Identification of effective inoculum of Rhizoctonia solani (Kuhn) for screening of Wa-Male-Sterile lines in rice against sheath blight disease

Occurrence of sheath blight disease (Rhizoctonia solani Kuhn) in rice (Oryza sativa L.) is a regular feature in tropical South Asia and causes huge economic losses. Till now, no resistant cultivar has been developed against the pathogen and gene(s) conferring complete resistance for the disease are not reported in the available rice germplasm. This necessitates the screening of new rice germplasm against the pathogen to identify the resistance source. Ten newly developed Wild Abortive (WA) cytoplasmic male-sterile (CMS) lines, along with a susceptible cultivar (cv. Pant Dhan-4) were evaluated under green-house conditions in a randomized complete block design, with three replications. Screening was done using fungal mycelia disc (15 mm size diameter) and sclerotia (0.2 mg) to determine the potentiality of inoculum. Genotypes were assessed for total lesion length (cm) at 3, 6, 9 and 12 days after inoculation. The response in relative lesion length varied significantly depending upon sclerotia or mycelium discs used and genetic background. During initial period of disease development, non-significant variation was observed but later on the response of genotypes was clearly distinguishable when lesion size gradually increased. Disease severity index was comparatively higher for sclerotial inoculation than mycelial inoculation indicating sclerotia are more effective. The genotype UPRM 271-8-5EUI-3-2A showed the lowest severity in both the methods of inoculation. The highest severity was recorded for UPRM 78-4-1A (mycelial inoculation) and UPRM 271-8-5EUI-6-4A (sclerotial inoculation). Disease appearance was more uniform and higher in sclerotial inoculation when compared with mycelial inoculation. Screening of rice germplasm using uniform sized sclerotia and disease assessment at one week after inoculation were found to be the most effective for screening of rice cultivars.