Pathogenicity of Three Isolates of Clubroot Fungus Attacking Clubroot-Resistant Cultivars of Chinese Cabbage

Clubroot-resistant(CR) cultivars of Chinese cabbage, surmised to possess a single CR gene introduced from European fodder turnip, and some crucifers were grown in soils infected with inoculation by five isolates of the clubroot fungus(Plasmodiophora brassicae) from Yamaguchi and Oita Prefectures. Three isolates from Yamaguchi Pref. showed the high pathogenicity to the CR cultivars and another isolate from Yamaguchi Pref. and one from Oita Pref. were non-pathogenic to these cultivars. All the three pathogenic isolates belonged to race 9, one of the unusual races in Japan, by WILLIAMS' method. Relatively mild virulence to a non-CR Chinese cabbage (cv. Nozaki-nigo）and non-virulence to cabbages were also observed commonly in tbese three isolates. On the other hand, two isolates non-pathogenic to the CR cultivars were judged to belong to dominant races (races 1 and 4) in Japan. These two isolates were apparently virulent to Nozaki-nigo and cabbages. All five isolates examined did not affect a line of European fodder turnip, 77b, which was one of CR-gene sources used for the breeding of the CR cultivars. These results suggest that ”77b” has multiple CR genes of which the CR cultivars of Chinese cabbage have only one, instead of a single CR gene, and that the clubroot incidence in these CR cultivars is due to missing out part of the CR genes during the breeding.

Plasmodiophora brassicae

clubroot-resistant cultivars

race

pathogenicity

Chinese cabbage