Strategies and Challenges in the Management of Clubroot Disease of Canola S.E. Strelkov, S.F. Hwang, M.D. Harding

Transcription

1 Strategies and Challenges in the Management of Clubroot Disease of Canola S.E. Strelkov, S.F. Hwang, M.D. Harding 14 th International Rapeseed Congress Saskatoon 2015

2 Outline of Presentation Introduction to clubroot Scope of the problem Clubroot management and the importance of genetic resistance Emergence of new virulence phenotypes Implications and conclusions

3 Clubroot of Crucifers Soilborne disease Caused by Plasmodiophora brassicae Attacks the roots, causes galls Galls interfere with normal uptake of water and nutrients by the plant Severe yield and quality losses

4 Clubroot in Canadian Canola Historically an issue in cruciferous vegetables in eastern Canada and BC First identified on canola in the Prairie Provinces in 2003 Cause for concern Nine million ha of canola Worth $15.4 billion to Canadian economy

5 Prevalence of Clubroot Number of clubroot-infested fields has increased rapidly Annual surveys have tracked spread

6 Most important: Mechanisms of Spread Movement of infested soil on machinery Secondary mechanisms: Windborne dust Soil tag on seeds and tuber (minor) Hwang et al. 2014

7 Clubroot Situation: 2015 Outbreak most severe in central Alberta A few cases in southern Alberta Isolated reports from Saskatchewan, Manitoba and North Dakota Most fields mildly to moderately infested, but 10-15% have heavy infestations Strelkov et al. 2015

8 Clubroot Management Management is inherently difficult P. brassicae produces large amounts of resting spores that serve as inoculum Resting spores can remain viable for more than 15 years Soil inoculum loads can increase quickly in the presence of susceptible hosts J.P. Tewari, U of Alberta

9 Management Strategies Few management options available when clubroot first appeared Long rotations out of Brassica crops Equipment sanitization These strategies were not popular with farmers and not widely adopted R.J. Howard, AARD

10 Development of Additional Management Strategies Focus on adapting strategies used in cruciferous vegetables or other systems Liming, fungicides (Terraclor), bait crops and biocontrol agents all were assessed Some gave inconsistent results Other approaches promising, but not practical or costeffective in canola cropping systems Wood ash Lime Lime Lime 5.0 t/ha 2.5 t/ha 7.5 t/ha 5.0 t/ha

11 Soil Fumigants Targeted application of soil fumigants (metam sodium) could help to contain isolated infection foci (< 100 m 2 ) Strategy only viable where clubroot is not yet well-established, very specific situations Control 0.39 ml/l 0.79 ml/l 1.56 ml/l

12 Genetic Resistance Development of clubroot resistant cultivars soon became focus of canola breeders Breeding activities guided by studies on pathotype structure of P. brassicae in Canada Pathotypes differ in ability to infect specific host varieties Important to know which pathotypes are predominant in regions where resistance will be deployed

13 Pathotype Classification Three systems used to classify P. brassicae into races or pathotypes Williams (1966) European Clubroot Differential (ECD) set Somé et al. (1996)

14 Populations vs. Single-Spore Isolates Pathotype designations can be obtained for populations or singlespore isolates of P. brassicae

15 Pathotype Diversity in Canada Province Pathotype(s) Populations Single-spore isolates Reference(s) Alberta 3, 5, 2 3, 8, 2, 6 British Columbia 6 6 Strelkov et al., 2006; Strelkov et al., 2007b; Xue et al., 2008; Cao et al., 2009 Strelkov et al., 2006; Williams, 1966; Xue et al., 2008 Manitoba 5 -- Cao et al., 2009 Nova Scotia 3, 1, 2 -- Hildebrand & Delbridge, 1995 Ontario 6 3, 5, 8 Quebec 2, 5 -- Saskatchewan 3 -- Reyes et al., 1974; Strelkov et al., 2006; Xue et al., 2008; Cao et al., 2009 Williams, 1966; Cao et al., 2009 S.E. Strelkov, unpublished data Pathotype designations on system of Williams (1966)

16 Pathotype 3 is Predominant in Alberta Pathotype 5 (3%) Pathotype 2 (7%) Pathotype 8 (14%) Pathotype 2 (7%) Pathotype 6 (7%) Pathotype 3 (90%) Pathotype 3 (72%) Field Populations Single-Spore Isolates Pathotype 3 (Williams) ECD 16/15/12 P 2 (Somé et al.) Howard et al. 2010

17 Clubroot Resistant (CR) Canola First CR canola became available in 2009 Quickly followed by other cultivars from multiple seed companies Basis of resistance not known publicly Screened against all known pathotypes (greenhouse), and evaluated in field trials Excellent performance

