Validation, Saturation, and Marker-Assisted Selection of Quantitative Trait Loci Conferring Adult Plant Resistance to Powdery

Powdery mildew caused by Blumeria graminis f. sp. tritici is one of the most devastating diseases in wheat (Triticum aestivum) worldwide. Hypersensitive, race specific genes primarily have been deployed to control the disease, however recent efforts have shifted to breeding for more durable resistance, such as ‘adult plant resistance’ (APR). Molecular markers and quantitative trait loci (QTL) associated with APR to powdery mildew must first be validated and QTL effects evaluated in different genetic backgrounds and breeding populations to be useful in marker-assisted selection (MAS) programs. Eighteen simple sequence repeat (SSR) markers previously mapped near the three QTL in Massey for APR to powdery mildew were evaluated for association with APR for powdery mildew in a recombinant inbred line (RIL) breeding population derived from a cross between USG 3209 and a moderately susceptible cultivar Jaypee, wherein Massey is the resistance source for APR in USG 3209. Thirteen new SSR markers were added to the pre-existing genetic linkage maps near the three QTL associated with APR in the Becker by Massey (BM) population. Interval mapping analysis of mildew severity data collected in 2002 (F5:6) and 2003 (F6:7) field experiments with marker genotype data obtained in 2003 (F6:7) confirmed the presence of three QTL for APR on chromosomes 1B, 2A, and 2B in the USG 3209 by Jaypee (UJ) population. The QTL on chromosomes 1B, 2A, and 2B explained 12% to 13%, 59% to 69%, and 22% to 48% of the phenotypic variance for powdery mildew severity in the UJ RIL populations, respectively, in the two field experiments. The efficiency of MAS was examined using powdery mildew data collected in 2002 and 2003 field experiments and also from a greenhouse experiment in 2004 (F7:8), wherein adult plants of the 293 RILs were evaluated for disease severity using a composite of five different isolates of B. graminis. Selection of RILs possessing the QTL on chromosome 2A and to a lesser extent the one on chromosome 1B was effective in identifying powdery mildew resistance in both greenhouse and field experiments, whereas the effect of the QTL on chromosome 2B was insignificant in the greenhouse. Overall, selecting RILs with QTL on chromosomes 2A and 2B was most successful in identifying highly resistant RILs compared to selecting RILs having other combinations of two or three QTL combinations. The RILs possessing both QTL on chromosomes 2A and 2B had mean mildew severities of 4.4% and 3.2% in 2002 and 2003 field experiments, respectively. Breeders implementing MAS programs for APR to powdery mildew via selection of RILs containing the two QTL on chromosomes 2A and 2B likely will obtain RILs having high levels of resistance in the field. However, combining all three QTL may ensure greater durability of APR, on the basis that resistance conferred by QTL on chromosome 2A and 1B are genetically stable across all environments in this study.