Are novel and co-xenic associations common in alien fungal and fungus-like plant pathogens?

Impacts of alien fungal and fungus-like plant pathogens depend on host interactions, including novel associations with native plants and co-xenic associations with alien plants that do not share the same area of origin, both largely unquantified globally. Combining global distribution and association data, we characterised associations across pathogens' introduced regions, calculated host species richness (SR) and phylogenetic divergence, and examined predictors of novel/co-xenic associations. Novel/co-xenic alien pathogen-plant association records were common (53%) and dominated by widespread generalists. They were found in about one-third of alien pathogens examined, mostly within their known phylogenetic host range. However, 46% of associations remained unclassified due to limited knowledge of pathogen biogeography. Novel/co-xenic associations were more likely for pathogens with larger introduced ranges, wider phylogenetic host range, economically used hosts co-occurring in the region, and host communities phylogenetically similar to the recipient regions' plant community. They were also more likely for host plants with no economic use and restricted native ranges, and in regions with higher plant SR. Novel/co-xenic associations are common and shaped by contact opportunity, phylogenetic host range, and regional community composition. Unravelling fungal and fungus-like pathogen biogeography is key to assessing their full magnitude.