Pathotype Classification of Plasmodiophora brassicae and its Occurrence in Brassica napus in Alberta, Canada

A field survey for clubroot of crucifers, caused by Plasmodiophora brassicae, was conducted in the regions surrounding Edmonton, Alberta, Canada, in 2005. The presence of clubroot was confirmed in 41 of the 112 canola (Brassica napus) fields surveyed. These P. brassicae- infested fields were located in Sturgeon, Strathcona, Leduc and Wetaskiwin counties, as well as in a rural area of northeast Edmonton. Infected roots were also received from an infested field in Flagstaff County, southeast of Edmonton. Although there was a significant negative correlation between index of disease and soil pH, the occurrence of clubroot was not restricted to fields with acidic soils. Populations of the pathogen were selected from 10 fields and used in pathotype classification on the differential hosts of Williams, Some´ et al. and the European Clubroot Differential (ECD) set. Kruskal–Wallis analysis indicated no significant differences in the virulence of the 10 populations tested, suggesting that they are relatively homogenous. If a disease index of 50% was regarded as the cut-off between a resistant and a susceptible reaction, then all P. brassicae populations tested were classified as ECD -⁄15⁄12 on the hosts of the ECD set, or as pathotypes 3 or P2 on the differentials of Williams or Some´ et al. respectively. However, it may be difficult to detect rare or infrequent pathotypes when field populations of the pathogen are used for characterization.