This is a downy mildew found on several species in the Ranunculaceae, most commonly recorded on Wood Anemone (Anemone nemorosa).  White down on the underside of the leaves causing pale yellowing patches above, eventually turning grey and killing the leaves.  It also occurs on several other plants in the Ranunculaceae,  including Aconitum, Pulsatilla, Hepatica and Anemone blanda.

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 

chiefly on Wood Anemone