A downy mildew affecting forget-me-nots (Myosotis and Brunnera) including those in gardens.  A greyish down is present on the leaf undersides; the corresponding upper side of the leaf has yellow-green patches.  

host

Photograph the mildew on the host plant; ensure you you have noted the host species in your comments 

Forget-me-nots: gardens, parks, roadsides, hedgerows, shady places, woodlands