Variation in virulence of Plasmodiophora brassicae in Japan tested with clubroot-resistant cultivars of Chinese cabbage (Brassica rapa L. ssp pekinensis)

The differential hosts of Williams (1966) and the European Clubroot Differe ntial (ECD) (Buczacki et al., 1975) have been used commonly to identify pop ulations of Plasmodiophora brassicae, which causes clubroot disease in Bras sica crops. However, some of these hosts showed intermediate and fluctuatin g scores to most populations from Japan. Therefore, these hosts could not b e used to provide a clear classification in Japan. We have tried to clarify the genetic diversity in pathogenicity of P. brassicae in Japan using Japa nese clubroot-resistant (CR) F-1 hybrid (F-1) cultivars and lines of Brassi ca rapa. The responses of some CR F-1 cultivars were very clear. Four group s of field populations in Japan were recognized using the CR F-1 cultivars. The clear response obtained here may depend largely on the genetic purity of the F-1 cultivars. Moreover, it is possible to classify some of these Ja panese populations into the same race using the Williams set and ECD 01 to ECD 05. The present differential hosts may be useful in the study of Europe an populations of P. brassicae. The response of the differential hosts sugg ests that there are several major CR genes in B. rapa. It is suggested that pyramiding CR genes would be useful in breeding CR cultivars that can over come the breakdown of the present CR cultivars of Chinese cabbage.