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Breakdown of clubroot resistance in Quebec provides an inference for resistance in Alberta.

Gossen, B.D., Pageau, D., Dalton, J., Yu, F., McDonald, M.R., and Peng, G. 2016. Breakdown of clubroot resistance in Quebec provides an inference for resistance in Alberta. Proc. 20th Crucifer Genetics Conf. & 19th Australian Res. Assembly on Brassicas. October 3 6, 2016, Melbourne, Australia.

Abstract

Genetic resistance to Plasmodiophora brassicae Wor. (cause of clubroot of brassicas) has been the cornerstone of clubroot management on canola (Brassica napus L.) in Canada since the release of the first clubroot-resistant cultivar in 2009. Erosion of this resistance was observed in several heavily infested fields in Alberta, Canada in 2013 and confirmed under controlled conditions. Several genotypes of this new and virulent pathotype have since been identified that differ in their clubroot reaction on lines in the European Clubroot Differential system. Outside of Alberta, clubroot is still found only infrequently on canola in Canada. However, it has been present at high levels in a field at Normandin, Quebec for several years. Pathotype 2 (William’s system) predominates at this site, rather than pathotype 3 (predominant in Alberta) or pathotype 6 (on vegetables in Ontario). Studies using clubroot-resistant cultivars were conducted at the Normandin site each year from 2013 to 2015. Limited clubbing was noted on a previously resistant cultivar in 2014, and severe clubbing developed in 2015, accompanied by a substantial increase in the concentration of resting spores in the soil (estimated using qPCR). Replicated and repeated inoculation with spores from clubs collected at Normandin onto the clubroot-resistant cv. 45H29 under controlled conditions resulted in moderate levels of clubbing on most plants. This demonstrates that a change to a virulent pathotype has also occurred at Normandin, which is separated from the sites in Alberta by > 2000 km. This provides strong support for the conclusion that the multiple new pathotypes observed in Alberta have developed independently, rather than being variants of a single change that has spread from an initial site.

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