Whole Genome Mapping of Stripe Rust Resistance QTL and Race-specificity Related to Resistance Reduction in Winter Wheat Cultivar Eltan.

Winter wheat Eltan has been one of the most widely grown cultivars in the U.S. Pacific Northwest. It has shown variable levels of resistance to stripe rust in different years since it was released in 1990. To map all currently effective and defeated resistance genes in Eltan and understand the factors causing the resistance changes, 112 F2:5 recombinant inbred lines (RIL) were developed from a cross of Eltan with Avocet S. The RILs were evaluated in fields of Pullman, WA in 2015, 2016, 2017, and 2018 and Mount Vernon, WA in 2016 and 2017 under natural infections; and in the greenhouse with races PSTv-4 and PSTv-40 of Puccinia striiformis f. sp. tritici (Pst). The RILs were genotyped with the 90K Illumina iSelect wheat SNP chip. A total of five quantitative trait loci (QTL) were identified in Eltan. Two major QTL on chromosome arms 2BS and 4AL were detected in the greenhouse tests, explaining up to 28.0% and 42.0% of phenotypic variation, respectively. The two race-specific QTL were also detected in some field experiments but with reduced effects. A minor QTL on 5BS was detected in the greenhouse and field tests, explaining 10.0-14.8% phenotypic variation. The other two minor QTL were mapped on 6AS and 7BL and detected only in field experiments, explaining up to 20.5% and 13.5% phenotypic variation, respectively. All stripe rust samples collected in the experimental fields in 2015 and 2016 were identified as Pst races virulent on seedlings of Eltan. The resistance reduction of Eltan was caused by changes of the Pst population from avirulent to virulent, overcoming the race-specific all-stage resistance in Eltan.