Characterization of clubroot (Plasmodiophora brassicae) from canola (Brassica napus) in the Peace Country of Alberta, Canada

Clubroot of canola (Brassica napus L.), caused by Plasmodiophora brassicae Woronin, has spread to more than 3000 fields in Alberta, Canada, since it was first identified in 2003, but the disease had not been reported in the Peace Country Region in the northwest of the province. In 2017, 21 cases of clubroot were found in Big Lakes County, the southeastern-most district in this region. In 2018, an additional five cases were identified. Surveys of the infested crops in 2017 detected a clubroot disease index (DI) of < 10% in 15 fields and > 10% in the other six, while in 2018, the DI was < 5% in four fields and > 50% in one. Canola had been grown 3–6 times over the preceding 7–8 years in all fields for which rotation histories could be obtained. A subset of 23 P. brassicae field isolates was evaluated for pathotype classification on the hosts of the Canadian Clubroot Differential (CCD) set. Of the isolates collected in 2017, 11 were pathotype 3H, two each were pathotypes 5I and 8N, and three had novel virulence patterns designated 5A, 8A, and 8B. The five field isolates tested from 2018 included pathotypes 6C, 8A and 8 N, plus two isolates of the novel pathotype 8C. On the differential system of Somé et al., 17 isolates were classified as pathotype P2 and six as P3. As the CCD pathotype 8C is virulent on clubroot-resistant canola, integrated disease management strategies will be required for resistance stewardship in this region.