A white mould affecting Goldilocks Buttercups, causing brown spots on the leaf wiith greyish caespituli (tufts of conidia) beneath.  The coniidia are long and formed in chains

Similar species of Ramularia affect other Ranunculus, and other organisms can cause the brown leaf-patches

host

Photograph the leaf spots and white tufts of conidia on the host.  Ensure you have noted the host-plant species in your comments

on Ranunculus including R auricomus; shady places and woodlands