The potential for invasive species to chemically interfere with natural symbioses is receiving increasing attention. Alliaria petiolata (garlic mustard), a Eurasian biennial forb invasive in eastern North American forests, produces anti-fungal organic compounds.  Alliaria petiolata has been shown to inhibit endomycorrhizal fungal colonization of plant roots, but little is known about its affect on ectomycorrhizae (ECM). Here we investigated whether Alliaria inhibits the colonization of ECM fungi on Quercus rubra (northern red oak). Acorns and bare root seedlings were planted in two stands of contrasting Alliaria density but similar canopy composition, and allowed to grow for one year. Seedlings were harvested and fine roots <1mm were cut into 3-4 cm pieces and randomly selected for analysis. A total of 250 root tips from each seedling were inspected and the proportion of colonized tips was recorded. Mycorrhizal morphotypes were categorized based on color and structure of the fungal mantle.

Results/Conclusions

The proportion of colonization was more than 3x greater on seedlings grown in stand with no Alliaria (ANOVA p = 0.0004). Mean ECM fungal morphotype richness differed between the two sites with the no Alliaria site having a richness of 4.24 and the high Alliaria site having a richness of 1.46 morphotypes/seedling (p = 0.0001). The two sites had considerable morphotype overlap based on presence/absence data (Sorenson Index = 76%), but when abundance was considered the sites were less similar (%Similarity = 12%). Important ECM fungal morphotypes, such as Cenococcum geophilum, were found on 94% of seedlings, accounting for 36% of all colonized root tips, on samples from the no Alliaria site. In contrast, this morphotype was of less importance at high Alliaria density and was only found on 23% of seedlings accounting for only 10% of colonized root tips. These data support the hypothesis that Alliaria negatively impacts ectomycorrhizal colonization.  This potentially indirectly affects establishment and growth of Quercus and other ectomycorrhizal trees, and may cause changes in fungal community composition. These results show an importance for continued investigation into the effect of Alliaria on ectomycorrhizae.