Barley stripe rust (BSR), caused by Puccinia striiformis f. sp. hordei, is an important fungal disease of barley in the Americas. Genetic resistance to BSR can be qualitatively or quantitatively inherited. Qualitative resistance alone is risky due to evidence that pathogen virulence can evolve faster than breeders can release new resistant varieties. Pyramiding of resistance QTL and combining quantitative and qualitative sources of resistance are effective strategies for lowering disease severity. The goal of this project is to measure the effects and interactions of genetic factors that determine BSR resistance. After identifying quantitative and qualitative traits affecting BSR resistance in three double haploid populations of barley derived from the crosses of Baronesse/BCD47, Shyri/Galena, and CI10587/Galena, we started a marker assisted backcross program to transfer the resistance alleles at three QTL (chromosomes 1H, 4H, and 5H), at all possible combinations of the three QTL (1H+4H, 1H+5H, 4H+5H, and 1H+4H+5H), and at a qualitative gene (chromosome 7H) into a susceptible Baronesse background. A set of backcross near-isogenic lines (BISON) is being created to allow measurement of resistance allele effects in the same genetic background and the systematic analysis of resistance allele interactions. The i-BISON lines, an intermediate step in the preparation of near-isogenic lines, represent the quantitative resistance alleles on chromosomes 4H and 5H from BCD47, a quantitative resistance allele on chromosome 1H from BCD12 (a double haploid line from the Shyri/Galena cross), and a qualitative resistance allele on chromosome 7H from D3-6/B23 introgressed, separately, into a Baronesse background. The average QTL effects were measured at multiple locations and four components of resistance: latent period, infection efficiency, lesion size, and sporulation density, were recorded at the seedling stage for i-BISON with individual QTL and combinations of QTL in order to identify a correlation between each resistance QTL and a component of resistance.