2023 Volume 65 Pages 6-15
Clubroot disease caused by a soilborne protist pathogen, Plasmodiophora brassicae (Woronin), is one of the most economically important diseases affecting plants of the Brassicaceae family. We investigated the effects of repeated cultivations using clubroot-resistant (CR) and clubroot-susceptible cultivars of cabbage (Brassica oleracea L. var. capitata) on clubroot severity and the population density of P. brassicae in soil under field conditions. Compared with the susceptible cultivar ‘Okina’, repeated cultivations of the resistant cultivar ‘BCR Ryugetsu’ significantly reduced the population density of P. brassicae. However, the same cycles of fallow treatment (no cultivation in the field) demonstrated no significant differences in the reduction compared with that in the resistant treatment (cultivation of ‘BCR Ryugetsu’). Four successive cycles of the resistant treatment in 2020–2021 could reduce clubroot severity in the final cycle (2022) when ‘Okina’ was planted compared with that in the continuous fallow treatments. ‘BCR Ryugetsu’ mostly suppressed the formation of large galls, which affect plant growth. Although a single individual of ‘BCR Ryugetsu’ formed a large clubroot gall in the final cycle of cultivation, a P. brassicae population isolated from the gall (Shimo-22DM) did not exhibit the typical pathogenicity on the same resistant cultivar, suggesting an avirulent population. In conclusion, this study demonstrated that repeated cultivations of the CR cabbage cultivar reduced the population density of P. brassicae to the level safe for the cultivation and harvest of the susceptible cultivar, although it could also be the contribution of natural reduction.
