Soil suppressiveness to clubroot disease of Chinese cabbage caused by Plasmodiophora brassicae

Two types of soil, Haplic Andosols (HA-soil) and Low-humic Andosols (LA-soil), collected from Fukushima in Japan were used to study soil suppressiveness of clubroot disease of Chinese cabbage (Brassica oleraces), caused by Plasmodiophora brassicae. Relationships between disease index and concentration of resting spores in both soils, i.e. dose response curves, revealed that LA-soil was more suppressive to the disease than HA-soil. The disease index in LA-soil was significantly lower than that in sterilized LA-soil (SLA-soil) even at 10(6) inoculum concentration, although there was no difference between HA-soil and sterilized HA-soil (SHA-soil) under severe disease pressure. The result suggests that biotic factors in LA-soil are responsible for disease suppression even at a high inoculum level of the pathogen. In addition, he disease index was lower in SLA-soil than in SHA-soil at all inoculum levels, suggesting that abiotic factors were also involved in the suppressiveness of LA-soil. These results indicate that the simultaneous effects of both biotic and abiotic factors are involved in the suppressiveness in LA-soil. The experiment with mixture of sterilized or non-sterilized LA-soils with infested HA-soil indicated that the suppressive factors, i.e. biotic plus abiotic factors, in LA-soil also function even in the soil mixture including HA-soil. The disease index was significantly higher in infested HA-soil diluted with SHA-soil than in infested HA-soil diluted with non-sterilized HA-soil at an inoculum level below estimated 10(3) resting spores g(-1) soil. The results suggest that biotic factors play an important role in disease suppression even in conducive HA-soil infested with a low inoculum level of the pathogen. (C) 2000 Elsevier Science Ltd. All rights reserved.