Isolate-specific QTLs of resistance to leaf stripe (Pyrenophora graminea) in the ‘Steptoe’ x ‘Morex’ spring barley cross

Leaf stripe caused by the fungus Pyrenophoragraminea represents a serious threat to grain yield in organicallygrown barley and in conventional Nordic andMediterranean districts, for which resistant cultivars arenecessary. A medium-density, molecular marker map derivedfrom a ‘Steptoe’ (partially resistant) × ‘Morex’(susceptible) spring barley cross and its derived doubledhaploidmapping population inoculated with the fungusmade it possible to identify QTLs of resistance to leafstripe. In order to investigate isolate-specificity of partialresistance, the ‘Steptoe’ × ‘Morex’ segregating populationwas inoculated with two highly virulent P. gramineaisolates, Dg2 and Dg5. The present study demonstratesthat partial resistance to leaf stripe of cv ‘Steptoe’ is governedin part by shared loci and in part by isolate-specificones. One QTL is common to the resistance for the twoisolates, on the long arm of chromosome 2 (2H), twoQTLs are linked on chromosome 3 (3H), and the remainingtwo are isolate-specific, respectively for isolate Dg2on chromosome 2 (2H) and for isolate Dg5 on chromosome7 (5H). The QTL in common is that with the majoreffect on the resistance for each isolate, explaining 18.3%and 30.9% R2 respectively for Dg2 and Dg5. The isolatespecificQTLs mapped in the ‘Steptoe’ × ‘Morex’ barleyreference map support the assumption of Parlevliet andZadoks (1977) that partial resistance may be due to minorgene-for-minor-gene interactions. Map comparisons ofthe QTLs with the known qualitative resistance genes toleaf stripe, Rdg1 (2H) and Rdg2 (7H), as well as with otherQTLs of partial resistance in barley, show that the QTLfor resistance to both isolates mapped on the long arm ofchromosome 2 (2H) does not coincide with the qualitativeRdg1 gene but is linked to it at about 30 cM. One isolate-specific QTL of resistance to P. graminea, mappedon the short arm of chromosome 2 (2H), is coincidentwith a QTL for resistance to Pyrenophora teres previouslymapped in the ‘Steptoe’ × ‘Morex’ cross.