| Summary: | The existence, nature and possible functions of double-stranded RNA (dsRNA) was studied in strains of <I>Rhizoctonia solani</I> anastomosis group (AG) 3, which infects potatoes, and in some other AGs of <I>R. solani. </I>The aim was to determine whether dsRNA might be exploited as a basis for reducing the virulence of <I>R. solani</I> strains, as occurs in <I>Cryphonectria parasitica,</I> a pathogen of chestnut trees. Isolates of <I>R. solani </I>AG 3 were obtained from potato tubers from a single field site, and from geographically distant sites. DsRNA was found to be ubiquitous, with multiple elements present in each strain, as determined by CF11 cellulose chromatography. Similar gel banding patterns were observed between strains isolated from separate tubers within a single field site; however, banding patterns differed between isolates from diverse sources. All the AG 3 isolates were assessed as being weakly virulent in seedling assays on six host crops (carrot, cress, lettuce, onion, radish, tomato). Attempts to "cure" strains of dsRNA by repeated hyphal tip, subculturing or by growing strains in the presence of cycloheximide were generally unsuccessful; although some individual dsRNA bands were lost, they sometimes reappeared, potentially indicating the presence of a chromosomally, integrated copy of the dsRNA. Partially cured strains were unaltered in virulence compared with their respective parental strains. To determine whether dsRNA elements might be transmitted throughout field populations by hyphal anastomosis, strains were paired in various combinations on agar and examined microscopically. Strains from single tubers were compatible with one another; but isolates from different tubers showed a high degree of incompatibility with one another, and isolates from separate fields were incompatible with each other.
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