Infection of Seed & Transmission of

Seed Borne PathogensLindsey du Toit

WSU Mount Vernon NWREC

Seed borne vs. seed transmittedSeed borne microorganisms:

- saprophytic- pathogenic- opportunistic

Seed borne microorganisms:- fungi- bacteria- viruses- nematodes

Classes of seed borne microorganisms

1.

Infected seed = primary inoculum source. If seed infection is controlled,

the disease is controlled2.

Important pathogen, but infected seed = minor source of inoculum

3.

Seedborne

microorganisms never demonstrated to cause disease

4.

Pathogens that infect seed in fields or in storage, & reduce seed quality

Attributes of seed transmitted organisms

•

Ability to gain access

to seed•

Ability to survive

commercial processes

–

Harvest, cleaning, treatment, storage•

Ability to establish on emerging seedlings–

Primary or secondary transmission

Biological factors that affect these attributes

•

Infection method(s)•

Timing of infection of seed crop

•

Nature of pathogen–

biotroph, necrotroph

•

Survival structures or states–

nematodes –

cryptobiosis, cysts

–

fungi and Chromistans

-

sclerotia, microsclerotia, oospores, chlamydospores

Ability to gain access to seed•

Active process of infection- within seed

•

Passive access–

present on seed surface

–

contaminant in seed lot

•

Affects ability to clean or treat seed

(from Maude, 1996)

Routes of active seed infectionstigmastyle

pollen tube

ovary wall(pericarp)ovule (seed)nucellusegg sac

testa(seed coat)

egg cell(embryo)micropylevascular tracefunicle(funicular scar = hilum)

A. Penetration through ovary wall

Routes of active seed infection

E.g.: Cladosporium

variabile (spinach), Botrytis spp. (onion)

C. Penetration through floral parts

E.g.: Ustilago nuda (grains)Cucumber mosaic virus

B. Systemic infection via vascular system

E.g.: Vascular wilt fungi,endophytesFrom Maude (1996)

Direct systemic infection via vascular system• Many viruses, e.g.:

• PSbMV

on pea• LMV on lettuce• AMV in alfalfa• PEBV in pea

• Some fungi, e.g.:•

Vascular wilts (Verticillium dahliae, Fusarium oxysporum)

• some downy mildews• Few bacteria, e.g.:

• X. campestris pv. campestris

• Direct connection between embryonic& endospermic tissue becomes disconnected as seed develops

• Potential for transmission affected bydegree of internal infection

Direct systemic infection of seedNeotyphodium coenophialum in seed next to the

embryo. Systemic infection of plants. Whole life cycle occurs inside tall fescue (mutualism).

K. Gwinn, UT

Seed borne Verticillium

dahliae in spinach

Non-inoculated control Inoculated with V. dahliae

Systemic infection from roots or seeddu Toit et al., 2005. Plant Dis. 89:4-11

Indirect systemic infection via stigma to embryo

• Pathogen moved from infected plants toflowers

• May follow pollen pathway to embryo sac• Examples:

•

pollen borne viruses –

LMV, CMV (some hosts)

• infected pollen may be less viable(poor fertilization)

• Nepoviruses• Loose smut fungi (Ustilago tritici) • Ergot fungi, not seed transmitted but

ergots can be mixed with seed• Lower rate of pollen vs. ovule transmission

(LMV)

Pollen tube entry into seedExamples:1.

loose smut of small grains

2.

CMV in spinach

Certification programs,host resistance,systemic fungicides

Indirect infection via flower or fruit• Weak necrotrophs:

•

Botrytis cinerea - infected petals remain attached to developing fruit

• Aggressive necrotrophs:• Attack floral parts directly•

e.g., Ascochyta pisi, Alternaria brassicicola

peduncle fruit coat(ovary) dorsal suture funicle*

seed coat

• Fleshy fruits (e.g., Solanaceae) –

seed attached to central placenta-

infect via calyx -

placenta –

funicle

–

embryo• Umbelliferae

& Liliaceae

–

flowers exposed in umbels• Seed transmission is typically discontinuous (infection outside

embryo)-

affected by intrinsic & environmental conditions

Brassica

seed

Alternaria brassicicola & A. brassicaeCorrelation of severity of pod spot with:

Seed germ = -0.89, Seed rot = +0.88du Toit & Derie, 2003. Fung. & Nem. Tests 58:V026

Leptosphaeria maculans(black leg)

Seed borne Stemphylium botryosum & Cladosporium variabile in spinach

Hernandez-Perez & du Toit. 2006. Plant Disease 90:137-145du Toit & Hernandez-Perez . 2005. Plant Disease 89:1305-1312

Botrytis scape

& flower blight of onion

B. aclada/B.alliiB. squamosaB. byssoidea

S.K. Mohan

Seed borneBotrytis aclada

& B. allii in onion

Bacterial leaf blight of carrot

Xanthomonas campestris pv. carotae

Pathogens use multiple points of access?

