Leaf scald, caused by the fungal pathogen Rhynchosporium commune, is still one of the major diseases in barley growing regions around the globe. The main control strategy today is the application of pesticides. A more sustainable alternative is the use of resistant cultivars. This requires the successful integration of known resistance genes into the elite barley gene pool, as well as the identification of new resistance genes to ensure sufficient variability for resistance management. A prerequisite for any breeding program are tightly linked markers for successful selection of resistant progeny and to avoid linkage drag of undesirable traits.Populations derived from three barley accessions of Spanish origin (SBCC145, SBCC154, CIho3515) were assessed for scald resistance in the greenhouse and field. Genetic maps were constructed to locate the resistance loci and identify markers tightly linked to these loci. SBCC145 and SBCC154 were selected from the Spanish Barley Core Collection for their outstanding resistance against several isolates of R. commune. Genetic markers HVM0027 and STSagtc17 showed strong linkage with the observed resistance, giving reason to the assumption that scald resistance gene Rrs1 might be present in both accessions. Subsequently both populations were genotype with the Illumina GoldenGate 1,536 SNP Assay and a QTL analysis was conducted. The analysis revealed a major QTL on the long arm of chromosome 3H very close to the centromere in both populations, confirming the Rrs1 locus. A panel of differential isolates indicated that the allele present in both SBCC145 and SBCC154 is Rrs1Rh4. Rrs1Rh4 was mapped as a binary trait and included in a consensus map of both populations. Flanking markers are 11_0010 proximally and 11_0823 distally with 1.2 and 0.9 cM distance respectively.CIho3515 has been well known for its reported consistently excellent performance in resistance tests elsewhere as well as in experiments conducted at the LfL. Many hypotheses had been postulated as to the resistance loci present in this accession. However an attempt to positively identify and map these loci has never been made. Assessment with five different R. commune isolates revealed two independent major resistance loci. SSR marker HVM0027 again showed strong linkage with the observed resistance, and a QTL analysis confirmed the presence of an Rrs1 allele in CIho3515. The differential reaction to R. commune isolate Rhy174 revealed that this allele of Rrs1 is different from the one present in SBCC145 and SBCC154. To avoid any confusion the designation Rrs1CIho3515 was selected. The second resistance locus present in CIho3515 was located on chromosome arm 6HS. The QTL for this resistance gene were located towards the distal end of the chromosome arm, refuting the initial assumption that CIho3515 might be carrying an allele of Rrs13. So far only QTL had been reported in this area. The apparently new resistance gene was therefore denominated Rrs18. The population derived from CIho3515 was assessed in the field as well. Subsequent MAS revealed that both loci are sufficiently effective in the field to keep the infection level below the threshold for pesticide application. The MAS also confirmed that 11_0205 and scsnp07305 are suitable markers for Rrs1CIho3515 and Rrs18, respectively.In total, two different alleles of the Rrs1 resistance gene, Rrs1Rh4 and Rrs1CIho3515, as well as a new scald resistance locus, Rrs18, have been identified in this work. In addition markers are presented making these genes accessible for practical breeding. From a scientific point of view this work presents sound basis to further investigate the complex nature of the Rrs1 locus, and to pinpoint the location of the newly identified locus Rrs18 in barley.
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