A downy mildew which parasitises Hedge Garlic, Alliaria petiolata, forming dense, well-defined white patches on the leaf undersides.  This is considered to be a gall-causer.

host

Photograph both sides of the leaf and the downy mildew on the host plant; ensure you have noted the host species in your comments 

Hedge Garlic, Alliaria petiolata

This species is part of the Hyaloperonospora parasitica s.lat. species complex; species in this group affect plants in the Brassicaceae