Neoerysiphe galeopsidis a powdery mildew which affects species in the Lamiaceae, including Dead-nettles (Lamium), Black Horehound (Ballota), Hemp-nettles (Galeopsis)  and Woundworts (Stachys).

host

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

certain plants in the Lamiaceae - refer to Woods et al (2024) for host-list.