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 shifts 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 6H was omnipresent in Australian cultivars, while the undesirable allele or the fourth 6H QTL was only found in ancestors and selections of Prior. The QTL on 3H co-located with resistance from Tifang, however further research is needed to ascertain whether the resistance is the same. Selection imposed on the northern region barley (NRB) breeding population has enriched the population for desirable alleles, however the genomic regions associated with resistance and susceptibility to Ptt are unknown. In order to identify QTL associated with desirable alleles, genome-wide association studies (GWAS) of 2012 and 2013 breeding population entries were conducted. Results discovered four QTL, one on 4H and three on 6H. The same reciprocal effect QTL from the previous chapter was re-identified, however the source of the undesirable allele was from the North Dakota (ND) germplasm pool, thus validating the effect of this QTL in unrelated germplasm. One of the other 6H QTL conditioned susceptibility to one isolate and was found to be derived from Moravian and English landraces, furthermore no genotype with this QTL is currently represented in any differential sets other than that detailed in previous research in this thesis. Tallon has been proposed as the representative genotype for this genomic region. The remaining QTL on 6H was contributed by CIho 5791 via the ND parents and is known to condition dominant resistance. The 4H QTL was contributed by PC 84 via the ND parents and is hypothesised to condition a resistance. Utilisation of a diverse panel of genotypes in tandem with both mapping studies was able to uncover genotype lineages that harbour QTL associated with resistance or susceptibility to Ptt, further increasing the direct relevance of the mapping studies to Australian and international germplasm. The knowledge generated in this thesis is will enable Australian barley breeders and researchers to further their understanding of the complex interaction between barley and Pyrenophora teres f. teres.