Aim: To advance our understanding of how foliar fungal communities are structured and assembled, we assessed to what extent leaf flush and latitude can explain the within- and among-tree variation in the foliar fungal community in a foundation tree species, the pedunculate oak Quercus robur.

Location: A latitudinal gradient spanning c. 20 degrees in latitude in Europe.

Major taxa studied: The foliar fungal community on Quercus robur.

Methods: We examined the main and interactive effects of leaf flush and latitude on the foliar fungal community by sampling 20 populations of the pedunculate oak Quercus robur across the tree’s range, spanning c. 20 degrees in latitude.

Results: Species composition, but not species richness, differed between leaf flushes. Across the latitudinal gradient, species richness was highest in the central part of the oak’s distributional range, and foliar fungal community composition shifted along the latitudinal gradient. Among fungal guilds, the relative abundance of plant pathogens and mycoparasites was lower on the first leaf flush, and the relative abundance of plant pathogens and saprotrophs decreased with latitude.

Conclusions: Changes in community composition between leaf flushes and along the latitudinal gradient were mostly a result of species turnover. Overall, our findings demonstrate that leaf flush and latitude explain 5-22% of the small- and large-scale spatial variation in the foliar fungal community on a foundation tree within the temperate region, with major consequences for plant health, species interactions and ecosystem dynamics.