Spindle galls, most likely caused by the eriophyid mite.
I've seen a lot of this on Black cherry leaves. This is what you have there, yes? So I looked around to find the best, most compact description. Found here: https://nhgardensolutions.wordpress.com/tag/plants/ was;
Finger galls on the leaves of black cherry (Prunus serotina) are caused by a tiny eriophyid mite (Eriophyes cerasicrumena.) Visually these galls aren’t very appealing but they don’t hurt the tree. They are small-maybe as long as a half inch. A blue butterfly called the cherry gall azure (Celastrina serotina) lays eggs on these finger galls in May, and when they hatch the resulting caterpillars eat the galls-mites and all. The caterpillars also leave behind sweet secretions that attract ants. The ants, in return for the sweets, protect the cherry gall azure caterpillars from wasps and other predators. Imagine-all of this happens on the surface of a single leaf.