The invasion of garlic mustard in forests across North America has been explained non-exclusively by the propagule pressure, novel weapons, and enemy release hypotheses, among others. As for the latter, garlic mustard has been known to escape its own European specialist herbivores and to resist North American specialist and generalist herbivores of mustard plants. We recently discovered a powdery mildew fungus attacking garlic mustard throughout southwestern Ohio, and showed that it can impact the growth, reproduction and competitive ability of individual garlic mustard plants from susceptible populations. In this study, we examined the identity, distribution, and host specificity of this fungus.
Sequencing ITS regions of the rDNA of this fungus confirmed it as a strain of Erysiphe cruciferarum, which we had previously determined based on morphological characters. Field surveys identified southwestern and westcentral Ohio as the center of its distribution. In the greenhouse, only one of 15 Brassica crop cultivars or species became infected with this fungus. Four co-occurring native forest understory mustards were susceptible under disease-favorable conditions in the greenhouse, but they displayed phenological escape from this fungus in the field. Finally, garlic mustard populations exhibited qualitative resistance to this species, and the strain examined appeared to be locally adapted to certain garlic mustard populations. This fungal pathogen may be an emerging natural enemy of garlic mustard in its invasive range, but further studies on strain specificity, spatial distribution, and effects on the population dynamics of garlic mustard are needed.