Classification of pathogenicity of Plasmodiophora brassicae field isolates in Japan based on resistance of F1 cultivars of Chinese cabbage (Brassica pekinensis L.) to clubroot.

摘要

A breakdown of clubroot resistance (CR) in CR F1 cultivars of Chinese cabbage, caused by variation in the pathogenicity of P. brassicae, has become a problem in Japan. For breeding of new CR lines, the pathogenicity of 15 P. brassicae field isolates collected from various areas in Japan was examined. The differential hosts of Williams and the European Clubroot Differential, which has been used commonly to identify populations of P. brassicae, showed intermediate and fluctuating scores to some field isolates and could not be used to classify them. In Japan, two CR F1 cultivars of Chinese cabbage, viz., Ryutoku and Utage70, were reported as differential hosts for the classification of Japanese field isolates. However, cv. Utage70 cannot be used now for testing clubroot resistance because seeds are not marketed any longer. Therefore, the degree of resistance of 18 CR F1 cultivars of Chinese cabbage was surveyed using standard pathogens, Yuki-01 and Rokunohe-01, and cv. Super CR Hiroki was used as a candidate cultivar for a differential host. The pathogenicity of 15 field isolates collected from various areas in Japan could be classified into 4 groups: group 1, which can infect both cv. Super CR Hiroki and Ryutoku; group 2, which can infect only cv. Ryutoku; group 3, which can infect only cv. Super CR Hiroki; and group 4, which cannot infect either cultivars. Two isolates classified into group 1 infected all Japanese CR F1 cultivars of Chinese cabbage tested. However, the two European fodder turnip cultivars (Siloga and Gelria R) were confirmed as a valuable source of CR genes because they were highly resistant to most field isolates..