Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) has posted a notice about the high incidence of leaf spot and leaf blight on several native species of maple this year. They explain that unusually cold spring, which prolonged leaf emergence, and the near constant wet conditions were perfect for supporting several fungal and bacterial diseases.
However, they also explain that these symptoms do NOT require any treatment, as the new leaves now emerging will be fine.
For full details and images, visit the OMAFRA blog here.