Reduction of resting spore density of Plasmodiophora brassicae and clubroot disease severity by liming

Relationships between the disease severity of clubroot caused by Plasmodiophora brassicae, the soil pH value and the concentration of exchangeable calcium associated with liming were investigated under controlled density of resting spores. Disease indices were lower in the plots treated with lime than in the control plots without lime application. The disease index was significantly lower when lime materials were mixed two weeks before sowing compared with four weeks before sowing. The reduction rate of the disease index was larger for a concentration of 2.0 g kg(-1) than 1.0 g kg(-1) of lime in soil. The density of the resting spores in soil at the time of sowing was significantly reduced by liming. The reduction rate was 17-31% for calcium cyanamide, 12-29% for dolomite, and 20-39% for calcium carbonate compared with the control plot. It was suggested that the disease severity was influenced by both the soil pH and the content of exchangeable calcium in soil based on the analysis of covariance.