QTL Analysis of a Novel Source of Barley Seedling Resistance Effective Against the Virulent North American Stem Rust Pathogen.

Wheat stem rust caused by the obligate biotrophic fungal pathogen Puccinia graminis f. sp. tritici (Pgt) is an important disease of barley and wheat worldwide. Alarmingly, the Pacific Northwest (PNW) contains a highly virulent Pgt population on barley. This population includes the Pgt isolate Lsp21 which is virulent on the barley stem rust resistance genes Rpg1, Rpg2, Rpg3, rpg4, Rpg5, and rpg8. The virulence on barley lines containing Rpg1 and the rpg4/Rpg5-mediated resistance locus (RMRL), when stacked together is a Pgt virulence profile on barley that had not been previously reported, thus, represents the most virulent Pgt isolates on barley R-genes characterized worldwide. The line Elliot (PI 592261) was identified from the world barley core collection as containing effective seedling resistance to Pgt isolate Lsp21. To genetically characterize the resistance present in Elliot, 129 recombinant inbred lines were developed by advancing a population from the cross Elliot (resistant) x Palmer (susceptible) to the F₆ generation. The population was phenotyped with Pgt isolate Lsp21 at the seedling stage and genotyped with the Illumina 50K bead express SNP chip, resulting in 7,284 high-quality SNP markers. Two significant resistance QTL (EPRpg_4H-1 and EPRpg_5H-1) contributed by Elliot were identified on chromosomes 4H and 5H, respectively. The major QTL, EPRpg_4H-1, is novel, while EPRpg_5H-1 localized to a region ~9 Mbp distal of RMRL within a region of the barley genome that contains previously identified stem rust resistance loci. These QTL should be useful in developing barley cultivars with resistance to the virulent PNW Pgt population.