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Effects of Cultivars and Fungicides on Rice Sheath Blight, Yield, and Quality

The development of sheath blight (Rhizoctonia solani)-resistant rice (Oryza sativa) cultivars will allow producers to use less fungicide and to avoid significant reductions in grain and milling yields. Among cultivars currently in cultivation in the southern United States rice-producing region, sheath blight resistance levels range from very susceptible to moderately susceptible. A study was conducted to determine the response of cultivars with different levels of susceptibility to sheath blight inoculations and fungicide applications and to determine the impact of sheath blight disease development on rice yield and quality. Sheath blight epidemics in field plots were initiated by inoculation at the panicle differentiation growth stage from 2003 through 2005. Azoxystrobin at 0.17 kg a.i. ha^--1 and flutolanil at 0.56 kg a.i. ha^--1 were applied in sequential applications at midboot and 50 to 70% heading. Inoculation significantly increased sheath blight severity and incidence and caused yield losses of 4% in moderately susceptible cv. Francis to 21% in very susceptible cv. Cocodrie. Milling yield was affected to a lesser extent. Fungicide treatments reduced sheath blight incidence and severity regardless of cultivar. Azoxystrobin was more effective than flutolanil in minimizing yield loss due to sheath blight in all cultivars except Francis.