from the incidence of take­all and the August-September rainfall the previousseason (r2=0.96) (Roget and Rovira 1991),

Biological control From my research experience onthe microbiology of the rhizosphere, I maintained aninterest in the interactions between root pathogensand rhizosphere microorganisms as a possible fac­tor in the biological control of root diseases, In1971-72 while a Senior Research Fellow at theUniversity of Bristol, UK, I worked with Dr RichardCampbell to show by scanning electron microscopythat, in axenic sand systems, bacteria of the Pseudo­monas spp. attached themselves to hyphae of thetake-all fungus growing on roots and caused the lysisof the hyphae (Rovira and Campbell 1975).

Dr Richard Smiley worked in my laboratory during1971 and 1972 and further developed his doctoralresearch on the effects of ammonium-N and nitrate­N on the pH of the rhizosphere. Smiley demon­strated that with ammonium-N there was a fall in thepH of the rhizosphere and a build-up of fluorescentpseudomonads suppressive to the take-all fungus(Smiley 1978a,b).

Pasture

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need for grass management in order to control take­all and improve yields. Figure 4 expresses thebenefits of take-all control in terms of the yield poten­tial as expressed in terms of the April-October rain­fall model of French and Schultz (1984). This modelhas proven a valuable tool to demonstrate the effectsof different practices on yield. I believe that it is auseful concept which plant pathologists could applyto other crops and diseases. Figure 4 shows that

Figure 4 Effect of take-all on the yield of wheat in rela­tion to potential yield, based on April-October rainfall, atKapunda, South Australia, in 1983 and 1987

Figure 3 Levels of rye grass and barley grass in peas andmedic sown with cultivation and by direct drilling and innaturally regenerated annual pasture at Avon, SouthAustralia. in 1978.

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Australasian Plant Pathology Vol. 19 (4) 1990 103

Figure 5 Effect of cultivation and rotation on root damagecaused by Rhizoctonia and on the area of patches in wheatat Avon, South Australia in 1983.

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this pathogen and account for this disease being aserious problem in direct drilled crops in southernAustralia (Rovira 1986).

Effects of cultivation, rotation and autumn chemicalfallow The practice of herbicides replacing theplough improves soil properties and prevents deg­radation; it is a necessary step to build up organicmatter in the move towards sustainable croppingsystems (Rovira 1990; Rovira et al. 1990). However,it became apparent from research and reports fromfarmers in Western Australia, South Australia, Vic­toria and southern New South Wales that Rhizoc­tonia bare patch was becoming a serious problemassociated with direct drilling in soils and districtswhere it had not previously caused obvious damage.

Research at Avon (Rovira 1986) demonstratedthat the damage to roots by Rhizoctonia and the con­sequent areas of crop lost as patches of poor growthwere greater in direct drilled wheat than in wheatsown following cultivation (Figure 5). There was noeffect of rotation on the damage to roots, but rota­tion affected the areas lost to patches; this area waslowest in wheat following medic pasture and peas.The higher available nitrogen in the soil followinggrass-free medic pasture and peas than followingwheat or grassy pasture enabled the plants to bet­ter tolerate root damage which is consistent with thereport by MacNish (1985) that fertiliser nitrogenreduced Rhizoctonia bare patch.

The practice originally recommended for directdrilling of cereals in southern Australia was basedon the 'Graze, Spray, Seed' message, wherebyfarmers used the growth of their newly germinatedautumn pastures as feed for sheep until it was timeto sow the crop. This early pasture is normallydominated by barley grass with 5000 to 10000 seed­lings/m2 and we have found that over 50% of rootsof these plants are infected by Rhizoctonia and the

Rhizoctonla bare patch

Rhizoctonia root rot or bare patch, which is a severedisease of cereals in many calcareous sandy soilsof South Australia (Samuel and Garrett 1932), hasa wide host range making it impossible to controlby rotation. The strong competitive saprophyticability of Rhizoctonia in soil allows it to colonise par­ticulate organic matter; Neate (1987) found thatpropagules of Rhizoctonia were some four timesmore abundant in the 0-5 cm layer of field soil thanin the 5-10 cm layer. Farming practices which con­serve organic matter on or near the surface favour

