Ecological spillover of coffee-associated foliar fungi from field to forest as a biotic edge effect in the Neotropics

Agricultural expansion creates mosaic landscapes where managed land borders undisturbed ecosystems, facilitating the ecological spillover of organisms between habitats. While fungal spillover from natural systems to crops is relatively well-studied, the reverse—agricultural fungi colonizing adjacent forests—remains underexplored. We investigated whether coffee fields act as reservoirs for foliar fungi that spill over into adjacent tropical forests, particularly affecting close relatives of coffee (Rubiaceae). Using transect-based field surveys, airborne spore sampling, and metabarcoding of fungal communities, we found that leaf spot incidence was much higher in coffee than in adjacent understory forest leaves and declined with distance into the forest for Rubiaceae plants. Pleosporales was the dominant fungal order in coffee leaves and declined in abundance with distance from the forest edge in both airborne samples and Rubiaceae leaves, suggesting propagule pressure from coffee fields led to changes in the phyllobiome of close coffee relatives in the adjacent forest. Among the 1,000 most abundant coffee-associated fungal taxa, 19 declined with distance into the forest, and 93 were more abundant on phylogenetically close relatives of coffee compared to non-Rubiaceae plants. Several of the closest matches for these taxa were potential plant pathogens. These results demonstrate that high-density crops can act as reservoirs for fungi capable of colonizing adjacent forest plants, disproportionately affecting plants closely related to the crop. This appears to disrupt natural plant pathogen dynamics in forests such that close relatives of the crop plant near the crop-forest border face increased disease pressure and may be at a competitive disadvantage as a result. These findings highlight an underappreciated edge effect of agriculture in fragmented landscapes.