We in the horticulture business believe in “the right tree for the right place”. This tree occurs naturally along stream and river beds where it can get 30-80 ft. tall and 20-50 ft wide. It needs rich, deep, moist, well drained soil. When a plant is not in its optimum environment, it tends to not be able to fend off diseases and pests. What you probably have is anthracnose. This is very common on sycamores. You will want to carefully remove and dispose of any litter from the tree. You might want to visit your local independent garden center to see what they would suggest as a preventative fungicide to use while the tree is in its dormant state. Sycamores can also have large root systems that can damage sidewalks and pipes.