Search

Clubroot of crucifers, caused by Plasmodiophora brassicae, is emerging as an important disease of canola (Brassica napus) in Alberta, Canada. Populations of the pathogen often consist of a mixture of different pathotypes. Therefore, a simple and efficient method to isolate single resting spores of P brassicae was developed, based on serial dilution of spore suspensions. The virulence of 24 single-spore isolates, representing five populations of the pathogen from Alberta, Ontario, and British Columbia, was characterized on the differentials of Williams and Some et al. Symptoms were rated 6 weeks after inoculation and Fisher's least significant difference (P < 0.05) was used to differentiate resistant from susceptible host reactions. The pathotype composition of P brassicae in Canada appeared more diverse when single-spore isolates were examined rather than populations of the pathogen. In Alberta, at least three and possibly four pathotypes were identified among the 14 isolates tested, whereas a maximum of only two pathotypes had been reported previously when populations of the pathogen were examined. Pathotype 3 or P2, as classified on the differentials of Williams and Some et al., respectively, was found to be pre-dominant in the province. The occurrence of other pathotypes at lower frequencies suggests that caution should be used in any breeding strategy, because rare pathotypes of P. brassicae may quickly become predominant if susceptible host genotypes are continuously grown. [ABSTRACT FROM AUTHOR]

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, Somé 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 Somé et al. respectively. However, it may be difficult to detect rare or infrequent pathotypes when field populations of the pathogen are used for characterization. [ABSTRACT FROM AUTHOR]