Searching for germplasm resistant to sheath blight from the USDA rice core collection

Sheath blight, caused by the fungal pathogen Rhizoctonia solani, is one of the most important and widely distributed diseases capable of infesting numerous crops including rice (Oryza sativa L.). Resistant germplasm with wide variation is essential for controlling this disease via breeding efforts, and genetic background helps design breeding strategies. We used the microchamber method to evaluate the USDA rice core collection, including 1794 accessions from 114 countries, for sheath blight resistance. Seventy two molecular markers were used for genotyping. Compared with 'Jasmine 85', which has been recognized as one of the most resistant germplasm accessions, 52 accessions were significantly more resistant at the 5% level of probability, and of these 17 were more resistant at the 1% level of probability. The resistant accessions originated from 26 countries in nine geographic regions and are diversified for 13 phenotypic characteristics. The resistant accessions all belonged to the cultivated species O. sativa. Genetic analysis using 72 molecular markers revealed that 45 resistant accessions (87%) were indica type, which were further classified in two groups. Three accessions were identified as aus, two as aromatic, and one each as temperate japonica and tropical japonica. Breeders could use these findings to choose sheath blight resistant accessions for cultivar improvement.