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

A breakdown of clubroot resistance (CR) of CR F-1 cultivars of Chinese cabbage, caused by variation in the pathogenicity of Plasmodiophora 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 (ECD), 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 F, cultivars of Chinese cabbage, 'Ryutoku' and 'Utage70', were reported as differential hosts for classification of Japanese field isolates. However, 'Utage70' cannot be used now for the test for clubroot resistance because seeds are not marketed any longer. Therefore, the degree of resistance of 18 CR F, cultivars of Chinese cabbage was surveyed using standard pathogens, Yuki-01 and Rokunohe-01, and 'Super CR Hiroki' was found 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 'Super CR Hiroki' and 'Ryutoku'; group 2, which can infect only 'Ryutoku'; group 3, which can infect only 'Super CR Hiroki'; and group 4, which can not infect both cultivars. Two isolates classified as group 1 could infect all Japanese CR F, cultivars of Chinese cabbage tested, showing the broadest host range. However, the two European fodder turnips, 'Siloga' and 'Gelria R', were confirmed as a valuable source of CR genes because they were highly resistant to most field isolates.