A downy mildew occurring on vetches, commonly on Common Vetch Vicia sativa 

Similar species can occur on other species of vetch or tare - e.g. Peronospora sepium on Bush Vetch and P mayorii on Tufted Vetch.  P ervi occurs on Hairy Tare (Vicia or Ervilia hirsuta)

host

Photograph the mildew on the host plant, both sides of leaf; ensure you have noted the host plant species in your comments.  Microscopic examination may be needed on some hosts.

on Vetches