Genome-wide population diversity in Hymenoscyphus fraxineus points to an eastern Russian origin of European Ash dieback. Ash_dieback_ddRAD

European forests are experiencing extensive invasion from the Ash pathogen Hymenoscyphus fraxineus, an ecological niche competitor to the non-pathogenic native H. albidus. Here 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 30-70 generation ago, leaving a genomic signature characterized by long segments of fixation, interspersed with “diversity islands”. As these characteristics are identical throughout Europe no effective secondary contact with native 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 Russian populations as a more likely source of the introduction than Japan. Negligible latency, rapid host-range expansion and viability of small founding populations specify strong biosecurity forewarnings against new introductions from outside Europe.