Technical Abstract: Why do some exotic plant species become invasive? Two common hypotheses, increased resource availability and enemy release, may more effectively explain invasion if they favor the same species, and therefore act in concert. This would be expected if plant species adapted to high levels of available resources in their native range are particularly susceptible to enemies, and therefore benefit most from a paucity of enemies in their new range. We tested this possibility for the first time by examining how resource adaptations influence pathogen richness and release among 243 European plant species naturalized in the United States. Plants adapted to mesic environments hosted more fungi than plants adapted to xeric environments, and plants adapted to nitrogen-rich environments hosted more viruses than plants from nitrogen-poor environments. Plant species previously classified as competitors (adapted to high resource availability) hosted more than four times as many species of fungi and viruses in Europe as did stress tolerators (adapted to low resource availability). Patterns of enemy release mirrored those of pathogen richness: competitors were released from many pathogen species, while stress tolerators and species from xeric and nitrogen-poor environments were released from relatively few pathogen species. These results suggest that enemy release contributes most to invasion by fast-growing species in resource-rich environments. Consequently, enemy release and increases in resource availability may co-occur, and act synergistically to cause invasion. Furthermore, anthropogenic increases resource availability may indirectly increase enemy release (by selecting for strongly-released fast-growing species), and therefore favor exotic over native species.