Density of Sclerotia ofRhizoctonia solaniand Incidence of Sheath Blight in Rice Fields in Mississippi

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 (.)