Characterization and Pathogenicity ofRhizoctoniaspecies from a Reduced-Tillage Experiment Multicropped to Rye and Soybean in Florida

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Abstract
Soil from a field maintained under reduced-tillage and multicropped to rye and soybean was sampled to a depth of 5 cm and assayed for Rhizoctonia spp. Nine anastomosis groups (AGs) or species of Rhizoctonia collected from the field were characterized. In general, six of these could be identified on the basis of cultural morphology on Difco potato-dextrose agar. In greenhouse studies, isolates from 2 of these groups (AG 4 [R. solani] and CAG3 [binucleate AG of Rhizoctonia spp.]) were pathogenic to soybean seedlings; only AG 4 isolates were pathogenic to rye seedlings. A positive correlation (r2 = 0.77) between the ability of AG 4 isolates to colonize autoclaved soil and their apparent virulence on soybean seedlings was observed during pathogenicity trials. Isolates of AG 4 were not recovered from noncultivated Arredondo fine sand in the vicinity of the experimental field but were frequently isolated from the same soil in which hosts susceptible to the pathogen were planted. Isolates of AG 4 and CAG 3 are probably found in this field as a result of their pathogenicity on soybeans or rye. Although the role(s) of other species of Rhizoctonia recovered from this field is not known, several possible reasons for their existence in this soil are suggested.