Isolation and identification of hypovirulent Rhizoctonia spp. from soil

Two-hundred and forty-eight isolates of Rhizoctonia 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 Rhizoctonia spp., 60 were R. solani and 45 were R. zeae. Three isolates of R. solani and 54 of binucleate Rhizoctonia spp, were hypovirulent on radish, whilst all isolates of R. zeae were highly virulent, Hypovirulent strains were isolated most frequently by the plant debris particles isolation method, Hypovirulent isolates of R. solani belonged to anastomosis group 4, whilst the hypovirulent binucleate Rhizoctonia isolates belonged to AG A, AG Ba, AG G, and AG O.
Thirty-two isolates of Rhizoctotria spp, selected for hypovirulence on radish were tested on cucumber in vitro. Only five binucleate Rhizoctonia isolates and one R. solani 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 Rhizoctonia spp.