I was visiting Toronto recently (Markham neighborhood to be more specific) and noticed some odd-looking leaves on the trees in the area. Most of the trees have these pea-sized lumps on the underside of their leaves.

Could these lumps be berries? Or is it diseased ?

My google search didn't find an answer. Appreciate it if someone can help solve this mystery.

Here is a link that might be useful: Photo of pea-sized lumps on tree leaves