When exposed to native or introduced herbivores and pathogens, invasive plants may become weaker competitors with more benign impacts on individual plants and plant communities. In a greenhouse pot study, we tested whether the presence of powdery mildew disease caused by Erysiphe cruciferarum could alter the competitive impact of garlic mustard on Impatiens pallida, a North American native understory plant. Target I. pallida plants were grown alone or with one, two, or three garlic mustard neighbors. Half of the pots exposed to garlic mustard were inoculated with conidia of E. cruciferarum. Competition with garlic mustard moderately affected aboveground growth of I. pallida, particularly at high garlic mustard density, but it strongly reduced seed output across all densities. In contrast, inoculation of garlic mustard plants with E. cruciferarum completely abolished their competitive impact on seed output of I. pallida across all densities, independent of effects on aboveground growth of target plants. This effect was likely due to alteration in the ability of garlic mustard to compete for belowground resources. Even without killing garlic mustard, these results indicate that the presence of powdery mildew disease in the field will likely dampen the competitive impact of garlic mustard on individual plants and plant communities. Escape from such attackers has likely contributed to the invasiveness and impacts of garlic mustard in North America.