Characterization of sclerotial germination of four rice sclerotial disease fungi, Rhizoctonia and Sclerotium spp.

Four rice sclerotial diseases, i.e., bordered sheath spot (causal fungus: Rhizoctonia oryzae), sheath blight (R. solani AG-1 IA), brown sclerotium disease (R. oryzae-sativae) and small globular sclerotium disease (Sclerotium hydrophilum), occur in worldwide rice-growing areas and severely affect the yield and quality of rice. In an attempt to understand the characteristics of sclerotial germination of these Rhizoctonia and Sclerotium species, the relationship between the germination and hyphal growth at 5-40degC, beginning times for germination, and the number of germinated hyphae was surveyed and microscopic observation of initial stages of germination was also done. Hyphal growth of these 4 fungi was observed at 5-15degC after 48 hr-incubation, but sclerotial germination was not observed at 5-15degC. Consequently, the temperature range for fine growth (25-35degC) showing 50-100% against the maximum growth was found to be narrow, compared to the range of higher sclerotial germination (20-35degC). Beginning times of sclerotial germination on the WA plates proved to be 6 hr for R. oryzae and R. solani AG-1 IA, and 12-24 hr for R. oryzae-sativae and S. hydrophilum. Moreover, R. oryzae was characterized by closely, radially, and linearly germinated hyphae, showing 26-32 in the number on the surface of sclerotia after 6-hr incubation. However, the hyphae of R. solani AG-1 IA, R. oryzae-sativae and S. hydrophilum were sparse, showing 7-17 in the early germination stage, 6-12 hr for R. solani AG-1 IA, and 12-18 hr for R. oryzae-sativae and S. hydrophilum with linearly growing hyphae. These characteristics in sclerotial germination are important findings for hasty identification of pathogens isolated from lesions on rice leaf sheaths and for understanding respective disease development within tillers as well as paddies.