The visit by Dr Jim Cook, from the USDAIARSCereal Root Diseases Laboratory, WashingtonState, for eight months in 1974 working on biologi­cal control of take-all, acted as a great stimulus tomy interest in the topic. This led to our hypothesisthat the fluorescent pseudomonads were a majorfactor in making long-term wheat soils suppressiveto take-all (Cook and Rovira 1976). This publicationhelped to create the interest in pseudomonads asbiocontrol agents and was followed by a review inwhich I presented an optimistic but realisticapproach to the manipulation of the rhizospheremicroflora to increase plant production (Rovira1985).

This research on biological control of take-all wasexpanded when Dr Graham Wildermuth undertookhis PhD at the Waite Institute with Dr Jack Warcupand me. Amongst other things, this research led usto postulate a mechanism for the suppression oftake-all in the rhizosphere associated with thephenomenon of take-all decline (Wildermuth et sl,1979; Rovira and Wildermuth 1981) in which pseudo­monads with suppressive activity build up on andin the lesions on roots caused by the take-all fungus.Wildermuth demonstrated that, in suppressive soils,hyphae of the take-all fungus are colonised by bac­teria as the hyphae grow from propagules towardsroots.

The visits in 1983 by Drs Jennifer Parke and DavidWeller from the USA as post-doctoral fellows furtherdeveloped our expertise in biological control of take­all and led to the appointment of Dr Maarten Ryderto apply modern biotechnological methods to thisarea of research with support from Monsanto Aust­ralia Ltd.

Of course, the ultimate test for biological controlof root diseases will be in the field with its wide rangeof edaphic and climatic conditions and, while thetechnique offers some exciting possibilities, thereare many hurdles to overcome before we can offerfarmers biological control as a method of controll­ing take-all. At this stage, we still lack knowledgeon many of the processes involved in biological con­trol in field soils and, hence, we can neither predictwhen it will succeed nor why it has succeeded orfailed in different seasons and different soils.

104 Australasian Plant Pathology Vol. 19 (4) 1990

Table 1 Effect of chemical fallow on Rhizoctonia damage to wheat roots (8 weeks after sowing) andon grain yield with two tillage systems and two rotations

Tillage Rotation in year Rhizoctonia rootpreceding wheat rot rating A

(0-5)

Grain yield(t/ha)

+B +B

Cultivated Peas 0.4 0.4 1.9 2.4Volunteer pasture 1.0 1.0 2.6 2.6

Direct drill Peas 2.7 1.7 2.2 3.0Volunteer pasture 2.9 2.1 1.2 2.2

LSD (P=0.05) Tillage 0.8 nsChemical fallow 0.7 0.7Rotation ns ns

'6

Number o! propagules<.g o~ so:

lished that these patches were due to R. solani AG-8(Neate and Warcup 1985), I decided that it wouldbe necessary to develop methods by which differ­ent levels of disease could be obtained in both glass­house and field experiments. McDonald and Rovira(1985) introduced propagules of Rhizoctonia grownon sterilised white millet seed at different rates intocalcareous sandy loam, incubated for O. 2 and 4weeks and grew wheat for 3 weeks at 1Q°C with8-hour days in a controlled environment cabinet.Figure 6 demonstrates that as the number ofpropagules in the soil was increased, less incubationtime was needed to produce the same level of rootdamage.

A problem confronting research on Rhizoctoniabare patch In the field is the uneven distribution andunpredictable location of patches. I thought that oneway of overcoming this problem would be to extendinto the field the technique Heather McDonald andI developed for pot experiments. Millet seed propa­gules were broadcast over the soil surface at 300and 1200 propaqules/rrr'. incorporated into the top

Figure 6 Effect of Incubation time and levels of artificialRhizoctonia propagules on damage to roots of wheat seed­lings grown in a calcareous sandy loam In a potexperiment.

