Characterization and pathogenicity of anastomosis groups of Rhizoctonia solani isolated from Beta vulgaris

Of 361 cultures of Rhizoctonia isolated from sugar beet seedlings, 36 isolates were binucleate. The remaining 325 cultures were multinucleate with characteristics typical of R. solani and were identified to six anastomosis groups (AG): AG-4 predominated and infected 44.3% of the seedlings, followed by AG-5 (27.1%), AG-2-2 (19.7%). AG-1 (1.8%), AG-2-1 and AG-3 (0.3% each); 6.5% of the cultures did not anastomose with any of the tester cultures (AG-1 through AG-9 and AG-B1). About 96% of the 163 cultures isolated from rotted roots greater than or equal to 8 wk old were AG-2-2, 3.7% were AG-4, and 0.6% were AG-5. Pathogenicity tests on sugar beet seedlings in the greenhouse showed that AG-1, 2-2, and 4 severely reduced stands, whereas AG-2-1, 3, 5, and the unidentified isolates were less pathogenic; the binucleate isolates of Rhizoctonia were nonpathogenic when the same cultures were inoculated on 8- to 9-wk-old sugar beet roots, only AG-2-2 isolates were highly virulent; AG-4 isolates produced superficial lesions near the point of inoculation, and isolates of the other anastomosis groups and binucleate isolates were nonpathogenic