Disease severity is greater with larger pathogen populations in the soil, earlier infection, wet environmental conditions, susceptible cultivars, and cultural management that favours thick canopies (Gangopadhyay and Chakrabarti, 1982 Savary et al., 1997 Groth and Bond, 2006). Under conditions favourable for disease development, rice grain yield losses, ranging from 4% to 50%, have been attributed to sheath blight (Lee and Rush, 1983 Marchetti, 1983 Marchetti and Bollich, 1991). The most important rice disease in the southern United States (Groth and Lee, 2003) and the second most important in the world (Ou, 1985) is sheath blight, caused by Rhizoctonia solani Kühn AG1-1A (Rush and Lee, 1992). Single azoxystrobin applications were effective in minimizing yield loss due to sheath blight in all cultivars. Fungicide treatments reduced sheath blight incidence and severity, regardless of cultivar except in Jupiter. Milling yields were affected to a lesser extent. Inoculation significantly increased sheath blight severity and incidence and caused yield losses of 8% in moderately resistant cv. Azoxystrobin at 0.17 kg a.i. ha −1 was applied at mid-boot. Sheath blight epidemics in field plots were initiated by inoculation at the panicle differentiation growth stage in 20. A study was conducted to determine the response of cultivars, with different levels of susceptibility, to sheath blight inoculations and fungicide application and to determine the impact of sheath blight disease development on rice yield and quality. Among cultivars currently in cultivation in the southern United States, sheath blight resistance levels range from very susceptible to moderately resistant. The increased number of sheath blight ( Rhizoctonia solani)-resistant rice ( Oryza sativa ) cultivars available will allow producers to use less fungicide and avoid significant reductions in grain and milling yields.
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