A powdery mildew affecting Willowherbs (Epilobium and Chamerion angustifolium); often recorded on Great Hairy Willowherb, covering stem, shoots and leaves in thick white mycelium, often causing thickening and distorted, stunted growth.  This is considered to be a gall-causer.  

host/gall

Photograph the mildew on host plant, and add host species to the comments for your record; note whether gall-causing.

Willowherbs

ref.: Redfern, M & Shirley, P.  2023.  British Plant Galls. FSC