The one constant for microbial pathogens in a dynamic natural world is change. It has been suggested that microbial genomes harbor large numbers of standing genetic variants because it is adaptive to have abundant raw material on which for selection to act (McInerney et al. 2017). One type of gene that we expect to find differentiating along with pathogen ecology are effectors. Effectors are secreted proteins that alter the local environment surrounding the cell, either by altering the microbiome/microbial ecology or the host directly. Often considered determinates of host range, effectors are an important class of gene for the understanding of infection outcomes. In addition to effector load, infection success is influenced by host ecology. It remains an open question whether strains classified into pathogenic varieties based on host-of-isolation are actually adapted to those hosts. With my dissertation, I ask a series of questions aimed at elucidating the ecological adaptation of Pseudomonas syringae. First, I perform a pangenome analysis on strains from multiple, ecologically distinct sources and ask how gene content delineates across groups. Next, I focused on five strains and performed a series of controlled growth assays and infections in common lab cultivation media as well as the pathogen’s own, and each other’s, hosts-of-isolation. Thus, I found evidence that local adaptation is present, but not universal, in this experimental system. Finally, I attempted to generate a TnSeq dataset that would enable me to identify a host-specific accessory genome per each of the five pathogens. However, I was unable to generate this dataset and instead present my best explication of my attempts in the hopes that it will benefit future attempts. This dissertation informs our general understanding of genome evolution and ecological adaptation in P. syringae.