Clubroot of Canola: Overview of

an Emerging Problem

Stephen Strelkov

2011 Manitoba Agronomists Conference

13th Dec. 2011, Winnipeg MB

Department of Agricultural, Food and Nutritional Science

410 Agriculture/Forestry Centre

University of Alberta

Edmonton AB

T6G 2P5

Outline of Presentation

• Introduction to clubroot

• Current disease situation

• Pathogen dispersal mechanisms

• Clubroot resistant canola and resistance

stewardship

• Integrated management of clubroot

• Conclusions

Clubroot

• Pathogen:

– Plasmodiophora brassicae

• Hosts:

– Canola, mustard, cruciferous vegetables and weeds

• Soilborne pathogen:

– Long-lived resting spores

• Occurrence:

– Traditionally BC & eastern Canada

– Cruciferous vegetables

Symptoms

• Below ground:

– Root galls (club-shaped

swellings)

– White at first, turn

grayish-brown to dark

brown

• Above ground:

– Stunting, wilting,

yellowing, shriveled

seed

Healthy Plants

Clubroot Patch

Disease Cycle

S.E. Strelkov

Resting Spores in Host Roots

J.P. Tewari

Clubroot on Canola

in Alberta

• Discovery of clubroot

in 2003 was a cause for

concern

• 12 fields near

Edmonton, AB

Strelkov et al. (2005)

Britannica Encycl.

Clubroot Situation

(Fall 2011)

• 831fields with confirmed P. brassicae infestations

• Mostly in central Alberta

– Few cases in southern Alberta and Saskatchewan

– A few infected plants in experimental plots in Elm Creek, MB (2005)

Strelkov et al.

Characteristics of Infested Fields

• pH from 4.8 to 7.6

(average = 6.2)

– Significant negative

correlation between

severity and soil pH

• Most heavily infested

fields generally in

– canola-cereal-canola-

cereal rotation

– canola-canola rotation

Strelkov et al. 2007

Clubroot Spread

• Principal

mechanism of

spread is on

machinery

• Other dispersal

mechanisms

have also been

implicated

8 7

6

5 4 3

2

9

1 Field Entrance

150 m

150 m

150 m

150 m

0.901

0.479

0.225

0.155 0.169

0.296

0.394

0.324 0.310

Cao et al. 2009

Seedborne Dispersal

• Clubroot cannot directly infect seeds or

potato tubers

• Can only occur as an external contaminant

• Developed and validated a qPCR-based

protocol to quantify inoculum loads on

seeds/tubers

– Combined with Evan’s blue viability staining

– Greenhouse bioassays

Seedborne Dispersal

• Assessed 46 seed or tuber lots harvested

from regions in AB where clubroot is

prevalent

• Quantifiable levels of infestation on:

– 6 of 16 non-cleaned samples

– 1 of 30 commercially cleaned samples

Resting Spore Loads

Spore loads as determined by qPCR on samples testing positive by conventional PCR

(Rennie et al. 2011)

Crop Spore Load

per 10 g Seed

(qPCR)

Viability

(Evan’s Blue

Staining)

Commercially

Cleaned?

Wheat 3.43 × 104 80% No

Canola 4.04 × 103 90% No

Pea <1,000 98% Yes

Potato 1.40 × 104 90% No

Pea (× 3) <1,000 97 – 100% No

Seedborne Dispersal

• Levels of infestation in some non-cleaned samples greater

than that required to cause clubroot in greenhouse

bioassays

– Seedborne dissemination could serve as secondary

mechanism of spread

• Seed cleaning seemed to be effective in reducing the risk

• Common seed treatments also effective in reducing the risk

Farmers should avoid planting of common, untreated seeds

harvested from clubroot-infested fields

Dispersal in Dust & Water

• Clubroot dispersal in

dust and water may

also occur

– Extent of problem not

well defined

• Epidemiological

studies to track and

quantify spread

Conventional PCR

M C Dust

Rennie et al.

