Effect of exogenous nitrogen, size and number of sclerotia on disease severity of sheath blight of rice.

In the study on the effect of inoculum density on disease severity, two sizes of sclerotia, 1.0-1.5 mm and 1.6-3.0 mm diameter, and five levels of sclerotial number, 30, 60, 120, or 480 sclerotia per square meter were used in the field. In the greenhouse test, 1, 2, 4, 4, 8 or 16 sclerotia per 10-cm diameter pot were used. The mycelial growth and lesion number and size increased with increasing inoculum size and/or level. Disease severity and sclerotia produced on IR58 plants were positively correlated with increasing inoculum level for both small and large sclerotia. Among the sources of nitrogen, sodium nitrate with a concentration of 2.5% and a 24-hour dipping time enhanced disease severity on rice plants. The scale for measuring sheath blight disease severity could change depending on the basic parameter used. The infectivity of the inoculum in flooded conditions was influenced by the kinds and populations of associated microorganisms and their antagonistic effect on the growth of Rhizoctonia solani. When sclerotia along with steamed or unsteamed straw were placed in the soil, germinability and colonizing ability of R. solani in inducible soil were greatly affected by straw condition, and/or nitrogen fertilizer application. There was, however, no sequential increase in the two parameters measured with increasing inoculum levels. Inoculum size was important only for initial growth or colonization. Ammonium sulfate improved viability and colonizing ability of R. solani in natural soil. Colonization was not affected till 20 days after burial. Sixty-six percent of a mass of sclerotia was rendered inviable within that period. R. solani possessed vigorous growth and infectivity in vitro. The disease potential was enhanced by the favorable conditions in the field. However, the pathogen's survival and growth in the natural soil were greatly suppressed by more competitive soilborne microorganisms. The subsequent lower inoculum potential could be the only reason for the lower disease of rice sheath blight in the field of present study.