Pathogenic and molecular characterization of single spore isolates of Plasmodiophora brassicae from Canada

Clubroot, caused by Plasmodiophora brassicae, is an important new disease of canola (Brassica napus) in Alberta, Canada. A simple and efficient method to isolate single resting spores of the pathogen was developed, and the virulence of 25 isolates from Canada was characterized on two differential sets. The pathotype composition of P. brassicae appeared more diverse when isolates rather than populations of the pathogen were examined. In Alberta, pathotype 3 was predominant, but at least three and possibly four pathotypes were identified. Additionally, diversity was assessed by RAPD analysis, with isolates found to cluster according to population rather than pathotype. The utility of cleaved amplified polymorphic sequence analysis for molecular differentiation of pathotypes was also demonstrated. It appears that the P. brassicae population in Alberta is fairly diverse, and caution should be used in any breeding strategy, since rare pathotypes may quickly become predominant if susceptible host genotypes are continuously grown.