Research Sites

BSNE (Dust) Samplers

105cm - 5

80cm - 4

60cm - 3

35cm - 2

10cm - 1

Sampling

Commercial Fields Research Plots

Wind direction

E

A

B

C

D

E

Sampler Conventional Detection Quantification

A No Resting Spores No Resting Spores

B No Resting Spores No Resting Spores

C Resting Spores Detected Marginal in C1

D Resting Spores Detected Highest in D5

E Resting Spores Detected Highest in E5

Wind direction

Management of Clubroot

• Initial focus was on exclusion of the

pathogen and long rotations out of

susceptible crops

• Work underway to evaluate efficacy of

fungicides, soil amendments and biological

control agents

• Intensive resistance-breeding efforts

– Public institutions and private industry

Genetic Resistance

• Widespread release of resistant cultivars in 2010

• All have good resistance to predominant pathotypes

• Represent most important tool for clubroot management

Resistance Stewardship

• Resistance will have to be well-managed:

– Pathogen populations can adapt in response to

selection pressure

Continuous cropping of a

resistance source

Variability in Virulence

Pathotype 3

(90%)

Pathotype 5

(3%)

Pathotype 2

(7%)

Pathotype 3

(72%)

Pathotype 6

(7%)

Pathotype 8

(14%)

Pathotype 2

(7%)

“Field Populations” Single-Spore Isolates

Classification on the differentials of Williams (1966)

Howard et al. 2010

Pathogen Cycling Experiment

• Objective: To assess the effect of multiple

infection cycles on the virulence of P.

brassicae

• Methodology:

– Population and single-spore isolate representing

pathotype 3

– Cycled 5× on a selection of R, MR and S host

genotypes

Methodology

Inoculate

with spores

6 weeks

Rate disease &

harvest spores Re-inoculate

6 weeks X 5

Pathogen Cycling

LeBoldus et al. (In Press)

Repeated cropping of a

resistance source can erode

the effectiveness of that

resistance

Resistance stewardship is

important!

CV-R CV-S BL

Field Situation - 2011

• Extensive surveillance revealed that all

canola products with genetic resistance to

clubroot were still fully effective against this

disease in 2011

– Disease severity on resistant canola crops was

low (0.2 – 10.2%)

– Severe clubroot found in many of the canola

crops sown to susceptible cultivars (severity

>60% in some)

Continued Monitoring

• We plan continued surveys for clubroot in

2012; this will include monitoring and field

sampling in clubroot affected regions to

follow the performance of clubroot-resistant

canola genotypes

Cross-Infectivity Experiments

• Objective: To assess whether various commercial

canola cultivars carry the same or different

sources of resistance

• Methodology:

– Cross-inoculate canola cultivars with P. brassicae

populations cycled on other Brassica hosts

• Rationale:

– If same source of resistance, then pathogen populations

cycled on one cultivar should show increased infectivity

on other cultivars

Cross-Infectivity Experiments

Canola

host

Cycled populations

CV-R BL ECD 05 ECD 15

W 5.5±9.4 1.9±7.7 4.6±8.9 5.5±9.4

X 8.6±2.9 0.0±0.0 0.0±0.0 0.0±0.0

Y 1.9±7.7 0.0±0.0 0.0±0.0 0.0±0.0

Z 11.1±9.5 0.0±0.0 0.0±0.0 0.0±0.0

LeBoldus et al. (In Press)

Pathogen

populations

cycled on one host

did not show

equivalent

increases in

virulence on other

hosts

Rotation of Resistance Sources

• Cross-infectivity experiments suggest that

some cultivars may be carrying different

clubroot resistance sources

• Potential for rotation of resistance sources

• Further work is ongoing

Resistance Stewardship

• Genetic resistance represents most effective

and economical clubroot management tool

• Sources of resistance will have to be well

managed

– 1 in 4 rotation with clubroot resistant canola is

recommended

– Rotation of cultivars

• Use resistance as part of an integrated

strategy

Integrated Clubroot Management

• Based on deployment of resistant cultivars

in combination with other strategies:

– Continued surveillance

– Proper sanitation

– Crop rotation

– Fungicides and soil amendments for ‘spot

treatments’?

Conclusions

• Clubroot now endemic to canola in central Alberta

• Disease appears to be spreading by a variety of mechanisms

• Management can be difficult

• Resistant cultivars represent an important new clubroot management tool

– Will have to be used as part of an integrated approach

Acknowledgments

• Collaborators: S.F. Hwang, T.K. Turkington, G. Peng, R.J. Howard & others

• Students & other research personnel: D. Rennie, V.P. Manolii, T. Cao, J. LeBoldus & others

• Funders: Canola Council of Canada through AAFC Clubroot Risk Mitigation Initiative, Alberta Crop Industry Development Fund, Agriculture & Food Council, ACPC, Canadian Seed Growers Association, SaskCanola, MCGA and other industry partners