Your sycamore tree is suffering from anthracnose. This fungal disease develops during the cool, wet conditions of springtime. The fungus overwinters on branch twigs and infects emerging buds, shoots, and leaves as they expand. Infected leaves develop brown patches along their veins, and drop a couple of weeks later. Infected shoots may develop cankers and dieback. By the time you see leaf drop and twig dieback, the infection period is usually over. Trees typically develop new leaves by early to midsummer. Reinfection is unlikely as warmer, drier weather conditions arrive. Repeat infections over the years can result in distorted growth from cankers and dieback, but fungicide sprays are seldom necessary. Cut down on the amount of fungus in the area by raking up and destroying fallen leaves. If your tree is small, you could prune out cankered twigs, but that is impractical on a large, established tree.