Reaction of selected Brassica vegetable crops to Canadian pathotypes of Plasmodiophora brassicae

Clubroot, caused by the protist pathogen Plasmodiophora brassicae, causes substantial yield and quality losses in Brassica vegetable crops around the world. Cultivars of several vegetable and oilseed Brassica crops with resistance to clubroot have been developed recently. This study compared clubroot incidence and severity on resistant and standard (susceptible) cultivars of broccoli (Brassica oleracea var. italica), Brussels sprouts (B. oleracea var. gemmifera), Shanghai pak choy (B. rapa var. communis) and napa cabbage (B.rapa spp. pekinensis) grown at field sites naturally infested with pathotype 6 of P.brassicae (P6) in Ontario and pathotypes 3 (P3) and 5 (P5) in Alberta. In Ontario, all of the susceptible cultivars had 100% clubroot incidence and very high severity. The resistant cultivars of Shanghai pak choy and Brussels sprouts had no symptoms of clubroot, and disease severity was low in resistant broccoli. For each crop species, yield of the resistant cultivar(s) was higher than in the standard cultivar(s) under moderate to high disease pressure. In Alberta, the standard cultivars of each Brassica crop were highly susceptible to P3, except for napa cabbage Bilko'. The clubroot reaction to pathotypes 2, 3, 5 and 6 of the cultivars grown in the field trials in Ontario was assessed under controlled conditions. The reaction to P6 was consistent with the field assessments, but several of the cultivars exhibited a differential reaction to pathotypes 2, 3 and 5. No symptoms were observed on Tillage Radish (R) (Raphanus sativus var. longipinnatus) in an infested (P5) field trial at Bassano, Alberta, but some clubroot symptoms (9%) developed on roots under controlled conditions.