Foliar fungi and bacteria are likely potent enemies, but in some cases, they may function as mutualists. We tested the hypothesis that rapidly growing annual plants would frequently be harmed by these microbes because annuals generally allocate fewer resources to defense. We also tested a component of the enemy release hypothesis whereby we predicted that native annuals would be more vulnerable to foliar enemies than exotic ones. We manipulated microbial communities on the leaf surfaces of three native (Panicum capillare, Ambrosia artemisiifolia, Acalypha virginica) and three exotic weeds (Setaria glauca, Barbarea vulgaris, Plantago lanceolata) through fungicide and antibiotic applications. The experimental design was a balanced three-way ANOVA, with two levels of foliar fungi (control and fungicide treatment), two levels of foliar bacteria (control and antibiotic treatment), and two levels of nitrogen (control and 40 kg N/ha), resulting in eight treatment combinations for each focal species. We had 10 replicates for each treatment combination, for a total of 480 plants. Plants were treated weekly with fungicide and/or antibiotics for 9 weeks, once with nitrogen, and dry biomass was measured after harvesting.
Biomass respectively increased by 74%, 28%, and 60% with fungicide application compared to the control treatments for S. glauca, A. artemisiifolia, and A. virginica. P. capillare biomass significantly decreased by 12% with fungicide and nitrogen application compared to the fungicide only treatment, suggesting this species may allocate nitrogenous defenses against pathogenic fungi. B. vulgaris biomass increased by 419% with fungicide application, while antibiotic and fungicide treatment decreased biomass by 13% compared to the antibiotic only treatment, suggesting bacteria have a positive effect on growth when fungi are absent. Fungicide applications reduced microbe numbers on plant leaves more effectively regardless of whether plants received nitrogen, whereas antibiotics, on its own or with fungicides, positively affected B. vulgaris biomass. Fungi appear to negatively impact these weed species more than bacteria based on the net increase in biomass in the majority of our focal species. There was no evidence supporting the enemy release hypothesis, possibly because of the large time period that these exotic species and microorganisms have coevolved. This study will be a significant step in understanding the potentially complex interactions between plants and the diverse foliar microorganisms that can alter competitive hierarchies, and thus, plant community composition as well as invasibility.