Clubroot of canola: Overview of an emerging problem
Stephen E. Strelkov, Dept. of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB T6G 2P5 E-mail: stephen.strelkov@ualberta.ca

Clubroot, caused by the obligate parasite Plasmodiophora brassicae, is an important soilborne disease of crucifers that was first identified on canola (Brassica napus) in central Alberta in 2003. Annual surveys have revealed that clubroot is spreading, with the number of fields with confirmed infestations increasing from 12 in 2003 to 831 in 2011. While most cases of clubroot are still confined to central Alberta, the disease has also been identified in a few fields in southern Alberta and in Saskatchewan. The primary mechanism of spread between fields is via the movement of P. brassicae-infested soil on farm equipment and other machinery. Although there is the potential for pathogen movement as an external contaminant on seeds and tubers, this risk is small and can be mitigated by proper seed cleaning and/or fungicidal treatments. The extent of pathogen movement by soil and water erosion is currently under study. In severely infected canola crops, yield losses caused by clubroot range from 30 to 100%. Until recently, the management of clubroot was restricted largely to the cleaning of equipment to prevent spread to new fields, and long rotations out of canola in those fields in which the pathogen was already present. However, a number of clubroot-resistant canola hybrids have been recently released onto the Canadian market, collectively representing an important tool for the successful management of this disease. Genetic resistance will have to be carefully managed, however, since greenhouse studies revealed that resistance can be quickly eroded after repeated exposure to the same pathogen population. Therefore, the sustainable management of clubroot on canola will require an integrated approach in which the deployment of resistant cultivars is combined with other strategies such as crop rotation, sanitation and continued pathogen surveillance.