Model of Vertical Development of Sheath Blight Lesions on Rice Plants Caused by Rhizoctonia solani Kühn

Abstract

Vertical disease development of sheath blight caused by Rhizoctonia solani Kühn was investigated with a rice cultivar, Koshijiwase, under different temperature and relative humidity. Model curves of the vertical development of lesions were made according to different temperature, relative humidity and susceptibility of the rice plants. Vertical development of lesions on rice plants under 100% relative humidity (RH) was 0.48cm at 20C; 1.13cm at 23C; 1.35cm at 25C and 1.58cm at 28C. The first model curve was obtained by a calculation of the vertical development of lesions under the average temperature. The ratios of vertical development of lesions under different relative humidity to that under 100% relative humidity (at 25C) were 0.99 at 98% RH; 0.96 at 95% RH; 0.87 at 90% RH and 0.38 at 86% RH. The second model curve was obtained as modified the first one according to the relative humidity. The susceptibility of leaf sheaths to a sheath blight fungus correlated with their maturity. The vertical development of lesions observed on 2-, 5-, 10-, and 15-day-old sheaths after heading under the most favorable conditions (at 28C, 100% RH) was 1.12, 1.32, 1.71, and 1.59cm per day, respectively. When the daily calculation values of vertical development of lesions were larger than the values calculated from the susceptibility of leaf sheaths, the values calculated from the susceptibility for each day were used. Thus, the final model curve of the vertical development of lesions calculated from the temperature, relative humidity and susceptibility of leaf sheath almost co-incide with the vertical development of lesions in paddy fields in 1971 to 1981. These models will be useful for forcasting of rice sheath blight.