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Effect of Temperature on Cortical Infection by Plasmodiophora brassicae and Clubroot Severity

A study was conducted to assess the effect of temperature on infection and development of Plasmodiophora brassicae in the root cortex of Shanghai pak choy (Brassica rapa subsp. chinensis) and on subsequent clubroot severity. Ten-day-old seedlings were grown individually, inoculated with resting spores, and maintained in growth cabinets at 10, 15, 20, 25, and 30°C. Seedlings were harvested at 2-day intervals, starting 8 days after inoculation (DAI) and continuing until 42 DAI. Roots were assessed at 4-day intervals for the incidence of cortical infection and stage of infection (young plasmodia, mature plasmodia, and resting spores), at 2-day intervals for symptom development and clubroot severity, and at 8-day intervals for the number of spores per gram of gall. Temperature affected every stage of clubroot development. Cortical infection was highest and symptoms were observed earliest at 25°C, intermediate at 20 and 30°C, and lowest and latest at 15°C. No cortical infection or symptoms were observed at 42 DAI in plants grown at 10°C. A substantial delay in the development of the pathogen was observed at 15°C. Resting spores were first observed at 38 DAI in plants at 15°C, 26 DAI at 20 and 30°C, and 22 DAI at 25°C. The yield of resting spores from galls was higher in galls that developed at 20 to 30°C than those that developed at 15°C over 42 days of assessment. These results support the observation in companion studies that cool temperatures result in slower development of clubroot symptoms in brassica crops, and demonstrate that the temperature has a consistent pattern of effect throughout the life cycle of the pathogen.