COMPARATIVE VIRULENCE OF STRAINS OF RHIZOCTONIA SPP ON LEAFY SPURGE (EUPHORBIA-ESULA) AND DISEASE REACTIONS OF CULTIVATED PLANTS IN THE GREENHOUSE

Six multinucleate and two binucleate strains of Rhizoctonia spp. patho genic to the weed leafy spurge (Euphorbia esula) were compared in aggr essiveness. Pathogenicity was tested by inoculating stems of leafy spu rge or planting roots or seeds in soil infested with Rhizoctonia strai ns (8 cfu/g). Two multinucleate strains were significantly more virule nt on roots of leafy spurge than the other strains. Eleven cultivated plant species were found to be susceptible to at least one of the eigh t Rhizoctonia strains, having mean disease ratings significantly diffe rent (P = 0.05) from those of control plants. Two or more strains caus ed significantly different mean disease ratings in eight of these host species, indicating that there was variation among strains. Four stra ins had equally broad host ranges of six plant species, but their resp ective host ranges were not identical. The two binucleate strains, whi ch ranked lowest in overall aggressiveness to leafy spurge, also had r elatively narrow host ranges of one and three species. The results ind icate variation in aggressiveness to leafy spurge and in host range am ong strains of Rhizoctonia spp., from which optimum biocontrol strains may be selected for appropriate use.