A powdery mildew affecting some cranesbills (Geranium species).  

Neoerysiphe geranii also affects some speciers of cranesbill - ref to host list in Woods et al (2024) for further details

host/microscopy

Photograph the mildew on the host plant, and add host species to the comments for your record.  Depending on host, microscopic examination may be needed to separate from similar species.

Cranesbills (Geranium species)