Validation and Characterization of Two QTL for Adult Plant Resistance to Stripe Rust on Wheat Chromosome Arms 6DS (Yr77) and 6BS (Yr78)

The spread of more virulent and aggressive races of Puccinia tritici f. sp. tritici (Pst), the causal pathogen of stripe rust since the year 2000 has caused substantial yield losses worldwide. To find new sources of resistance, we previously performed a genome-wide association study and identified two strong QTL for adult plant resistance on the short arms of chromosomes 6D (QYr.ucw-6D) and 6B (QYr.ucw-6B). In this study, we validated QYr.ucw-6D in six biparental populations and mapped the peak of the QTL closely linked to IWA167, between barc54 and cfd188. Since no named Pst resistance gene has been mapped before to this region, QYr.ucw-6D was assigned the gene name Yr77. We also validated QYr.ucw-6B in ten biparental populations, and mapped it 0.6 cM proximal to IWA7257 and 3.9 cM distal to IWA4408. We showed that QYr.ucw-6B is located >15 cM proximal to the seedling resistance gene Yr35, and that none of the resistant lines carry the previously cloned Yr36 gene. Based on these results, QYr.ucw-6B was assigned the name Yr78. Neither Yr77 nor Yr78 was effective against Pst at the seedling stage, indicating that they are adult plant resistance genes. Yr77 can be a useful tool to diversify the sources of Pst resistance in Western wheat breeding programs, where this gene is present at very low frequencies. Conversely, Yr78 is predicted to be present in few Asian wheat varieties, and will be worth to test its effectiveness against Pst races from this region.