Identifying novel resistance genes in newly introduced blast resistant rice germplasm

Blast, Magnaporthe oryzae B. Couch, and sheath blight, Rhizoctonia solani Kühn, are major fungal diseases of cultivated rice (Oryza sativa L.) in the USA. Resistance to U.S. M. oryzae races was observed in 91 newly introduced rice accessions, suggesting these accessions are possible sources of novel blast resistance genes (Pi-genes) that could be incorporated into U.S. rice cultivars. The genes Pi-ta and Pi-b have been introduced into U.S. cultivars and characterized molecularly. The objective of this research was to identify new Pi-genes in the aforementioned accessions by differentiating known Pi-genes, determining relatedness of the accessions with SSR markers, and identifying associations of SSR markers with blast resistance and sheath blight resistance. Twenty-seven accessions were identified with resistance to U.S. blast races and as having neither the Pi-ta nor Pi-b gene. Based on 125 SSR markers distributed over the rice genome, 11 of the 27 accessions were closely related to each other, but the remaining 16 accessions had varying levels of genotypic diversity, including two accessions selected from crosses of the Asian cultivated species, O. sativa, with the African cultivated species, O. glaberrima. Blast resistance traits were associated with 32 of the 125 SSR markers and sheath blight resistance traits with 19 markers. Of the 32 blast-associated markers, 20 were located in chromosomal regions previously identified as containing Pi-genes. The remaining 12 markers will provide the basis for discovering additional Pi-genes.