Effects of Rate and Timing of Fungicide Applications on Incidence and Severity of Sheath Blight and Grain Yield of Rice. Elly Van Eeckhout, Graduate Research Assistant, Department of Botany, Louisiana State University, Baton Rouge 70803. M. C. Rush, and Meredith Blackwell. Professor, Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center; and Professor, Department of Botany, Louisiana State University, Baton Rouge 70803. Plant Dis. 75:1254-1261. Accepted for publication 15 June 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1254.

Incidence and severity of rice sheath blight, caused by Rhizoctonia solani, were monitored in 1989 and 1990 from the green ring growth stage to maturity in nonsprayed, inoculated plots and inoculated plots sprayed with propiconazole and other fungicides at different rates and growth stages. Single green ring applications of propiconazole at 0.18, 0.32, or 0.48 kg a.i./ha and sequential green ring and boot sprays (0.18 + 0.18 and 0.32 + 0.32 kg a.i./ha) effectively reduced disease incidence over time. Severity of sheath blight over time, expressed as percent lesion height to total sheath height, was increased by green ring applications. With the loss of fungicidal activity over time, lesions on treated tillers progressed upward at a faster rate and reached higher levels than on nonsprayed tillers. Single boot applications at 0.18, 0.32, or 0.48 kg a.i./ha; boot applications of propiconazole at 0.32 kg a.i./ha followed by heading applications of either benomyl, iprodione, or pencycuron; or boot and heading sprays of benomyl (0.57 + 0.57 kg a.i./ha) had little effect on disease incidence. In contrast, disease severity was significantly reduced by all boot and heading treatment combinations. Disease progress measured as incidence or severity was reduced most effectively by the application of pencycuron at boot and heading (0.18 + 0.18 kg a.i./ha). Consistent positive yield responses compared with the nonsprayed control resulted from all boot and heading applications, whereas green ring applications generally resulted in a negative yield response. Yields were significantly negatively correlated with areas under the disease progress curves (AUDPC) based on weekly severity measurements. Yields were not correlated with AUDPC for disease incidence over time.