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. pathogenic to the weed leafy spurge (Euphorbia esula) were compared in aggressiveness. Pathogenicity was tested by inoculating stems of leafy spurge or planting roots or seeds in soil infested with Rhizoctonia strains (8 cfu/g). Two multinucleate strains were significantly more virulent on roots of leafy spurge than the other strains. Eleven cultivated plant species were found to be susceptible to at least one of the eight Rhizoctonia strains, having mean disease ratings significantly different (P = 0.05) from those of control plants. Two or more strains caused significantly different mean disease ratings in eight of these host species, indicating that there was variation among strains. Four strains had equally broad host ranges of six plant species, but their respective host ranges were not identical. The two binucleate strains, which ranked lowest in overall aggressiveness to leafy spurge, also had relatively narrow host ranges of one and three species. The results indicate variation in aggressiveness to leafy spurge and in host range among strains of Rhizoctonia spp., from which optimum biocontrol strains may be selected for appropriate use.