ISOLATES OF RHIZOCTONIA-SOLANI ANASTOMOSIS GROUP 2-2 PATHOGENIC TO SOYBEAN

Two highly virulent isolates of Rhizoctonia solani, 61D-3 and 65L-2 (ATCC 66489), from diseased soybean (Glycine max) plants grown in Illinois were identified as members of anastomosis group (AG) 2-2. They caused crown and root rot on inoculated plant species in the Chenopodiaceae, Fabaceae, and Poaceae at an inoculum level of 100-mu-g of fresh mycelium per gram of soil in the greenhouse. Both isolates produced few loose, thick-walled sclerotia and maintained their pathogenicity to soybean after 2 yr in storage. The minimum, optimum, and maximum growth temperatures for both isolates were 8, 30, and 40 C, respectively. Single hyphal cells of isolate 65L-2 from fragmented mycelium germinated directly, similar to spores of other fungi. A previously undescribed gobletlike structure developed before a viable germ tube was formed. The number of nuclei per cell varied. Both isolates were thiamine auxotrophic. They differed from most sugar beet isolates at loci aco, est-1, est-2, and pgm as determined by isozyme analysis.