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Isolation and Variation in Virulence of Single-Spore Isolates of Plasmodiophora brassicae from Canada

March 2008 , Volume 92 , Number  3
Pages  456 - 462

S. Xue and T. Cao, Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB T6G 2P5, Canada; R. J. Howard, Alberta Agriculture and Food, Crop Diversification Centre South, Brooks, AB, Canada; S. F. Hwang, Alberta Agriculture and Food, Crop Diversification Centre North, Edmonton, AB, Canada; and S. E. Strelkov, Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton



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Accepted for publication 2 November 2007.
ABSTRACT

Clubroot of crucifers, caused by Plasmodiophora brassicae, is emerging as an important disease of canola (Brassica napus) in Alberta, Canada. Populations of the pathogen often consist of a mixture of different pathotypes. Therefore, a simple and efficient method to isolate single resting spores of P. brassicae was developed, based on serial dilution of spore suspensions. The virulence of 24 single-spore isolates, representing five populations of the pathogen from Alberta, Ontario, and British Columbia, was characterized on the differentials of Williams and Somé et al. Symptoms were rated 6 weeks after inoculation and Fisher's least significant difference (P < 0.05) was used to differentiate resistant from susceptible host reactions. The pathotype composition of P. brassicae in Canada appeared more diverse when single-spore isolates were examined rather than populations of the pathogen. In Alberta, at least three and possibly four pathotypes were identified among the 14 isolates tested, whereas a maximum of only two pathotypes had been reported previously when populations of the pathogen were examined. Pathotype 3 or P2, as classified on the differentials of Williams and Somé et al., respectively, was found to be predominant in the province. The occurrence of other pathotypes at lower frequencies suggests that caution should be used in any breeding strategy, because rare pathotypes of P. brassicae may quickly become predominant if susceptible host genotypes are continuously grown.


Additional keywords:Chinese cabbage, oilseed rape, races

© 2008 The American Phytopathological Society