Effect of temperature on primary infection by Plasmodiophora brassicae and initiation of clubroot symptoms

A study was conducted to assess the effect of temperature on infection and development of Plasmodiophora brassicae in root hairs of Shanghai pak choi (Brassica rapa subsp. chinensis) and on initiation of clubroot symptoms. Ten‐day‐old seedlings were grown in liquid‐sand culture, 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 2 days after inoculation (DAI) and continuing until swelling of the tap root was observed (maximum 28 days). Roots were assessed for root hair infection (RHI), stage of development of infection (primary plasmodia, zoosporangia, release of zoospores, secondary plasmodia), symptom development, and for clubroot severity at 24 DAI. Temperature affected every stage of clubroot development; RHI was highest and visual symptoms initiated earliest at 25°C, intermediate at 20 and 30°C, and lowest and latest at 15 and 10°C. Root hair infection was observed at every temperature, but clubroot symptoms developed only above 15°C. A substantial delay in the development of the pathogen was observed at 10 and 15°C. No symptoms were observed at 28 DAI in plants grown at 10°C. Swelling of the tap root was visible at 28 DAI in plants at 15°C, 14 DAI at 20 and 30°C, and 10 DAI at 25°C. These results support and explain the observation in companion studies that cool temperatures result in slower development of clubroot symptoms in brassica crops.