Is this caused by an insect or disease?
click to enlarge:
Not certain because I'm not in your region. But I bet that's a leaf gall. If so, not damaging other than unsightly.
But let's wait for someone else to comment. Perhaps rhizo?
I think jean001 is on the right track. You might google "vein pocket gall" and compare.
Doesn't really look like the pocket gall to me. But I don't have any alternate suggestions right off the bat. Wish I could dissect them to see what the heck is inside, lol!
Well, OP could do that and then post an image. Right?
I don't know if this will help but I broke one open and took a picture.
I saw a couple very tiny black flies but I'm not sure they came out of it.