Interaction between the herbicide chlorsu/furon andRhizoctonia Research. both overseas and in Aus­tralia. has demonstrated that there can be a 'down­side' to the use of certain herbicides in the presenceof soilborne root diseases caused by fungi andnematodes (Altman and Rovira 1989). We found inboth farmers' crops and in glasshouse trials that thepresence of extremely low residues of the sulfonylurea herbicide chlorsulfuron (Glean) led to increaseddamage from Rhizoctonia to cereal roots resultingin yield losses of up to 1 tlha (Rovira and McDonald1986). The interactions between herbicides and rootdiseases give cause for concern because of thewidespread use of herbicides. but awareness ofthese effects should encourage cereal growers toplant crops into soils in which disease levels havebeen controlled by management strategies. e.g.rotations for take-all and cereal cyst nematode.rotation and tillage for Rhizoctonia.

Development of artificial inoculation techniques forRhizoctonia research One of the major stumblingblocks in research on Rhizoctonia bare patch hadbeen an inability to reproduce the 'patch' symptomseither in the field or in the laboratory using soil frompatches. When the bare patches appeared in ourdirect drilled wheat plots in 1981 and we had estab-

ARhizoctonia root rot rating: a = no disease. 5 = maximum diseaseB + = chemical fallow (sprayed 32 days before seeding)

- = no chemical fallow

take-all fungus. Such root material provides highinoculum sources in the soil into which wheat is tobe direct drilled. Roget et al. (1987) developed theconcept of a short chemical fallow in which thispasture is killed with herbicide some weeks beforesowing to reduce inoculum levels: the impact ofchemical fallowing on root damage caused byRhizoctonia and grain yield is shown in Table 1.

MacNish (personal communication) has not beenable to reproduce the benefits of chemical fallow­ing and, hence. further research is required todetermine the effects of density of grass plants. theeffects of grass species. and the length of autumnchemical fallowing on Rhizoctonia damage.

Australasian Plant Pathology Vol. 19 (4) 1990 105

Table 2 Effect of Rhizoctonia inoculum on root damage rating and wheat yield at Avon in 1984

Number of Disease Number of Dry weight/plantB Number of Grain yield?propagules/m2 ratingAB plantsB/m (mg) heads/plant (Uha)

o 1.6 31 52 1.55 2.11300 2.2 33 50 1.50 1.92

1200 3.1 30 41 1.36 1.74LSD (P= 0.05) 0.6 ns 8 ns 0.23

Note: Millet seed propagules of R. so/ani (McDonald and Rovira 1985) were broadcast over the soil surfaceand rotovated through the top 10 cm; wheat was direct drilled into the soil with the SIRQDRILL 4 weeks later.A Disease rating for Rhizoctonia damage: 0 = no damage, 5 = maximum damageB 8 weeks after plantingc At maturity

5 cm with a rotary hoe, allowed to stand for 4 weeksbefore direct drilling wheat with the SIRODRILL.Table 2 shows that although there was a significantbackground of Rhizoctonia in this soil, the dis­ease level could be increased and the grain yielddecreased by this method. The disease level on theroots was uniform over the whole of the treated area,which indicated that this technique may reduce theproblem of variation in disease levels and patchesin the field. Neate (1989) has used this techniquein field trials to screen cereal cultivars for resistanceto Rhizoctonia: he has also used the technique toscreen fungicides as control agents (Neate, personalcommunication). We have used the technique todemonstrate the effects of Rhizoctonia on medic.ryegrass, peas and oats (Rovira and Neate, unpub­lished).

Cereal cyst nematode

In their review on the impact of CCN on wheatproduction and the development of integrated con­trol methods, Rovira and Simon (1982) discussedvarious control strategies e.g. chemical control, timeof sowing, cultivation and rotation with non-hostsand resistant cultivars, and their effects on cropyields. The effects of rotation on CCN were demon­strated by Meagher and Rooney (1966) andMeagher and Brown (1974).

