Mapping Sheath Blight and Blast QTLs In An Advanced Backcross Bengal/Oryza Nivara Population.

An advanced backcross mapping population was developed from a cross between ‘Bengal’, a popular southern U.S. tropical japonica rice (Oryza sativa L.) cultivar, and an accession of the rice ancestral species, O. nivara Sharma & Shastry (IRGC104705). Previous studies identified this O. nivara accession as resistant to two important rice fungal diseases, sheath blight (SB), caused by Rhizoctonia solani Kuhn and leaf blast, caused by Magnaporthe oryzae B. Couch. To identify quantitative trait loci (QTLs) associated with these resistances, 253 BC₂F_2:3 progeny were genotyped with 131 SSR markers distributed throughout the 12 rice chromosomes. Sheath blight disease was evaluated in the greenhouse using a micro-chamber method and under field conditions using a randomized complete block design with two replications over two years (2008 and 2009). In addition, days to heading, plant height and plant type, which can be confounding factors for rice SB disease in the field, were recorded. Reaction to blast disease was determined using two U.S. blast races, IB-1 and IB-49, in greenhouse screening. The linkage map was developed using JoinMap 4.0 software and QTL analysis was conducted in QGene 4.3.6 software using the single trait multiple interval mapping option. Major QTLs for SB resistance were identified on chromosomes (chr.) 1, 2, 3 and 6 in the field screening. The QTLs on chr. 3 and 6 were identified in both years. The main QTL for days to heading was on chr. 6, in the same location as the SB-QTL. Similarly, the SB-QTL on chr. 3 was also near a QTL for days to heading. The major QTL for plant height was at the location of the semi-dwarf (sd-1) gene on chr. 1, and the major QTL for plant type was on chr. 9 at the location of a previously reported QTL for tiller angle. Only SB-QTLs with low LOD scores were identified on chr. 1, 5, 6, 11 and 12, using the micro-chamber method and some of these SB-QTLs were near SB-QTLs identified in the field evaluation. Major blast QTLs were noted on chr. 8 and 12 for both blast races, and in the location of previously reported blast genes. A minor QTL was found on chr. 5 for race IB-49 and on chr. 6 for race IB-1. Through additional backcrossing of lines containing the putative blast and SB-QTLs from O. nivara with Bengal and subsequent selection using molecular markers, we are developing germplasm with improved blast and SB resistance.