Horse chestnut is an important amenity tree species which has been significantly affected over the past decade by a widespread outbreak of bleeding canker disease. Symptoms include rust-coloured or blackened bleeding cankers on the stem and branches, which can lead to tree mortality. The causal agent of this disease is the pathogenic bacterium Pseudomonas syringae pv. aesculi, which is believed to have originated in India on Indian horse chestnut. Development of a real-time polymerase chain reaction diagnostic test for P. syringae pv. aesculi has enabled its rapid detection in symptomatic trees and provides a useful tool for studying host infection and survival outside the host. The pathovar can survive in soil for up to one year and can tolerate lengthy periods of freezing. To better understand the evolutionary history and genetic make-up of this aggressive tree-infecting bacterium, draft genome sequences were generated for seven isolates of P. syringae pv. aesculi from Europe, and a type strain from India. Genomic comparisons suggest that this bacterium probably spread to Europe in the early 2000s via an unknown pathway, with the epidemic across several countries resulting from the introduction of a single bacterial strain. Future genomic comparisons with other P. syringae pathovars combined with functional analyses of genetic pathways should help unravel the key host–pathogen interactions that underlie bacterial diseases of trees.