Characterization of Rhizoctonia Species Complex Associated with Rice Sheath Disease in Karnataka

Abstract

Sheath disease of rice is a complex biotic stress, caused by Rhizoctonia solani (sheath blight), R. oryzae (sheath spot) and R. oryzae-sativae (aggregate sheath spot). To determine the extent of distribution of these pathogen species, eighteen isolates of Rhizoctonia spp. were collected from different rice growing agroclimatic zones of Karnataka, India. Causal organisms were isolated from the diseased samples and identified based on morphological and molecular tools. Mycelial characters and sclerotial morphology were considered for species differentiation at culture level. Later, species identity was confirmed through polymerase chain reactions using previously standardized Rhizoctonia species-specific primers. Out of eighteen isolates, 14 isolates characterized as R. solani, two as R. oryzae and remaining two as R. oryzae-sativae. The results indicated that all three species of Rhizoctonia, viz. R. solani, R. oryzae and R. oryzae-sativae, are involved in causing rice sheath disease in Karnataka and R. solani was found predominantly distributed. Though sheath diseases caused by R. oryzae and R. oryzae-sativae are regarded as minor, their complementary role with R. solani in causing severe sheath disease epidemics has to be investigated. Accuracy in distinguishing these pathogens is essential to ensure the success of breeding programmes, which aim to develop rice varieties with resistance to sheath disease complex. As per the information available, this is the first report on the distribution of three species of Rhizoctonia associated with rice sheath disease in Karnataka.