ROBERTS, KARL J,1 & ROGER C. ANDERSON.2
1Department of Biology, Prince George's Junior College, 3001 Largo Road, Largo, Maryland 20772-2199, USA.
2Department of Biology, Illinois State University, Campus Box 4120, Normal, Illinois 61790-4120, USA.
Garlic mustard (Alliaria petiolata) is an invasive, exotic plant species in northeastern United States and adjacent Canada, where it displaces the indigenous understory flora in native woodlands. Garlic mustard is a non-mycorrhizal species; however, about 75% of the native forest understory plants are mycorrhizal. Water leachates of garlic mustard added to Bacto-agar prevented germination of azygospores of the AM fungus Gigaspora rosea. Leachate was prepared by soaking 50 g of air-dried plant tissue in 1000 ml of sterile distilled water for 24 h. The leachate was passed through a series of coarse sieves to remove large particles and then through a 0.2 m millipore filter to remove microorganisms. Surface sterilized spores were placed on the agar surface in petri dishes with tomato seedlings (four spores per seedling). In ten control plates, Bacto-agar without leachate, 33 of 40 azygospores germinated. Seven of ten tomato seedlings grown in control plates were colonized by G. rosea. None of the spores germinated in the ten treatment plates. Under field conditions we found a significant negative correlation between the density of garlic mustard and the mycorrhizal inoculum potential of the soil in which the plants grew. These results suggest that garlic mustard may reduce the competitive abilities of native plants by interfering with the formation of mycorrhizal associations.