Abstract
Damping-off disease of radish seedlings declined in the soil into which normal field isolate 1271 belonging to Rhizoctonia solani AG-4 was repeatedly (3 times or more) inoculated. Diseased isolates which show abnormally slow growth were isolated from this disease declined soil. By incubating normal and diseased isolates in pair, diseased agent(s) of diseased isolate could be transferred to isolate 1271 by hyphal anastomosis, but not to other isolates of AG-4 and AG-2-2. This shows transmission of diseased agent is isolate-specific. Diseased isolates produce acidific substances, particularly oxalic acid. In comparison with that of isolate 1271, pathogenicity of diseased isolates is extremely low, though it varies from high to low. Using as inocula mixtures of isolate 1271 and diseased isolates, damping-off disease of radish seedlings decreased when inoculum of isolate 1271 was low, and cross protection could be seen among isolates.
