Temperature sensitivity of resistance to two pathotypes of Plasmodiophora brassicae in Brassica oleracea

Several methods were evaluated in an attempt to develop a greenhouse screening procedure that would predict field resistance of brassica breeding lines to clubroot disease caused by Plasmodiophora brassicae. Several Brassica oleracea cultivars and breeding lines bred for resistance to Plasmodiophora brassicae and a susceptible Chinese cabbage cultivar were exposed to high levels of inoculum of both pathotypes PB 6, PB 7 at 12, 15, 20, 25 and 30°C. No infection occurred on any host at 12°C. Chinese cabbage was heavily diseased from 15-30°C. Bagder Shipper cabbage, a cauliflower deriving resistance from this variety, and Oregon CR-1 broccoli were resistant to pathotype PB 6 at 15 and 20°C and partially resistant at 25 and 30°C. They were resistant to pathotype PB 7 and 15°C and almost totally susceptible at 20, 25° and 30°C. Oregon cabbage line OR 123 was resistant to pathotype PB 6 at 15°C at almost completely susceptible at 20, 25 and 30°C. It was resistant to pathotype PB 7 at all temperatures. Temperature sensitivity of resistance can partially explain why breeding lines are resistant in field trials and susceptible in greenhouse tests