Collection and identification of Plasmodiophora brassicae pathotypes from western Canada in 2021-2023

Clubroot disease, caused by Plasmodiophora brassicae, is a major threat to canola (Brassica napus) production in the Canadian Prairies. One of the most effective clubroot management strategies is the deployment of clubroot-resistant cultivars. 'First-generation' resistant cultivars, with resistance derived from B. napus cv. 'Mendel', provided farmers with an initial line of defence. However, the emergence of more virulent pathotypes of P. brassicae has led to an increasing number of cases where these novel pathotypes overcome resistance, resulting in severe clubroot symptoms in previously resistant cultivars. In this study, 206 field isolates of the pathogen were collected between 2021 and 2023 from Alberta (194 isolates), Saskatchewan (eight isolates) and Manitoba (four isolates). Pathotype designations were determined using the Canadian Clubroot Differential (CCD) set, leading to the identification of 31 unique pathotypes. Among these were 10 novel 'resistance-breaking' pathotypes, designated as 1D, 1E, 1G, 3F, 3I, 3J, 5D, 6F, 8K and 9G, along with pathotypes 1H and 3G, still controlled by first-generation resistance. The novel pathotype 5D, virulent on 'Mendel', represents the first instance of a resistance-breaking pathotype in Saskatchewan. Despite the large number of pathotypes detected, pathotypes 3A (25%), 3D (17%) and 3H (15%) remain the most prevalent in Alberta, consistent with previous years. Other notable pathotypes, including 8E, 8N, 8P and 9E, continue to be detected, albeit at lower frequencies (4-8%). The rapid diversification of pathotypes and the spread of P. brassicae in the canola growing regions of Canada underscore the importance of continued surveillance.