The research conducted in this thesis sought to fill knowledge gaps with regard to pathogenicity of Pyrenophora teres f. teres (Ptt) in Australia, knowledge of genomic regions in Australian differential cultivars that interact with prevalent isolates and identification of resistance and susceptibility QTL in Australian barley breeding germplasm. To successfully breed cultivars with resistance to pathogens within the target growing region, knowledge of the pathogen population is critical. Large

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... hifts in the barley breeding structure in Australia over the last decade has meant that breeders often target broad adaptation of cultivars that allows them to be grown across the entire country, meaning that stable resistance to multiple pathotypes is relevant now more than ever. A collection of Ptt isolates from five Australian states was assayed on differential genotypes at seedling stage. Hierarchical cluster analysis revealed that isolates belonged to four main groups that were each typified via differential virulence to four barley genotypes, Maritime, Prior, Skiff and Tallon. Further differentiation was observed within each of the four groups, suggesting that each group was not equivalent to a single pathotype. Different proportions of virulence were observed in each state and also between eastern, southern and Western Australia and adaption of isolates on locally grown cultivars was considered to be the driving force behind the state based diversity. Prior and Skiff were found to differentiate the greatest number of isolates and isolates from the widest geographic range. The genetics of resistance and susceptibility in these genotypes had not been previously studied. Subsequently, a Prior x Skiff cross was used to develop a population of recombination inbred lines, which was phenotyped at seedling and adult growth stages with two Ptt isolates. Analysis discovered a total of five quantitative trait loci (QTL) on two chromosomes. All QTL in this Chapter co-located with that of previously published studies. Four QTL were located on 6H with two QTL closely linked in repulsion interacting with both isolates in a reciprocal manner, inspection of 256 diverse genotypes confirmed Skiff as the donor of susceptibility of one QTL and Prior was the donor of susceptibility to the other QTL. The undesirable allele for another QTL on