Biogeographic variation in resistance of the invasive plant, Alliaria petiolata, to a powdery mildew fungus, and local influences on the prevalence of resistance

Background/Question/Methods

While apparently benefiting from enemy escape for several decades in North America, the invasive plant Alliaria petiolata has increasingly shown evidence of being attacked by several pathogenic fungi and bacteria. A fungal pathogen has been identified through morphological and molecular analyses as a strain of Erysiphe cruciferarum, the causal agent of powdery mildew disease in crucifers, and susceptibility of different garlic mustard populations to this pathogen varies in a qualitative fashion. To examine large-scale patterns in the distribution of powdery mildew resistance, we screened populations from throughout its invasive and native ranges under common greenhouse conditions for susceptibility to powdery mildew disease.  To examine local influences on the distribution of the resistance trait within populations, we examined the outcome of competition between individuals from resistant and susceptible populations from the invasive range in the presence and absence of the pathogen. 

Results/Conclusions

A screen of over 80 populations from both the invasive and native range revealed that 55 of them were resistant and 25 susceptible to a strain of powdery mildew from southwestern Ohio. The susceptibility trait appears to be randomly distributed among populations in the invasive and native range, but there appear to be some large areas that hold only resistant populations. Individuals from resistant populations outcompeted individuals from susceptible populations in the presence of the powdery mildew fungus, but the outcome of competition in the absence of the pathogen was less predictable.  No clear biogeographic pattern in distribution of the resistance trait emerged, suggesting that the current distribution of resistance to the pathogen is influenced by multiple introductions of garlic mustard and the outcome of competition between resistant and susceptible individuals.  Efforts are continuing to screen 30 additional populations for mildew resistance and to model the spatial distribution of resistance in the field.