Acidovorax avenae subsp. citrullibacterial fruit blotch of watermelon

Possible mechanisms of seed infestation by A. avenae subsp. citrulliA. Penetration through ovary wall

C. Penetration through floral partsB. Systemic infection via

vascular system

??No evidence

Role of female watermelon blossoms in Acidovorax avenae subsp. citrulli seed infection

Walcott et al. Phytopathology 93:528-534

Seed borne pathogens:Implications for risk analysis

• Epidemiology of the pathogen- Potential for seed transmission- Conditions for disease development & spread- Ease of eradication

• In-field control measures- Resistant cultivars- Fungicides & forecasting- Crop rotation- Plant spacing, row orientation- Irrigation (system & schedule)

• Threshold(s) for seedborne

inoculum- Environmental conditions?- Resistant vs. susceptible cultivars?

• Alternative sources of inoculum-

Infested residues-

Soilborne

inoculum-

Infected adjacent or overwintering crops or weeds

Potential for seed transmission• Environmental factors

- temperature*- moisture*- light- pH- soil microflora- degree of internal infection of seed

• Genotype- host resistance, maternal vs. paternal- pathogen isolate/strain

• Inoculum

potential- amount of inoculum

& conduciveness of environment

- transmission rate- threshold(s) for seedborne

inoculum

- complex, lack of adequate field research

Seed borne inoculum

thresholdsXanthomonas campestris pv.

campestris• 1 in 30,000 seed•

Increases asymptomatically

before epidemic in field

Cylindrocladium parasiticum in peanuts

•

1 in 400 seed = VA threshold

Significance of seed borne carrot pathogensUmesh

et al., 1998. Plant Dis. 82:1271-1275. Bacterial blight in central CA.Threshold of ~104-105

CFU/g seed in semi-arid central CA.

Seed borne pathogens: ImplicationsAlternative inoculum

sources for seed borne carrot diseases

- Infested residues (Alternaria dauci in CA) (Gilbertson et al.)- Soil borne inoculum

(Alternaria radicina = 8 years)- Infected

adjacent or overwintering crops or related weed hosts(X. campestris pv. carotae in carrot seed crops in PNW)

du Toit et al., 2005. Plant Dis. 89:896-907.

Mean % seed transmission(% of infected seed planted)

Fungus Trial 1 (RH ~95%) Trial 2 (RH ~75%)C. variabile 5.1 (18.1) 0.1 (0.4)S. botryosum 9.1 (10.3) 3.3 (3.7)

Influence of relative humidity on

transmission of leaf spot fungi from spinach seed

Alternative sources of inoculum

for S. botryosum in spinach seed crops

Pleospora herbarum on surface spinach stem residuesdu Toit & Derie, 2003. Phytopathology

93:S22

0

10

20

30

40

50

60

1 10 20

1973 (dry summer)1974 (wet summer)

% Seed infected with B. allii

% B

ulbs

wit

h ne

ck r

ot

Relationship between infected seed & post-harvest neck rot in the UK

(from Maude & Presly, 1977b)Tichelaar, 1967;Maude & Presly,

1977aEllerbrock

& Lorbeer

(1977): Field infection sources were more important in NY

than infected seed.

Influence of environment on seed transmission ofBotrytis aclada/B. allii in onion

0

10

20

30

40

50

60

1973 1974 1975 1977 1978 1979

Treated with benomylor benomyl + thiramNon-treated

Control of neck rot in stored onion bulbs in the UK using seed treatments(from Maude, 1983. Seed Sci. & Technol. 11:829-834)

% B

ulbs

wit

h ne

ck r

ot in

sto

rage

Influence of environment on seed transmissionof Botrytis aclada/B. allii in onion:

2004 trials in WA & NY

Lot

Seedborne

Botrytis (%)

Stand count/

2 m bed(6 May)

% Plants infected with Botrytis spp.

