The white mould Ramularia lapsanae causes pale yellowish spots on the live leaves of Nipplewort (Lapsana communis).  Conspicuous white tufts of conidia emerge from both sides of the leaf.

microscope

Photograph on the host plant, both sides of leaf.   Note host species in your comments.  Microscopic examination is needed.  We recommend that your record is verified from a specimen by an expert before submission to NatureSpot