These balls or raised areas on the leaves of your maple tree are leaf galls caused by the maple gall mite. The galls are abnormal, rapid growth formed in response to mites feeding on the leaf and in an effort to stop damage by the mites. While they can look unsightly, the galls do not harm the tree. No treatment is recommended, nor is any treatment possible this time of year. More information about maple gall mites can be found here: http://ento.psu.edu/extension/factsheets/leaf-galls-maple

A leaf gall! I would never have discovered that by myself! Thank you so much for your reply!!! Yes, the galls are green and then red - haven't seen the black color yet. I was afraid I would have to lose the tree. So glad I do not. I appreciate your time and expertise! The website is awesome Marlene Kopper Turtle Creek, PA