Crop rotation, cultivar resistance, and fungicides/biofungicides for managing clubroot (Plasmodiophora brassicae) on canola

Select biofungicides and fungicides, used alone or with cultivar resistance or crop rotation, were assessed for their potential in integrated management of clubroot disease. The synthetic fungicides pentachloronitrobenzene, fluazinam and cyazofamid showed activities against Plasmodiophora brassicae . The biofungicides Serenade ® and Prestop ® also suppressed the disease on canola via antibiosis and induced host resistance under controlled-environment conditions. Granular and seed-treatment formulations were developed to facilitate the delivery of biofungicide in field trials. Where P. brassicae resting spore populations were large in the soil, neither biofungicides nor synthetic fungicides were sufficiently effective when applied in the seed furrow. They occasionally reduced clubroot severity on Chinese cabbage. More than 5000 soil microbial isolates indigenous to the Canadian prairies were screened for potential clubroot control, but none showed consistent efficacy. Resistant cultivars reduced clubroot severity and canola yield losses significantly. A 2-year break from canola reduced P. brassicae resting spore concentrations by 90% relative to growing continuous canola or only a 1-year break in heavily infested field plots. This 2-year break alleviated disease impact on plant growth and development in a susceptible canola cultivar. Despite the substantial inoculum reduction after 2 years, the levels were still too high to obtain commercially acceptable yields in a susceptible cultivar. In a resistant cultivar, >2-year breaks increased yields by up to 25% relative to continuous growing of canola. A 2-year interval with non-hosts between canola crops, together with use of resistant cultivars, is recommended to reduce the inoculum load of P. brassicae in soil and achieve maximum yields of canola.