Garlic mustard (Alliaria petiolata) is a non-native, allelopathic herb that is invading many forests in the eastern and midwestern US. It has been shown to kill mycorrhizal fungi in laboratory experiments. We hypothesized that garlic mustard would reduce the presence of soil mycorrhizal fungi in soil of forest understories, and that therefore reduce the fitness of native plants, which have symbiotic relationships with mycorrhizal fungi. We collected roots from 6 native herbaceous species that were growing in the presence and absence of garlic mustard and found that the presence of garlic mustard is correlated with lower inoculation of mycorrhizal fungi. In addition, we performed a greenhouse experiment in which the native herb, Impatiens capensis, was grown with garlic mustard, a non-allelopathic mustard species, and alone (control). We find evidence that the presence of garlic mustard results in fewer mycorrhizal fungi and reduced fitness of Impatiens capensis. We conclude that invasion by garlic mustard will degrade the soil and detriment native plants species which rely on mycorrhizal fungi.