1Department of Crop and Soil Science, Oregon State University, Corvallis, OR 97331 USA; 2ICARDA/CINZM Apdo. 6-641 Mexico 6, DF Mexico

Background and objectives
We are using genome mapping techniques to develop barley germplasm with durable resistance to stripe (a.k.a. yellow) rust, a disease caused by Puccinia striiformis fsp. hordei. Germplasm developed by the ICARDA/CIMMYT program in Mexico is resistant to the spectrum of virulence encountered in South America, Mexico, and the United States. When this germplasm is crossed with susceptible North American cultivars, phenotypic classes in the progeny are not discrete. We have, accordingly, used quantitative trait locus (QTL) techniques to determine the number, location, and allele phase of genes determining the resistance phenotype. Our longer-term objective is to develop a catalog of mapped quantitative and qualitative stripe rust resistance genes. This catalog should lead to a more complete understanding of resistance mechanisms and pathways.

Results and conclusions
We have developed five doubled haploid populations for QTL mapping and one doubled haploid population for mapping qualitative resistance. Resistance QTLs were mapped to chromosomes 4, 5, and 7 in three of the populations. The QTLs on 4 and 7 were mapped in one population and the QTL on 5 was mapped in another (1). Mapping is underway in the other two populations, which do not have QTLs on 4, 5, or 7. Mapping is underway in the qualitative resistance population: the single gene does not map to the QTL regions on 4, 5, or 7. We introgressed the chromosome 4 and 7 QTLs into two different susceptible genotypes, using marker-assisted backcrossing. In one case, target levels of resistance were recovered (2). In the other case, the target level of resistance was not recovered. We have developed a new doubled haploid population to test the hypothesis that a suppressor is limiting expression of resistance in the genetic background where target levels of resistance were not recovered. We have also developed doubled haploid lines that represent pyramids of the resistance QTL alleles on chromosomes 4, 5, and 7. Preliminary data indicate that at least two resistance QTL alleles are required to achieve target levels of resistance. Finally, we are developing pyramids of quantitative and qualitative resistance genes. All our genetic stocks and data are available, and we would welcome cooperators in this effort to characterize, and benefit from, oligogenic and polygenic resistance to barley stripe rust.

1. Hayes PM, Prehn D, Vivir H, Blake T, Comeau A, Henry I, Johnston M, Jones B, Steffenson B, 1996. J. Quant Trait Loci. http://probe.nalusda.gov:8000/otherdocs/jqtl/index.html
2. Toojinda T, Baird E, Booth A, Broers L, Hayes P, Powell W, Thomas W, Vivar H, Young G, 1998. Theor. Appl. Genet. (in press).