The pathogenesis of twenty-eight isolates of Plasmodiophora brassicae WORONIN obtained from thirteen prefectures, to the WILLIAMS' race differential hosts and clubroot-resistant (CR) cultivars of Chinese cabbage were investigated. The pathogenesis of seven isolates from Chugoku and Kyushu areas were tested also to CR cultivars of cabbage. Twenty-six isolates were identified as race 4, suggesting that race 4 is the most dominant in Japan. Remaining two isolates were race 9 (or race 3) and race 1, respectively. Seven race 4 and one race 9 (or 3) isolates showed pathogenicity to CR cultivars of Chinese cabbage, Kukai 70 and/or Horai San-go, whereas nineteen race 4 and one race 1 isolates hardly showed pathogenicity to these cultivars. Of seven isolates from Chugoku and Kyushu areas, three (race 4) were moderately pathogenic to CR cultivars of cabbage, YCR Tae and YCR Tamu; three isolates (race 4) were moderately pathogenic to YCR Tae but non-pathogenic to YCR Tamu; and one (race 9 or 3) was non-pathogenic to both cultivars. The results indicate that race 4 isolates are not uniform in pathogenicity to CR cultivars of Chinese cabbage and CR cultivars of cabbage.
