The influence of virulence of Plasmodiophora brassicae populations on epidemiology of Chinese cabbage [Brassica chinensis] clubroot and efficacy of fungicides

Biological characteristics of three field populations of Plasmodiophora brassicae Woronin differing in virulence to a susceptible cultivar of Chinese cabbage were examined. The relationship between their virulence and the epidemiology of Chinese cabbage clubroot, as well as the efficacy of fungicides was also investigated. Highly virulent populations resulted in a high frequency of root hair infections and a high density of resting spores in the clubroot gall. Highly virulent population also caused severe damage to Chinese cabbage even at a low inoculum density under short-day-length conditions. The efficacy of three fungicides (trichlamide, flusulfamide and fluazinam) was high against the weakly virulent population, but low against the highly virulent population. These results suggest that the frequency of root hair infections, the density of resting spores in the clubroot gall, the effect of day length on clubroot incidence and the efficacy of fungicides vary depending on the vir