Learn to Live with Tar Spot
Maple tar spot (Rhytisma acerinum) is a disease of the Norway maple (Acer platanoides) that causes pale yellowish spots on the leaf surface in early summer which become, in the month of August, large black spots. In years where the disease is very present, there is also a massive loss of leaves in August, well before normal leaf fall in October. When the spring is rather dry, the disease is still present, but less obvious.
Tar spot is usually considered a cometic disease. Usually, infested maples survive without any significant effect on their long term health. On the other hand, if the disease has begun to become to be a yearly problem in your area, planting new Norway maples is begging for trouble.
No treatment is really possible, because you would have to spray a very large tree with toxic pesticides several times a year, year after year, which is not only difficult and expensive, but environmentally irresponsible. Apply the Laidback Gardener’s 15 Steps Rule: step back 15 steps and if you do not see the symptoms, it is not worth treating!
Note that other species of tar spot can affect other maples, but they rarely cause significant damage.