I am submitting this for a friend who just moved to Bethesda, MD. They are wondering what these "galls" are on the leaves of their trees and if they need to do something about it. Their property includes woodlands and forest of tulip poplars and other trees that back to the Cabin John Trail. Thanks!
From the photo you submitted it looks like galls on the foliage. The galls are not harmful to the tree and does not need to be controlled. See our website for photos and more information http://extension.umd.edu/hgic/shade-tree-galls