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Pathogenicity of Rhizoctonia spp. and Sclerotium rolfsii on wheat and peanut and genetic variation among Rhizoctonia isolates

Choppakatla, Vijaykumar
Scope and Method of Study: Studies were conducted to determine the pathogenicity of two soilborne fungi, Rhizoctonia spp. and Sclerotium rolfsii which cause important diseases on peanut and wheat. In Rhizoctonia study, pathogenicity of isolates G-24 ( Rhizoctonia solani ) and Fellers ( Rhizoctonia cereali) were determined on three winter wheat cultivars (Jagger, 2137 and 2174) and four peanut genotypes (Okrun, Tamspan 90, Southwest Runner and C 209 6-60). Disease severity was measured on a 1-6 scale and also by quantifying the number of infection cushions. In Sclerotium rolfsii study, pathogenicity of four isolates of S. rolfsii from peanut (Melouk, Ft. Cobb, Power St and Durant) and two from wheat (Wheat and ZP-3082) was determined on cultivars previously described for Rhizoctonia . In addition, in vitro and in vivo assays were conducted to determine the activity of endo-polygalacturonase (endo-PG) and oxalic acid produced by two isolates of S. rolfsii (Melouk and Wheat).
Findings and Conclusions: Rhizoctonia isolate G-24 from peanut was the most virulent on all peanut and winter cultivars with significantly more number of infection cushions as compared to the R. cerealis isolate Fellers. Results from Rhizoctonia study suggest that quantification of infection cushions can be a useful technique for screening cultivars for resistance to infections caused by Rhizoctonia . In S. rolfsii study, all isolates from peanut caused significant damage on peanut and wheat cultivars used in the study. Among peanut cultivars, Southwest Runner showed significant resistance to S. rolfsii infection. In vitro Activity of endo-PG and oxalic acid produced by S. rolfsii isolates differed based on the type of carbon source used. In vivo study suggested a definite correlation between the activity of endo-PG and oxalic acid and the virulence of the S. rolfsii isolate.