Abstract:
A study was conducted at Department of Plant Pathology, College of Agriculture, Vellayani to explore the variability in Rhizoctonia so/ani with a view to detect hypoviruient strains of the fungus, and evaluate the efficacy of these isolates in management of sheath blight disease of rice. Six hypovirulent isolates were selected after screerung 28 isolates obtained from different rice growing tracts of Kerala. Screening for hypovirulence was . based on cultural characters like mycelial growth rate and sclerotial production under ill vitro conditions, and pathogenicity tests through cut stalk assay. The hypovirulent isolates were non-sclerotial whereas the virulent isolate produced profuse sclerotia. The hypovirulent isolates produced tan mycelia appressed to the medium In Petri dishes. Antibiosis and hyperparasitism were absent in the interaction between the hypovirulent and virulent isolates in vitro. The character of hypovirulence could not be transmitted from hypovirulent to the virulent isolate. The hypovirulent isolates had no effect on the germination and seedling vigour of rice plants. They did not produce symptoms of sheath blight on susceptible variety. Two of the six hypovirulent isolates selected viz A 13 and GR23 which were binucleate and did not anastomose with the selected virulent isolates, offered the maximum protection to the rice plants from sheath blight. The plants treated with GR23 showed immune reaction to challenge inoculation with virulent isolate, showing no symptom production on challenge inoculation with virulent isolate. The protective effect was found to be higher than that of chemical and biological control treatments. The phenol oxidase activity in the plants treated with the hypovirulent isolates was found to be significantly higher than that of the plants treated with virulent isolate while the peroxidase activity was higher in the plants treated with virulent isolate. Among the different carrier materials tested for hypovirulent Rrsolani rice bran was found to be good for long term storage and coirpith was found to be a cheap indigenous material for its mass multiplication.