Effectof soil temperature on CCNdamage The rela­tionship between CCN levels in soil and damage tocrops is confounded by a number of factors includ­ing time of sowing. Many farmers know that they canovercome a CCN problem by sowing in early to mid­May. Simulating different seasonal conditions in con­trolled environment cabinets I demonstrated how asoil with 10 eggs/g could give root damage ratingsover the range 1 to 4 depending upon temperaturesbefore and after planting. Table 3 and Figure 7demonstrate that with a simulated early 'break' andearly sowing (soil kept moist at 20°C before sowingand at 15°C for 4 weeks after sowing) ten eggs/greduced total root length by 14%, whereas with the

106

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i

Figure 7 Effect of soil temperatures before and after plant­ing on damage to wheat roots by CCN. Treatments - Left:Soil kept moist at 20°C for 4 weeks before planting. growthat 15°C for 4 weeks. Centre: Soil kept moist at 15°C for2 weeks before planting. growth at 15°C for 4 weeks. Right:Soil kept moist at 15°C for 4 weeks before planting, growthat 10°C for 4 weeks.

simulated late break (soil kept moist at 15° beforesowing) with late sowing, viz. equivalent to earlyJune (soil kept at 10°C for 4 weeks after sowing)the reduction in root growth by CCN was 81%. Inthese experiments, the direct effect of soil temper­ature on root growth in the absence of CCN damageto the roots was assessed from pots to which anematicide (aldicarb) was added at sowing.

S/RONEM soil bioassay for CCN When we com­menced our research on CCN the standard methodsin Australia of estimating eelworm levels in soil, ordamage on plants was to count either the numbersof eggs in sailor the numbers of 'white cysts'(immature females) on root systems at anthesis.This has its limitations, e.g. specialist knowledgeand equipment is required for egg counts in soiland as a routine procedure egg counting is tediousand time-consuming, whilst counting 'white cysts'reflects the population build up on roots rather thanactual damage to roots by CCN. For this reason we

Australasian Plant Pathology Vol. 19 (4) 1990

Table 3 Effect of soil temperatures before and after planting on damage to wheat roots byHeterodera avenae

Soil temperature after wetting and15Abefore planting (0C) 20A 15A

Time soil was wetbefore planting (weeks) 4 2 4

Soil temperatureafter planting (0C) 15B 15B 10B

Total length of primaryNSroot axes/plant (em) 37(46)C 26(38) 27(57)

Total length of lateralroots/plant (em) 582(671) 417(815) 112(403) 153

Root damage ratinq? 1.0 2.5 4.0 0.5

Dry weight tops/plant (mg) 59(86) 52(77) 29(45) 13

A Equivalent seasonal conditions:120°C for 4 weeks, 15°C after planting - early break in season (rains), early seeding:15°C for 2 weeks, 15°C after planting - mid break, early seeding;15°C for 4 weeks, 10°C after planting - late break, late seeding.

B Plants grown at indicated temperature for 4 weeks.C Figures in brackets obtained when aldicarb at 10 mg/kg was mixed through soil before planting tocontrol H. avenae.° Root damage rating: 0 = no damage, 5 = maximum damage.

Table 4 Control of Heterodera avenae with in-furrow applications of low rates of Counter (terbufos),Temik (aldicarb), Vydate (oxamyl), Furadan (carbofuran), Benlate (benomyl) and Nemadi (liquidethylene dibromide) at Calomba, South Australia, in 1980

Chemical Rate Disease ratingA

(kg a.i.lha) on rootsNo. of

'white cysts"Grain yield

(t/ha)

Nil 4.4 85Temik GC 2.0 0.5** 11 * *Counter G 0.4 2.9* * 45* *Counter G 0.6 1.9* * 27* *Vydate SD 0.125 2.3* * 59* *Vydate SD 0.250 1.1* * 37**Furadan SD 0.5 2.2* * 45* *Nemadi L 7.4 2.6* * 34* *Benlate P 0.5 2.6* * 48* *

A Disease rating: 0 no damage, 5 maximum damageB 'white cysts' = immature femalesC G = granules, L = liquid, SD = seed dressing, P = powder.", * * Significantly different from 'Nil' treatment at P = 0.05 and 0.01, respectively.

