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Miconia calvescens is a highly invasive shrub in tropical oceanic island ecosystems and the fungus Colletotrichum gloeosporioides f. sp. miconiae (Cgm) has been widely introduced as a biocontrol against it. On the island of Moorea, French Polynesia, Cgm exhibits differential success along an elevation gradient, with highest effectiveness in controlling M. calvescens at higher elevations. We examined the association between the fungal biome of M. calvescens and Cgm biocontrol success using field surveys, microbiome sequencing, and in vitro competition experiments. Our results demonstrate that: (1) quantifiable differences in foliar damage occur across the elevation gradient despite the presence of Cgm at all elevations; (2) these differences correlate to differences in community structure of leaf-associated fungi in spite of close proximity of surveyed sites; and (3) endophytic fungi isolated from plant tissues exhibit different levels of competitive ability against Cgm in vitro, with higher competitive ability displayed by fungi isolated from lower elevations. Together, these results suggest an important role of the leaf microbiome in determining the success of biocontrol efforts made against invasive plants. [ABSTRACT FROM AUTHOR]