Experts and BHG readers answer.
Leaf drop in Sycamore Tree
Rake up those diseased leaves, bag them and throw them away. It is most probably sycamore anthracnose, which usually appears in spring. As your sycamore puts out new leaves toward summer and the weather gets warmer and drier, this fungus will subside. But do get rid of the leaves so it won't reappear next year.