Comparative germination ability and specific gravity of sclerotia of Rhizoctonia and Sclerotium spp., causing rice sclerotial diseases

Four rice sheath blight-like disease fungi, i.e., Sclerotium hydrophilum, Rhizoctonia fumigata, R. oryzae-sativae, and R. oryzae, were compared with sheath blight fungus, R. solani AG-1-IA regarding three sclerotial characteristics, specific gravity, incubation time until germination and intermittent germination ability. Sclerotia produced on the rice straw medium (RSM) and on the leaf sheaths of mature rice plants were used. Specific gravity of RSM-sclerotia were 0.9 for S. hydrophilum and 1.3-1.4 for the other sclerotial disease fungi, but that of sclerotia produced on rice plants was 0.7 for S. hydrophilum and 1.1-1.2 for the other fungi. Sclerotia of sheath blight-like disease fungi germinated very little or not at all at 6 hrs after incubation on the water agar medium, while R. solani AG-1-IA showed 19-40% at 6 hrs, the highest germination among the 5 fungi. All fungi showed 85-100% of germination at 24 hrs in RSM sclerotia and at 48 hrs in sclerotia produced on rice plants. Sclerotia were intermittently incubated on the water agar medium to learn the number of germination times. Sclerotial germination of the each fungus decreased with the number of incubation times, ranging from 1st to 10th. Regardless of sclerotia differently produced, R. solani AG-1-IA sclerotia consistently exhibited the highest germination percentage among the 5 fungi, and showed 53-97% at the 6th time and 27-86% even at the 9th time. Sclerotia of sheath blight-like disease fungi, especially S. hydrophilum, R. fumigata and R. oryzae-sativae, resulted rapidly in a low germination percentage, 16-67% at the 2nd and 0-14% at the 3rd time, followed by 0-5% at the 4th time or later. These findings regarding physiological characteristics of sclerotia were considered to be important factors in understanding the dispersal of pathogens and disease development of rice sclerotial diseases in paddy fields.