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Density of Sclerotia of Rhizoctonia solani and Incidence of Sheath Blight in Rice Fields in Mississippi. J. P. Damicone, Mississippi Cooperative Extension Service, P.O. Box 68, Stoneville 38776. M. V. Patel, and W. F. Moore. Department of Plant Pathology and Weed Science, Mississippi State University, Mississippi State 39762. Plant Dis. 77:257-260. Accepted for publication 3 November 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0257.

Sixty-five fields in various rotation sequences of Lemont rice with soybean were surveyed over a 2-yr period. Density of sclerotia of Rhizoctonia solani was determined prior to planting, and incidence of sheath blight was assessed at midseason. Sclerotia were detected in 80% of fields sampled. Sheath blight was present in 79.5% of the 39 fields cropped to rice during the study. Disease incidence in 33% of the fields exceeded the current threshold for fungicide treatment of 15% incidence of disease foci. Density and viability of sclerotia and disease incidence increased linearly (P < 0.05) with number of years cropped to rice in the 3 yr prior to sampling. Relationships for years cropped to soybean were converse with those of rice. No viable sclerotia and few disease foci were detected in fields following three years of soybean culture. Rotation with soybean in an area where occurrence of soybean aerial blight is rare did not increase inoculum of R. solani or the incidence of sheath blight in rice. Densities of total sclerotia were positively correlated with incidence (%) of diseased tillers (r = 0.540.63) and incidence (%) of disease foci (r = 0.640.67). However, variability interfered with the development of models to accurately predict sheath blight incidence. When density of viable sclerotia was used, there was an increase in the degree of correlation with disease incidence in only 1 yr.