Resistance to powdery mildew in a doubled haploid barley population and its association with marker loci

The genetic basis of resistance to powdery mildew (Erysiphe graminis DC. f. sp. hordei Marchal) was analyzed using doubled haploid barley (Hordeum vulg are L.) lines from the cross Harrington/TR306. Based on infection types obs erved after inoculation with defined single-conidium isolates, the lines we re classified into four groups. The observed phenotypic ratio fit a two-loc us model. The two putative loci were mapped relative to molecular markers. One coincided with the previously mapped dMlg locus on chromosome 4. Based on the observed infection types, Harrington carries the Mlg resistance alle le, and TR306 carries a second locus on chromosome 7 (5H); this was tentati vely designated Ml(TR). It is the first reported race-specific powdery mild ew resistance gene located on that chromosome. These two loci were also det ected by simple interval mapping of disease severity data from naturally in fected field plots. Composite interval mapping with the first two resistanc e loci as co-factors detected an additional locus on chromosome 6, with a m inor effect on resistance. Finally, superimposing the race-specific classif ication onto the field data provided evidence for a minor-effect locus on c hromosome 7 (5H). The Mlg locus had the largest effect, the Ml(TR) locus ha d an intermediate effect and the other two loci had very small effects. Thi s study demonstrates the effectiveness of an integrated approach to identif ying and mapping resistance loci using classification data from inoculated experiments and quantitative data from field experiments.