Field-to-forest fungal spillover is a biotic edge effect of coffee in Costa Rica.

Agricultural expansion creates mosaic landscapes where managed land borders natural ecosystems, facilitating the movement of organisms between habitats. Ecological spillover from natural systems to crops has been previously examined, but the extent to which populations amplified by agriculture move into adjacent forests remains largely unexplored. We investigated whether Costa Rican coffee fields act as reservoirs for foliar fungi that, because of high crop density and phylogenetic signal in plant-fungal associations, may particularly affect close relatives of coffee (Rubiaceae) in the forest understory. Field surveys, airborne propagule sampling, and metabarcoding along transects at the coffee-forest border revealed declining leaf spot incidence with distance into the forest for Rubiaceae but not non-Rubiaceae hosts, in parallel with declines in the dominant fungal order on coffee leaves, Pleosporales, and 19 coffee-associated taxa. Ninety-three coffee-associated taxa, including many potential plant pathogens, showed higher modeled abundance on forest Rubiaceae than forest non-Rubiaceae hosts. Our findings suggest that crops can act as reservoirs for fungi colonizing adjacent forest plants, disproportionately affecting hosts more closely related to the crop. These plants may face increased disease pressure, leading to changes in forest understory composition near the crop-forest border. Phylogenetically structured spillover represents an underappreciated edge effect in fragmented landscapes.