Abstract
European forests are experiencing extensive invasion from the Ash pathogen Hymenoscyphus fraxineus, an ecological niche competitor to the non-pathogenic native congener H. albidus. We report the genome-wide diversity and population structure in Asia (native) and Europe (the introduced range). We show H. fraxineus underwent a dramatic bottleneck upon introduction to Europe around 30-40 generations ago, leaving a genomic signature, characterized by long segments of fixation, interspersed with “diversity islands” that are identical throughout Europe. This means no effective secondary contact with other populations has occurred. Genome-wide variation is consistently high within sampled locations in Japan and the Russian Far East, and lack of differentiation amongst Russian locations suggests extensive gene flow, similar to Europe. A local ancestry analysis supports Russia as a more likely source population than Japan. Negligible latency, rapid host-range expansion and viability of small founding populations specify strong biosecurity forewarnings against new introductions from outside Europe.