A downy mildew parasitising Cow Parsley and Rough Chervil (Anthriscus sylvestris and Chaerophyllum temulentum).  White down on the leaf undersides with corresponding pale green patches above, turning bronze.   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 

Cow Parsley and Rough Chervil - woodlands, hedgerows, shady places

This is part of the Plasmopara crustosa s.lat. species complex of downy mildews that affect host plants in the Apiaceae (umbellifer) family.  The compex has recently been split into several species depending on host.  See https://www.aber.ac.uk/waxcap/downloads/Chater20-DownyMildewsWales.pdf