Golovinomyces vincae is a powdery mildew which affects the leaves of Periwinkle (Vinca).

Other species of powdery mildew may affect this host - microscopic examination is needed

host

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

Periwinkle (Vinca)