This is a powdery mildew that affects the leaves of maples and sycamore (Acer)

Sawadaea tulasnei is listed in Woods et al (2024) affecting the Norway Maple, Acer platanoides, and various ornamental, non-native Acer.  Phyllactinia marissalii also affects Acer species

host/microscopy

Photograph the mildew and chasmothecia on host plant, and add host species to the comments for your record. Depending on host species of Acer, microscopic examination or verification by an expert before submission to NatureSpot will be needed

Acer species - maples and sycamore