Biological Agents Affecting the Viability of Resting Spores of Plasmodiophora brassicae Wor. in Soil without Host Roots

Abstract

Viability of the resting spores of Plasmodiophora brassicae incubated in natural and autoclave-sterilized soils without host roots, was compared between the two soils. Although the increases of nonviable spore frequency were found in both natural and sterile soils, they were suppressed in natural soils in spite of almost the same pH values as that of the sterile soils in each test soil. The suppression in natural soils, however, varied among four test soils with an approximately optimum pH range for the frequency increase of nonviable spores. The suppression in natural soil from an uncultivated area, which was lower in populations of microorganisms, but higher in the content of total C and humus and the C/N ratio than those in the other three natural soils from fields, was less pronounced than the suppression in the others. The suppression was eliminated by gas-sterilization of natural soils. Mixtures of natural soils with sterile soils reduced the suppression. Addition of glucose in natural soils also reduced the suppression. These results suggest that the suppression of increase in the frequency of nonviable resting spores in soil is influenced by biological agents associated with the activity of microorganisms.