Mapping of resistance loci in wheat line Milan/S87230//Babax to South African races of Puccinia striiformis f. sp. tritici

The continuous emergence of new Puccinia striiformis Westend f. sp. tritici Erikss (Pst) races requires a sustained search for useful sources of resistance. A CIMMYT bread wheat line (pedigree: Milan/S87230//Babax; renamed NSRPan4) was identified as highly resistant to South African Pst races. A recombinant inbred line (RIL) mapping population was developed by crossing NSRPan4 with Avocet S. Stripe rust data collected for RILs, using Pst race 6E22A+, include seedling infection types determined under controlled conditions as well as adult plant responses determined in field trials over three consecutive seasons. Quantitative trait loci (QTL) mapping identified stripe rust resistance genes/QTL on chromosomes 2A and 2B, temporarily designated as QYr.ufs-2A.2(Yr17) and QYr.ufs-2B.1, respectively. The QTL interval on chromosome 2A, which includes the Lr37/Yr17/Sr38 gene complex (also referred to as the 2NS/2AS or 2N^VS segment), conferred a significant seedling (R² 23.5%) and adult plant resistance effect (R² up to 54.5%). The QTL on 2B contributed up to 46.4% of phenotypic variance observed. Three minor QTL, temporarily designated as QYr.ufs-1B.1 (possibly Lr46/Yr29), QYr.ufs-3D.1 and QYr.ufs-7A.1 were also detected. Our results support findings that notwithstanding the reported virulence for Yr17, the Lr37/Yr17/Sr38 segment remains useful in combination with other sources of resistance in certain genetic backgrounds and environments. Together with QYr.ufs-2B.1 and in the presence of the detected minor QTL and yet unidentified resistances, it was a consistent major contributor to the high level of stripe rust resistance in NSRPan4 to South African races of Pst.