Spread of Plasmodiophora brassicae on canola in Canada, 2003-2014: Old pathogen, new home

Clubroot caused by Plasmodiophora brassicae has been reported at sites across North America on brassica vegetables for more than 50 years. However, it had not been reported on canola (Brassica napus) on the Canadian prairies until the initial discovery of a cluster of 12 infested fields near Edmonton AB in 2003. The purpose of this review is to consolidate and summarize the data on the spread of P. brassicae on canola in Canada since 2003, to compare this pattern of distribution with observations from an infested site in Ontario, and to draw inferences about the relative importance of short- and long-distance transmission of the pathogen on clubroot distribution in the prairie region. Over the last decade, P. brassicae has spread across central Alberta, with the leading edge of the epidemic moving at about 20 km per year, resulting in more than 1850 fields confirmed infested. DNA of the pathogen has also been detected from soil collected at sites across Saskatchewan and Manitoba, and very slight clubroot symptoms have been observed at isolated sites across the prairies. Transport of resting spores in soil carried on farm equipment has been shown to be an important mechanism of short-distance dissemination in this region. Dispersal of resting spores with wind-borne soil may also have an important role in both short- and long-distance dissemination. Dispersal on seed does not appear to be an important factor in clubroot spread. In contrast to the rapid spread observed in Alberta, P. brassicae is spreading very slowly, if at all, at the site in Ontario. This likely reflects the relatively small size and strength of the inoculum source and the absence of susceptible hosts nearby at the site in Ontario, relative to the thousands of hectares of heavily infested fields that provide a large, strong inoculum source in central Alberta.