The Sycamore problem sounds like anthracnose. Three sprays of
contact fungicide spaced ten days apart while the leaves are expanding
will help control this disease. After the leaves have expanded, they are no
longer as vulnerable. Diseased branches should be pruned out and burned, and
leaves should be raked up and burned as well. This disease is not usually
fatal, but can be very unattractive. If the tree is very large, this can
lead to a lot of maintenance work. Removal would not be out of the question.
A resistant cultivar, 'Columbia,' is available.