Origin and evolution of the kiwifruit canker pandemic

Recurring epidemics of kiwifruit (Actinidia spp.) bleeding canker disease are caused by Pseudomonas syringae pv. actinidiae (Psa). In order to strengthen understanding of population structure, phylogeography, and evolutionary dynamics,we isolated Pseudomonas from cultivated and wild kiwifruit across six provinces in China. Based on the analysis of 80 sequenced Psa genomes, we show that China is the origin of the pandemic lineage but that strain diversity in China is confined to just a single clade. In contrast, Korea and Japanharbor strains frommultiple clades. Distinct independent transmissioneventsmarkedintroduction of thepandemic lineage into New Zealand, Chile, Europe, Korea, and Japan. Despite high similarity within the core genome andminimal impact of within-clade recombination, we observed extensive variation even within the single clade from which the global pandemic arose.