Some microbial properties of suppressive soil induced by successive inoculations of Rhizoctonia solani anastomosis group 2-2

Abstract

Cultivated soil collected in Hokkaido became suppressive to damping-off caused by Rhizoctonia solani AG2-2-IV and AG4 after 4-5 successive inoculations of R. solani AG2-2-IV or AG4 at 2-wk intervals. It is necessary to apply living mycelium of R. solani into the soil for suppressiveness but cultivation of sugar beet is not always necessary. Growth rate of mycelium of R. solani in the suppressive soil was reduced to about one-half of that in the naturally cultivated soil after 5 days of incubation. No correlation was detected between the suppressiveness of the soil and the population of total fungi, bacteria, actinomycetes and Trichoderma spp. in the soil. However, the suppressiveness was enhanced by incubation of natural soil mixed with living R. solani mycelium and a small amount of suppressive soil. This fact indicated that microorganisms parasitic to R. solani may be involved in the suppressiveness. Suppressiveness was eliminated by aerated steam treatment at 55℃ for 30 minutes, indicating that heat-resistant bacteria are not the major suppressive factors. The suppressive factor was not sensitive to benomyl mixed in soil, although members of Trichoderma, Gliocladium and Verticillium, which are well known to be parasitic to R. solani, were sensitive to benomyl. These results suggest that heat-sensitive and benomyl-tolerant microorganisms are the main factors for the suppressive soil.