A Powdery Mildew Fungus Modulates the Impact of an Invasive Plant in North America

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Garlic mustard (Alliaria petiolata) is a European biennial herb that is invasive in North America, where it can impact native plant and microbial populations. In North America, this plant has both escaped European specialist herbivores, and is substantially resistant to North American mustard specialist and generalist herbivores. The powdery mildew fungus, Erysiphe cruciferarum, will infect garlic mustard in the field in North America, where it can strongly inhibit growth and reproduction. Here, we show that garlic mustard infected with powdery mildew also becomes a much weaker competitor with Impatiens pallida, a North American annual herb that co-occurs with garlic mustard in the field. In pot grown plants in the greenhouse, garlic mustard suppressed the growth of Impatiens in the absence of powdery mildew infection. In the presence of infection, Impatiens strongly suppressed garlic mustard. In addition, the presence of mycorrhizal inoculum altered the competitive relationships of Impatiens and garlic mustard and affected the response of Impatiens to mildew infection of competing garlic mustard plants. Impatiens plants transplanted near infected garlic mustard in the field grew substantially larger than transplants placed near healthy garlic mustard. This study reveals that powdery mildew infection of garlic mustard should substantially reduce its competitive impact on native plants in the field, and that healthy mycorrhizal populations will modulate the impact of garlic mustard. Along with specialist herbivores, powdery mildew is present in Europe where garlic mustard is native and may contribute to the regulation of its populations, an effect that may eventually be seen in North America.


Presented at the ESA/SER Joint Meetiing, San Jose, CA.