Several isolates of Rhizoctonia solani (= Thanatephorus cucumeris) were shown to be affected by Rhizoctonia decline, a degenerative disease.  Studies were made with the isolate 189 (= ATCC 13248), anastomosis group 1, and 189a, a diseased isolate obtained from 189 during routine transfers.  Cultural characteristics of 189a are a white-tan color, floccose texture, irregular appearance, production of few or no sclerotia, and a slow growth rate.  Healthy 189 cultures are brown with mycelium appressed to the agar surface, are uniform in appearance, produce numerous dark sclerotia, and grow rapidly.  Healthy cultures were recovered at a low frequency (1-5%) from 189a by hyphal-tip isolations; these cultures were repeatedly hyphal-tip-transferred and to date, no symptoms of disease have been observed.  Attempts to cure 189a using elevated incubation temperatures, hot water exposures, anitibiotics, and acridine dyes failed.  None of these treatments was any more efficient in the recovery of healthy cultures than hyphal tipping.  The disease was not due to bacterial contamination or toxins.  Basidiospore analysis of 189a showed inheritance of the disease agent to be cytoplasmic.  Transmission of the agent occurs by hyphal anastomosis.  Fusion between 189a and healthy 189 results in the conversion of healthy 189 cultures into the disease-type.  The movement of the disease agent throughout the hyphae is complete since all transfers from infected 189 are diseased.  Attempts to transmit the disease agent from 189a to other anastomosis-group-1 isolates failed.

Additional key words: cytoplasmic inheritance, mycovirus, double-stranded RNA.