Genetic mapping of sheath blight resistance QTLs within tropical japonica rice cultivars

Most commercial cultivars of rice (Oryza sativa L.) are susceptible to sheath blight (SB), a devastating fungal disease causing significant losses in grain yield and quality. There are limited sources of genetic resistance adapted to U.S. growing conditions, and no commercial long grain cultivar of rice is currently available in the United States with a high level of SB resistance. Sheath blight resistance has been reported to be horizontal and quantitative. A population of 279 [F.sub.2:3] progeny rows derived from a cross between two tropical japonica U.S. rice cultivars, Rosemont (semi-dwarf, SB susceptible) and Pecos (tall, SB resistant), was used to map SB resistance. Progeny families were evaluated for disease reactions, plant height (PH), and heading date (HD) in replicated field trials for 2 yr and genotyped with 149 simple sequence repeat markers. Correlation analysis between SB ratings with PH and HD showed that both agronomic traits were significantly correlated with SB resistance. Four significant (logarithm of odds ratio [greater than or equal to] 3.6) quantitative trait loci (QTLs) were identified for SB resistance, with individual effects explaining 5.6 to 33.4% of the total phenotypic variation. Plant height appears to have a direct influence on SB resistance, with QTLs for these traits colocated on chromosome 1. Consistent results across years indicate the stability of the identified QTLs and their potential for improving rice SB resistance using marker-assisted selection.