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<div>Commercial planting practices and strategic breeding for traits that are desirable for commercial production (fast growth, high fiber content) may inadvertently shape the evolution of pathogens that exist on these trees. Domestication of wild pathogens alongside their host can generate new diseases that can have devastating consequences. The Dothideomycete fungus, <i>Sphaerulina musiva </i>is an endemic fungus that occurs naturally on wild poplar in north-eastern and north-central North America where it causes innocuous leaf infections. In domesticated poplars this fungus causes a new disease that results in wounds (cankers) on the stem of the trees- in the most extreme cases trees literally snap in half. In order to manage disease spread, prevent further incursions and identify genes involved in plant attack, a detailed review of historical records and the genomes of 83 strains of the pathogen were decoded. Genes involved in adaptation across all populations and regional adaptation generating local geographic populations were found. These geographic populations also displayed significant difference in virulence profiles on different genotypes of <i>Poplar trichocarpa</i>. Modeling using genomic profiles shows that the pathogen originated in the US, a center of diversity and has repeatedly spread or been introduced into Canada. The most likely vector enabling these repeated introductions is symptomless poplar material exchanged for commercial plantations or breeding programs.</div>