Pyramiding quantitative trait locus (QTL) alleles determining resistance to barley stripe rust: Effects on resistance at the seedling stage

Durable disease resistance may be achieved by pyramiding multiple qualitative resistance genes in single genotypes and by using quantitative resistance (QR) genes. Quantitative trait locus (QTL) analysis tools can be used to rind determinants of QR. Resistance QTL pyramids may also lead to durable resistance, and they provide independent validation of QTL effects and QTL interactions. We used molecular markers to identify allelic architectures at three previously mapped QTL conferring resistance to barley stripe rust (caused by Paccinia striiformis Westend. f. sp. hordei) in a set of barley (Hordeum vulgare L. subsp. vulgare) doubled haploid (DH) lines. The three QTL are located on three different chromosomes. One parent contributed the resistance alleles at two QTL and another parent contributed the resistance allele at the third QTL. In this report, we focus on resistance at the seedling stage; resistance at the adult plant stage will be addressed in a future report. The DH population was phenotyped for resistance by means of four pathogen isolates that show different patterns of virulence on a set of differentials. We used molecular markers to infer the resistance QTL allele architecture of each DH line. There was no significant QTL X race interaction, although some DH lines showed differential responses to isolates. The effects and locations of two QTL, each tracing to a different parent, were validated. The third QTL did not have a significant effect on disease symptom expression. To maximize the probability of recovering the resistant phenotype, resistance alleles are necessary at both QTL.