Abstract
Diseased isolates of Rhizoctonia solani (DIRS) which show abnormally slow growth in the early stage (less than half of the hyphal growth of healthy isolates during 5 days on potato dextrose agar) were isolated from the artificially inoculated sugarbeet field soil. DIRS were also obtained from the soil in the sugarbeet root-rot disease declined field. Among DIRS though the degree of disease is different, disease characteristics such as slow growth, hypal distortion, lysis of protoplasm, thin density of hypae in the apical part of the colony, and bad formation of sclerotia are commonly observed. Hyphal growth rate from the basal part of the colony of DIRS is better than that from the apical part of DIRS. DIRS could not be cured by incubation in the medium containing various amounts of streptomycin, ethydiumbromide, chloramphenicol, or cycloheximide. Disease agent(s) of DIRS could not be transferred to healthy isolates by hyphal anastomosis, and dsRNA also could not be detected from DIRS. Pathogenicity of DIRS varies from isolate to isolate, but is rather lower than that of healthy isolates. DIRS could be obtained in any of the R. solani anastomosis groups preserved by routine transfers. It has been suggested that if DIRS increase in field soil, inoculum potential becomes low as their low pathogenicity and slow growth, and consequently, disease is reduced and declines.
