Clubroot in the Canadian canola crop: 10 years into the outbreak

Clubroot, caused by the obligate parasite Plasmodiophora brassicae, is an important soilborne disease of the Brassicaceae. In Canada, clubroot has long been a problem in cruciferous vegetables, but was not reported on the Prairie canola (Brassica napus) crop until 2003, when 12 clubroot-infested fields were identified in central Alberta. Continued surveillance has shown that the disease is spreading, and at least 1064 fields in the province had confirmed clubroot infestations as of 2012. While the outbreak is still mainly confined to central Alberta, isolated clubroot infestations and the presence of viable P. brassicae inoculum have been confirmed in southern Alberta, Saskatchewan and Manitoba. Dissemination of the parasite appears to be predominantly through the movement of infested soil on farm and other machinery, although secondary mechanisms of spread, such as by wind and water erosion and soil contamination of seeds, also have been implicated. Numerous strains of P. brassicae occur in Canada, but pathotype 3 or 16/15/12, as classified on the differentials of Williams or the European Clubroot Differential series, predominates on canola. Given the significant economic value of Canadian canola, the emergence of clubroot on this crop has caused major concern and led to the initiation of a large, coordinated research effort aimed at understanding and managing this disease. This review summarizes the extent and nature of the clubroot outbreak in the Canadian canola crop, 10 years after it began, and provides a context for ongoing research and management efforts.