Spotting

I suspect that these are some type of gall - and possibly (stress possibly) induced by eriophyid mites. Have a look at the underside of the leaf, and see if there are little openings associated with the bumps. You might even cut one or two open with a razor blade and see if there is anything moving around inside (although the mites could be quite difficult to see without a microscope or magnifying glass).

I noticed this on a lot of leaves in my area a month or two ago - I'm also curious if anybody knows what it is. Seems like some kind of fungus, but I can't figure it out.