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Abstract
Clubroot, caused by Plasmodiophora brassicae, has emerged as an important disease of canola (Brassica napus) in Alberta, Canada. Annual surveys from 2005 to 2008 revealed 405 clubroot-infested fields, mainly in central Alberta, but also in the southern part of the province. Within-field surveys suggested that the main mechanism of clubroot spread is through the movement of infested soil on farm machinery, as the frequency of infestation was highest at the field entrances and decreased sharply at distances of 150 and 300 m from the entrance. Characterization of P. brassicae populations from Alberta on the differentials of the European Clubroot Differential (ECD) series, P.H. Williams, and A. Somé et al. revealed that pathotype ECD –/15/12, 3, or P2, respectively, is predominant in the central part of the province, a finding consistent with earlier reports. However, novel virulence phenotypes were also observed. Two populations from southern Alberta were classified as pathotype 5 or P3 on the hosts of P.H. Williams and A. Somé et al., respectively, or as pathotypes ECD –/6/8 and –/4/0 on the ECD set, suggesting independent introductions of P. brassicae to this region or greater diversity than previously thought. Pathogen populations collected from infected canola plants from Quebec and Manitoba also yielded unique virulence patterns but were still virulent on most B. napus hosts, and an Ontario population induced significant disease only on a few of the vegetable differentials. Virulence levels overall were highest for the P. brassicae populations from central Alberta, which may help to explain the rapid increase in clubroot observed in this province.