18 CR Canola as a Management Tool CR canola soon became most important clubroot management tool In many cases, became only tool Grown over a wide area often in short rotations Short rotations may be risky: Past experience from Europe, Japan and U.S. indicates that clubroot resistance can be defeated

19 Pathogen Adaptation to Host Genotypes Greenhouse studies showed that repeated exposure to a resistance source led to loss in effectiveness of that resistance Highlighted the need for proper resistance stewardship! CV-R BL CV-S LeBoldus et al. 2012

20 Resistance in the Field In 2013, six canola fields growing a CR cultivar were found to have patches with high clubroot severity

21 Virulence of Strains Tested on CR Canola Varieties Extracted spores from field-collected galls, and re-inoculated onto same varieties under greenhouse conditions Individually evaluated 3 galls from each field of concern Each canola variety also inoculated with pathotype 3 (not exposed to resistance sources)

22 Strain of P. brassicae Virulent on Resistant Canola Spores from galls from one of these fields were able to cause severe clubroot on the CR variety that had been planted in that field Indices of disease severity 99% 100% vs. 1.9% in response to pathotype 3

23 Infectivity of New Strain Virulence of this new strain was tested on CR canola varieties representing all companies in western Canada All were susceptible In most cases, indices of disease severity > 90% Serious threat to canola production in areas where clubroot is common

24 Pathotype Classification New strain of P. brassicae behaves like pathotype 5 based on classification system of Williams (1966) But this does not reflect its increased virulence on CR canola! New strain is referred to as pathotype 5x for now Another strain from a 2 nd field in 2013 also showed some increase in virulence on CR canola, but not as dramatic

25 Questions 1. Is the defeat of resistance a more widespread issue? 2. Are we dealing with a single new pathotype or are there multiple virulence phenotypes?

26 Evaluation of Additional Strains Additional surveying carried out in 2014 Focused on CR canola crops Identified 27 fields with higher than expected levels of clubroot CR canola field with significant clubroot

27 Characterization of 2014 Collections Pathogen populations extracted from individual galls for testing in a stepwise manner: 1. Assessed virulence on cultivars from which populations were recovered 2. If virulent, then tested on suite of CR canola cultivars available on the market 3. Determined virulence phenotypes on differential hosts

28 Testing of 2014 Collections Increased virulence confirmed in P. brassicae populations from 16 of 27 fields of concern Most caused indices of disease severity ranging from 90%-100% on CR canola varieties Resistant Canola Inoculated with New Strains of P. brassicae M. Nawrot, U of Alberta

29 Emergence of New Highly Virulent Strains Not restricted to immediate vicinity of 2013 case New cases separated by up to ~600 km Strelkov & Cao, unpublished

30 Strains Characterized on a Suite of Differential Hosts Represented host genotypes that are being evaluated for inclusion in a putative Canadian clubroot differential set Selected hosts of the ECD set Differentials of Williams (1966) and Somé et al. (1996) Mendel, Westar and a CR cultivar

31 Virulence Patterns on Differentials Nine distinct virulence phenotypes detected P. brassicae population Somé et al.: P 2 & P 3 Williams: pathotypes 2, 3, 5, 6 & 8 Inclusion of additional differentials revealed multiple variants of pathotypes 3, 5 and 6

32 Implications Virulent strains not restricted to small geographical area Distinct virulence phenotypes suggest that at least some arose independently of each other via selection pressure in each field Problem is more widespread than we hoped

33 Implications Emergence of new strains able to overcome resistance highlights continued vulnerability to P. brassicae Loss of resistance would represent loss of most effective clubroot management tool Resistance stewardship is very important Need longer rotations out of canola, especially where clubroot is an issue!

34 Next Steps Continue studies with new strains of P. brassicae Obtain & characterize single-spore isolates Use molecular tools to clarify relationships and develop markers for differentiation Work with breeders to coordinate rational approaches to resistance screening Continue clubroot surveillance activities and monitoring of performance of CR canola

35 Conclusions Clubroot well established in Alberta and continues to spread Various management strategies evaluated, but resistance is most important New highly virulent strains of P. brassicae have been identified in some fields Warning that we must use resistance responsibly!

36 Acknowledgements Financial support from ACIDF, GF2 Program (AAFC/CCC), ACPC, SaskCanola & MCGA V. Manolii, T. Cao, M. Nawrot (U of A) Q. Zhou, G. Turnbull, M. Vadnais, D. Feindel (AAF) CCC Agronomists & Agricultural Fieldmen Other industry partners