Virulence Spectrum of Single-Spore and Field Isolates of <i>Plasmodiophora brassicae</i> Able to Overcome Resistance in Canola (<i>Brassica napus</i>)

Clubroot, caused by Plasmodiophora brassicae Woronin, is an important disease of canola (Brassica napus L.) that managed mainly planting clubroot-resistant (CR) cultivars. Field isolates P. can be heterogeneous mixtures various pathotypes, making assessments the genetics host–pathogen interactions challenging. Thirty-four single-spore were obtained from nine field pathogen collected CR The virulence patterns and assessed on 13 host genotypes Canadian Clubroot Differential (CCD) set, which includes differentials Williams Somé et al. Indices (IDs) severity 25, 33, 50% (±95% confidence interval) compared as potential thresholds to distinguish between resistant susceptible reactions, with ID giving most consistent responses for pathotype classification purposes. With this threshold, pathotypes could distinguished based CCD system, 7 Williams, 3 hosts highest correlations observed among matrices generated using three threshold IDs set. Genetically homogeneous gave a clearer profile structure. Novel not reported in Canada previously, identified isolates. This large collection serve reference screening breeding clubroot resistance.