Edge of woodland heavily trodden I think this is powdery mildew on garlic mustard. Erysiphe cruciferarum is a likely pathogen but I won't put that into the taxo.
Seems to have destroyed a clump or two of this Pseudofumaria lutea.
Powdery mildew on dames rocket
Type of powdery mildew, perhaps on Alliaria petiolata