A powdery mildew affecting Meadowsweet (Filipendula ulmariae), usually distorting the leaves, stems and shoots.  Fibrosin bodies (seen as tiny reflecting flecks under high magnification) are present within the conidia.  The thickening and distortion caused by the mildew is a gall.

Erysiphe ulmariae also affects Meadowsweet but does not have fibrosin bodies and does not cause thickening or distortion - it is not 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.  Unless the species is causng galls (thickening and distortion of stems etc.) microscopic examination of conidia or verification by an expert before submission to NatureSpot will be needed

Meadowsweet

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