The over-arching objective of this study was to explore the potential impact of novel A1 and A2 isolates of Phytophthora infestans on the existing population in Ireland and the impact of these new strains on potato blight management efforts. Towards this, activities were executed to determine P. infestans population structure and genetic diversity and afterwards to investigate the potential for changes in P. infestans populations through sexual recombination or host resistance associated adaptive change. Simple sequence repeat (SSR) marker based analysis of the sampled Irish population (in 2009) revealed: (i) moderate genetic diversity (ii) two co-existing clusters delineating with mating type and metalaxyl phenotypes (iii) on-going displacement of existing A1 clonal lineages by new genotypes. In parallel, higher foliar blight aggressiveness on four potato cultivars was recorded for isolates with new genotypes compared to those isolates of ‘traditional’ genotypes. SSR analysis of F1 progeny derived from crosses between the new A1 and A2 isolates revealed levels of genetic diversity, typically expected from mating. However, the F1 progeny were recorded to be less fit on a moderately resistant potato cultivar Cara compared to parental isolates. Following several reiterative host-pathogen interactions, the new A1 and A2 isolates demonstrated the potential to adapt to several of the transgenic potato lines tested, which are equipped with the ‘resistance to blight’ (RB) gene. Adapted isolates showed amino acid substitutions in the carboxylic terminal of the avirulent ipiO protein, which is the complementary target of the RB protein. Separately, efforts to develop a quantitative PCR assay for P. infestans were also explored culminating in a technique capable of detecting asexual propagules of P. infestans in infected tubers. In light of the emergence of novel A1 and A2 strains of P. infestans in Ireland, this study provides valuable information both to the research community and the Irish potato sector as a whole. This is timely in light of the high blight pressure continuously recorded throughout this study and in regards to the developing debate on the testing of GM blight resistant potatoes in Ireland.