0.962.00**1.33**1.46* *1.34* *1.61* *1.43* *1.46* *1.25*

developed the root damage rating scale of 0 to 5 forseedlings between 6 to 8 weeks after sowing; thisrating system has since been used as part of thesoil bioassay technique (Simon 1980).An essentialelement of this bioassay was to incubate the soil ata low temperature (15°C) to promote hatching(Banyer and Fisher 1971) and then grow plants ata low temperature (10°C) to exacerbate rootdamage. This SIRONEM bioassay is available as acommercial soil test and is used by many farmers

Australasian Plant Pathology Vol. 19 (4) 1990

and advisers. One disadvantage of the SIRONEMbioassay is that a rating of 5 is caused by 20 or moreeggs/g, so that in soils with very high CCN popula­tions the application of strategies to reduce CCN,e.g. rotation or resistant cereals, may not be reflec­ted immediately by a fall in the rating despite adecline in egg levels. Nevertheless, the bioassay isbeing used by many farmers to monitor trends inCCN levels in individual paddocks as part of theirstrategy to control this disease.

107

SE. I

Galleon WeeahWheal

Red loam

CCN root damage (0-5) to wheat2.0 38 1.0 29

+ + + +

:ft..

Galleon WeeahWheal

Grey loam

2.5r-----==--------------,

05

til 2.0s:2-'0 1.5Qi';;'

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o1983 Crops:1986 Crop:Soil Type:

demonstrate how soil type affects both the damageof CCN to roots and yield loss from CCN.

Figure 8 Effect of CCN resistant barley (cv. Galleon) andCCN susceptible barley (cv. Weeah) on the CCN damageto roots and yield of wheat grown in the same field threeyears later. The field had two soil types, viz. friable greyloam and a hard setting red loam in a mallee environment.

Effect of tillage on CCN Unexpected benefits ofdirect drilling which we have found in our trialsinclude the reduction of root damage by CCN, thelower numbers of white females (and hence lesscarryover of cysts and eggs into following seasons)and increased yields with direct drilled wheat com­pared with wheat sown following cultivation (Table6). Subsequent field experiments havedemonstratedthat the higher damage from CCN with cultivationis probably due to the mixing and spreading of cystsand hatched nematodes during cultivation and/orthe lower soil bulk density facilitating the movementof nematodes towards the roots.

This result has been confirmed in a number oftrials conducted at different locations and also intillage trials conducted at Roseworthy College andby the South Australian Department of Agriculture.

Chemical control of CCN Following the success­ful demonstration by Brown (1973) that low rates ofaldicarb (Temik) applied in furrow at sowing con­trolled CCN and increased yields, we worked withUnion Carbide, ICI, DuPont, Cyanamid and AgChemto test a range of chemicals for the economic con­trol of CCN. Some of the results obtained over threeyears are reported in Table 4.

Effect of CCN resistant cereals on the carry-over ofCCN Chemical control of CCN demonstrated thelarge yield losses caused by CCN in Victoria andSouth Australia and promoted the interest of cerealbreeders in developing resistant cultivars. When wewere conducting trials on chemical control of CCNthe only commercial cereals known to be resistantwere Avon and Swan oats (Cook 1974; O'Brien andFisher 1974) and Festiguay wheat (McLeod 1976).

McLeod's report on Festiguay went unnoticeduntil this cultivar was grown around the perimetersof wheat fields in South Australia because of itsresistance to stem rust following the rust epidemicof 1973 which severely affected local wheat culti­vars. Mr Trevor Dillon of the South AustralianDepartment of Agriculture and I observed that infields where Festiguay had been grown around theperimeters, with other wheat cultivars in the centresand then 2 or 3 years later a CCN susceptible culti­var grown over the whole field, there was less CCNdamage and better growth of wheat where the Fes­tiguay had been grown. The results from two suchfields are presented in Table 5.

