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Key Message: In two Spanish barley landraces with outstanding resistance to scald, the Rrs1 Rh4 locus was fine mapped including all known markers used in previous studies and closely linked markers were developed. Scald, caused by Rhynchosporium commune, is one of the most prevalent barley diseases worldwide. A search for new resistance sources revealed that Spanish landrace-derived lines SBCC145 and SBCC154 showed outstanding resistance to scald. They were crossed to susceptible cultivar Beatrix to create large DH-mapping populations of 522 and 416 DH lines that were scored for disease resistance in the greenhouse using two R. commune isolates. To ascertain the pattern of resistance, parents and reference barley lines with known scald resistance were phenotyped with a panel of differential R. commune isolates. Subpopulations were genotyped with the Illumina GoldenGate 1,536 SNP Assay and a large QTL in the centromeric region of chromosome 3H, known to harbour several scald resistance genes and/or alleles, was found in both populations. Five SNP markers closest to the QTL were converted into CAPS markers. These CAPS markers, together with informative SSR markers used in other scald studies, confirmed the presence of the Rrs1 locus. The panel of differential scald isolates indicated that the allele carried by both donors was Rrs1 Rh4 . The genetic distance between Rrs1 and its flanking markers was 1.2 cM (11_0010) proximally and 0.9 cM (11_0823) distally, which corresponds to a distance of just below 9 Mbp. The number and nature of scald resistance genes on chromosome 3H are discussed. The effective Rrs1 allele found and the closely linked markers developed are already useful tools for molecular breeding programs and provide a good step towards the identification of candidate genes.

Twenty-two landrace-derived inbred lines from the Spanish Barley Core Collection (SBCC) were found to display high levels of resistance to a panel of 27 isolates of the fungus Blumeria graminis that exhibit a wide variety of virulences. Among these lines, SBCC145 showed high overall resistance and a distinctive spectrum of resistance compared with the other lines. Against this background, the main goal of the present work was to investigate the genetic basis underlying such resistance using a doubled haploid population derived from a cross between SBCC145 and the elite spring cultivar Beatrix. The population was genotyped with the 1,536-SNP Illumina GoldenGate Oligonucleotide Pool Assay (Barley OPA-1 or BOPA1 for short), whereas phenotypic analysis was performed using two B. graminis isolates. A major quantitative trait locus (QTL) for resistance to both isolates was identified on the long arm of chromosome 6H (6HL) and accounted for ca. 60% of the phenotypic variance. Depending on the B. graminis isolate tested, three other minor QTLs were detected on chromosomes 2H and 7H, which explained less than 5% of the phenotypic variation each. In all cases, the alleles for resistance derived from the Spanish parent SBCC145. The position, the magnitude of the effect observed and the proportion of phenotypic variation accounted for by the QTL on 6HL suggest this is a newly identified locus for broad-based resistance to powdery mildew.