Hello, This is not a serious disease. This appears to be anthracnose, a fungal disease that appears early in the season when there are frequent rains. You can rake the leaves and bag them to reduce the pathogen available but there is no need for further action. Here is a good article that discusses this common disease: https://www.canr.msu.edu/news/maple_leaf_blister_and_anthracnose_two_foliar_diseases_that_can_appear_on_s I hope this is helpful. Thank you for contacting Ask Extension. Ruth