A method for inoculation and evaluation of rice sheath blight disease

Sheath blight of rice, caused by Rhizoctonia solani, is one of the most important rice diseases worldwide; however, no rice cultivar has been found to be completely resistant to this fungus. To facilitate detailed analysis of sheath blight resistance at genetic, molecular, biochemical, and functional genomic levels, new methods were developed for effective and uniform infection and accurate evaluation of the disease. The efficiency of R. solani infection was tested on two resistant (Tetep and Jasmine 85) and two susceptible (Chucheongbyeo, Junambyeo) cultivars using three different inoculum types (agar block, liquid cultured mycelia ball, and mycelia suspension). By covering the inoculated sheaths with aluminum foil to maintain humidity, 100% infection rate was achieved in this study. Liquid cultured mycelia balls caused significantly longer lesions (5.4 cm) than other types of inoculum, including agar block (2.4 cm) and mycelia suspension (1.6 cm). An improved method for evaluating sheath blight disease was selected by comparing two methods for evaluating disease severity among three partially resistant cultivars and five susceptible cultivars inoculated with liquid cultured mycelia balls. In addition, a new formula was developed to calculate the disease susceptibility index. Lesion length and the susceptibility index generally were correlated in each leaf, but there were discrepancies between the two evaluation methods due to differences in plant architecture among the cultivars. The susceptibility index calculated using the new formula was the most accurate method for evaluating sheath blight disease across all cultivars. The effect of heading date and panicle number also was evaluated in relation to sheath blight resistance. Cultivars with late heading dates generally were more resistant to sheath blight than those with early heading dates.