The breeding by Sparrow, Fisher and Dube at theWaite Agricultural Research Institute and South Aus­tralian Department of Agriculture of the resistantbarley, Galleon, has been of tremendous value incontrolling CCN in South Australia. This is illustratedin Figure 8 from a field sown in two halves in 1983with two barley cultivars, Weeah which is CCN sus­ceptible and Galleon which is resistant; after annualpasture in 1984 and 1985, the field was sown to asingle cultivar of susceptible wheat. The results also

Table 5 Yields of wheat in 1978 in fields in which wheats resistant and susceptible to Heteroderaavenae were grown on Yorke Peninsula, South Australia in 1975

Farm Wheat cultivar Wheat cultivargrown in 1975 grown in 1978

Number of Grain yield'white cysts' in 1978

per plant in 1978 (t/ha)

2

Sabre (S)A Halberd 58 2.0Festiguay (R) Halberd 2B 3.1cAldirk (S) Kite NAD 1.7Festiguay (R) Kite NA 2.3c

Note: In the two years between wheat crops the fields had grass-Medicago spp. annual pasture.A S = susceptible to Heterodera avenae

R = resistant to Heterodera avenae and grown around the perimeter of the fieldB Reduced number of cysts significantly different (P=0.001)c Increased yield significantly different (P =0.01)D NA = not assessed

108 Australasian Plant Pathology Vol. 19 (4) 1990

Acknowledgements

I thank my colleagues in CSIRO who have worked

Table 6 Effects of cultivation on the damage towheat roots by CCN, on the numbers of 'white cysts'at anthesis and on grain yield at Calomba, SouthAustralia, in 1980

Concluding remarks

I have presented results which indicate the philos­ophy which has driven my proqrarn, viz. to conductsound science with a strong field component so thatthere are practical outcomes from the research aswell as good scientific publications. I believe that thesuccess of this program has been demonstrated bythe widespread adoption by farmers of many of thetechniques which reduce root diseases and increaseyields. The publications quoted in this paperrepresent some of our scientific publications in thisfield indicating that it is possible to marry the twogoals which I set out to achieve in 1975.

Biological control of CCN In England Dr Brian Kerry(1980) demonstrated that biological control of CCNby nematophagous fungi was responsible for thedecline of CCN in fields which had been croppedwith wheat for several consecutive years. This ledto similar studies in Australia by Kerry while with theCSIRO Division of Soils Laboratory in Adelaide ona Reserve Bank Fellowship. Sterling and Kerry(1983) demonstrated that Verticillium chlamydo­sporium, which parasitises CCN females and eggs,was widespread but less numerous than in Englishsoils. In an attempt to manipulate the population ofVerticillium in the rhizosphere of wheat as a biocon­trol agent for CCN, Kerry et a/. (1984) demonstratedthat introduction of the fungus into soil reduced thenumbers of CCN by up to 80%. Different isolatesof Verticillium colonised wheat roots to differentextents, indicating the potential for selecting isolateswith greater rhizosphere competence which couldimprove biocontrol activity. One problem with bio­control of CCN in Australia is that the populationthreshold for economic losses from CCN is 2-5eggs/g soil compared with 10 eggs/g soil in England.Hence, the biological control agent has to performmore efficiently under Australian conditions if it isto reduce yield losses from CCN.

so hard to make the program succeed; their namesappear in papers quoted in the publication list. I alsothank the many farmers for their input into theprogram. Collaboration from colleagues in the StateDepartments of Agriculture and from Industry hashelped the program enormously. Finally, I amgrateful for the support from the Wheat ResearchCouncil and the SA Wheat and Barley ResearchCommittees which has enabled us to conduct thelong-term field experiments so essential for a pro­gram of this type.

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