Virulence and genetic structure of Plasmodiophora brassicae populations in Alberta, Canada

Abstract

Clubroot, caused by Plasmodiophora brassicae Woronin, is one of the most important diseases of canola (Brassica napus L.) in western Canada. While the disease is managed most effectively by planting clubroot resistant (CR) canola cultivars, genetic resistance has been overcome in more than 200 fields in Alberta since 2013. Thirty-four single-spore isolates of P. brassicae were purified from 9 field isolates collected from CR canola crops across the province, and evaluated for pathotype classification on the Canadian Clubroot Differential (CCD) set, which includes the differentials of Williams and Somé et al. Using an index of disease of 50% (+/- 95% confidence interval) to distinguish between resistant and susceptible host reactions, 13 pathotypes could be distinguished based on the CCD system, seven on the differentials of Williams, and three on the hosts of Somé et al. Novel pathotypes, not reported in Canada previously, were identified among the isolates. The genetic structure of the isolate collection was evaluated by simple sequence repeat (SSR) marker analysis, which indicated a low level of genetic diversity. Polymorphisms were detected in 32 loci with the identification of 93 distinct alleles. Haploid linkage disequilibrium and number of migrants suggested that recombination and migration were rare or almost absent in the tested P. brassicae population. A relatively clear relationship was found between the genetic and virulence structure of some of the pathotypes, and isolates from northern and southern Alberta were genetically distinct from each other and from those from central Alberta. Further testing on a suite of seven CR canola cultivars indicated that the field isolates consisted of a mixture of virulent and avirulent pathogen genotypes. Significant genetic differentiation was detected among the pathotypes and between virulent and avirulent populations. Genetically homogeneous single-spore isolates provided a more complete and clearer picture of P. brassicae virulence and genetic structure. The deployment of genetically resistant canola should be combined with other management tactics, such as longer rotation intervals out of susceptible hosts, for the sustainable management of clubroot in Alberta.