Understanding the routes of pathogen introduction contributes greatly to efforts to protect against future disease emergence. Here, we investigated the history of the invasion in North America by the fungal pathogen Microbotryum lychnidis-dioicae, which causes the anther smut disease on the white campion Silene latifolia. This system is a well-studied model in evolutionary biology and ecology of infectious disease in natural systems. Analyses based on microsatellite markers show that the introduced American M.lychnidis-dioicae probably came from Scotland, from a single population, and thus suffered from a drastic bottleneck compared with genetic diversity in the native European range. The pattern in M.lychnidis-dioicae contrasts with that found by previous studies in its host plant species S.latifolia, also introduced in North America. In the plant, several European lineages have been introduced from across Europe. The smaller number of introductions for M.lychnidis-dioicae probably relates to its life history traits, as it is an obligate, specialized pathogen that is neither transmitted by the seeds nor persistent in the environment. The results show that even a nonagricultural, biotrophic, and insect-vectored pathogen suffering from a very strong bottleneck can successfully establish populations on its introduced host.