This downy mildew affects Lesser Celandine (Ficaria verna) producing a greyish down on the undersides of the leaf.  Affected leaves are often yellowed and with the leaf blade curled back; leaves may have longer petioles and stand up from the unaffected leaves.  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 

Celandines - woods, parks, hedges and damp shady places

early spring to midsummer