I have a sycamore that develops the same problem each spring. The leaves
are just fine then develop small spikes on the top surface. The margin of
the leaf seems to be most effected. Eventually the leaves distort and fall
off. The tree also develops anthracnose later in the year. Neither disease
seems to be killing the tree. While I have searched Google for ideas, I
can't find anything that seem relevant. I posted some pictures to
alt.binaries.pictures.garden under the title: "Sycamore disease ID request"