(latent)% Neck rot

(Feb/Mar 2005)SeedCo.

WSUlab*

WA(19 Jul)

NY(20 Jul)

WA(7 Feb)

NY(15 Mar)

A 0.0 6.0 a 105 a 96.5 a 73.3 a 3.7 a 9.1 aB 14.6 16.3 b 92 a 96.6 a 74.1 a 4.0 a 0.0 aC 20.6 33.6 c 93 a 100.0 a 54.6 a 3.7 a 16.8 aD 1.9 55.3 d 92 a 98.8 a 72.6 a 2.8 a 13.5 aE 41.1 61.3 d 90 a 98.8 a 69.5 a 3.1 a 21.8 a

* = % surface-contamination detected at WA State Univ. (% ‘internal’ infection detected for lots A, B, C, D, & E

=

0, 2, 3, 7, & 6%,

respectively).

Similar results in WA in 2002 & 2003

Alternative sources of inoculum

of Botrytis for infection of onion bulb & seed crops

• Physical -

kill pathogens, not seed

• Chemical–

protect seed/seedling against pathogens

• Biological–

protect seed/seedling against pathogens

-

induce systemic resistance-

improve plant growth

Management of seed borne pathogens: Seed treatments

- Hot water- Chlorine- Aerated steam- Hot, dry air- Fungicides

-

conventional, organic, biological-

Efficacy, potential phytotoxicity:

- infected vs. infested seed- volume of seed treated- parameters- pathogens, cultivars, seed quality- drying seed- shelf-life

Management of seed borne pathogens: Seed treatments

Hot water seed treatment for spinach leaf spot

0

10

20

30

40

Inci

denc

e (%

) of s

eed

with

Cla

dosp

oriu

m v

aria

bile

0

1

2

3

0102030405060708090

100

Seed

ger

min

atio

n (%

)

40 C 45 C50 C55 C60 C

0

10

20

30

40

50

0

1

2

3

4

5

6

7

0 10 20 30 40

Duration (min)

Inci

denc

e (%

) of s

eed

with

Ste

mph

yliu

m

botr

yosu

m

0

10

20

30

40

50

60

70

80

0 10 20 30 40

Duration (min)

Trial 1 Trial 2

(du Toit & Hernandez-

Perez. 2005. Plant Disease 89:1305-1312)

0

10

20

3040

50

60

70

80

90

100

Cont

rol

Sere

nade

ASO

Mer

tect

340F

Tops

in-M

70W

PDy

nast

y 10

0FS

Thira

m42

-SM

axim

4FS

Pris

tine

XCF

Rovr

al4F

Natu

ral I

I

Trial 1Trial 2

Evaluation of fungicide seed treatments for control of seed borne Stemphylium botryosum

(du Toit et al., 2007. Plant Dis. Mgmt Reports 1:ST003)

% S

eedb

orne

S. b

otry

osum

a a

a

bb b

a

cd c d

w w w w

x

y

x

z

z

z

Exp #1

Exp #2Kodiak

Micro 108

Mycostop Mix

Natural II

Natural X

PGPR Galaxy

Subtilex

T-22 Plntrbox

Yield ShieldApron

NonTrt

Mertect

Per

cent

age

Ver

ticill

ium

0

10

20

30

40

50

60

Evaluation of organic seed treatments for spinachEvaluation of organic seed treatments for spinach (Cummings, 2007)

= Significantly less seed borne Verticillium than non-treated seed

Cmpst TExp #1

Exp #2Kodiak

Micro108

Mycostop

Natural II

Natural XPGPR

Prestop

Soilgard

SubtilexT-22

YldShldApron

Ntrt Inoc

Ntrt NonInoc

AU

DP

Cto

tal

0

500

1000

1500

2000

2500

3000

Greenhouse evaluation of organic seed/drench Greenhouse evaluation of organic seed/drench treatments for SOIL BORNE treatments for SOIL BORNE PythiumPythium ultimumultimum

(Cummings, 2007)

= Significantly less disease than non-treated seed= Significantly more disease than non-treated seed

Challenges presented by seed borne pathogens

•

Determine

definitively

if a seed borne pathogen is seed borne & seed transmitted

•

Develop thresholds

for seed borne inoculum, applicable to a range of environments

•

Regulatory implications

for seed transmitted vs. seed borne status

•

Management

of diversity of seed borne pathogens under diverse production environments

Thank you!