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Pathotypes of Plasmodiophora brassicae collected from clubroot resistant canola (Brassica napus L.) cultivars in western Canada in 2017-2018

Abstract Clubroot, caused by Plasmodiophora brassicae Woronin, is an important soilborne disease of canola (oilseed rape; Brassica napus L.). In Canada, clubroot management relies heavily on the planting of resistant cultivars, but since 2013, resistance has been broken in an increasing number of fields. Prior to the introduction of resistance, P. brassicae pathotype 3H, as defined on the Canadian Clubroot Differential (CCD) set, was predominant in Alberta. In testing of pathogen collections from 2014 to 2016, however, pathotype 3A was most common, indicating rapid shifts in the pathogen population. Up-to-date knowledge of pathotype composition is important for effective resistance breeding and stewardship. Therefore, clubbed roots were collected from clubroot-resistant canola crops from 166 fields in Alberta, Saskatchewan, and Manitoba in 2017 and 2018, with one isolate per field evaluated for pathotype designation on the CCD set and the differentials of Somé et al. Seventeen pathotypes were detected on the CCD set, including the previously reported pathotypes 3A, 3D, 3H, 5 L, 5X, 8E, 8N and 8P, plus the novel pathotypes 2C, 6D, 8D, 9A, 9B, 9C, 11A, 13A and 13B. Five pathotypes were identified on the hosts of Somé et al. including P1, P2, P3, P4 and P5, with P4 and P5 reported here from Canada for the first time. The majority of the isolates, representing 39 fields in 2017 and 92 fields in 2018, could overcome genetic resistance. The results suggest significant diversity in the virulence of P. brassicae populations and an increasing prevalence of resistance-breaking P. brassicae strains.