Isolation and identification of hypovirulent <i>Rhizoctonia</i> spp. from soil

<jats:p>Two‐hundred and forty‐eight isolates of <jats:italic>Rhizoctonia</jats:italic> spp, were obtained from 13 locations in Gifu Prefecture in Japan using the plant debris particles isolation, colonization of bait tissue, and soil‐clump plating methods. Of the isolates, 143 were binucleate <jats:italic>Rhizoctonia</jats:italic> spp., 60 were <jats:italic>R. solani</jats:italic> and 45 were <jats:italic>R. zeae.</jats:italic> Three isolates of <jats:italic>R. solani</jats:italic> and 54 of binucleate <jats:italic>Rhizoctonia</jats:italic> spp, were hypovirulent on radish, whilst all isolates of <jats:italic>R. zeae</jats:italic> were highly virulent, Hypovirulent strains were isolated most frequently by the plant debris particles isolation method, Hypovirulent isolates of <jats:italic>R. solani</jats:italic> belonged to anastomosis group 4, whilst the hypovirulent binucleate <jats:italic>Rhizoctonia</jats:italic> isolates belonged to AG A, AG Ba, AG G, and AG O.</jats:p><jats:p>Thirty‐two isolates of <jats:italic>Rhizoctotria</jats:italic> spp, selected for hypovirulence on radish were tested on cucumber <jats:italic>in vitro.</jats:italic> Only five binucleate <jats:italic>Rhizoctonia</jats:italic> isolates and one <jats:italic>R. solani</jats:italic> isolate were hypovirulent on both species, and these isolates were also hypovirulent on seven other crop species. Cucumber showed wide variation in disease susceptibility to different isolates but hypovirulent isolates exhibited a consistent reaction on five different host cultivars, Pathogenicity tests using cucumber grown in soil also showed consistent reactions with isolates selected either for hypovirulence or virulence. The results support the use of cucumber in bioassays for identifying hypovirulent isolates of binucleate <jats:italic>Rhizoctonia</jats:italic> spp.</jats:p>