Introgression of rpg4/Rpg5 into barley germplasm provides insights into the genetics of resistance to Puccinia graminis f. sp. tritici race TTKSK and resources for developing resistant cultivars.

Stem rust (incited by Puccinia graminis f. sp. tritici, Pgt) is a devastating disease of wheat and barley in many production areas. The widely virulent African Pgt race TTKSK is of particular concern because most cultivars are susceptible. In order to prepare for the possible arrival of race TTKSK in North America, we crossed a range of barley germplasm - representing different growth habits and end-uses- with donors of stem rust resistance genes Rpg1 and rpg4/Rpg5. The former confers resistance to prevalent races of Pgt in North America, and the latter confers resistance to TTKSK and other closely related races from Africa. We produced doubled haploids from these crosses and determined their allele type at the Rpg loci and haplotype at 7,864 SNP loci. The doubled haploids were phenotyped for TTKSK resistance at the seedling stage. Integration of genotype and phenotype data revealed that: (1) Rpg1 was not associated with TTKSK resistance; (2) rpg4/Rpg5 was necessary but not sufficient for resistance; and (3) specific haplotypes at two quantitative trait loci (QTLs) were required for rpg4/Rpg5 to confer resistance to TTKSK. To confirm whether lines found resistant to TTKSK at the seedling resistance were also resistant at the adult plant stage, a subset of doubled haploids was evaluated in Kenya. Additionally, adult plant resistance to leaf rust and stripe rust (incited by P. hordei and P. striiformis f. sp. hordei, respectively) was also assessed on the doubled haploids in field trials at three locations in the USA over a two-year period. Doubled haploids were identified with adult plant resistance to all three rusts, and this germplasm is available to the research and breeding communities.