I have multiple wild plums trees on my property. I was checking them to see if there were any plums on them today and as I looked at them I noticed these on many of the leaves of the plum trees. I will attach a photo. I have never seen anything like this before and would like to know what it is, if it harms the tree, and if so what I can do about it. There are other deciduous trees all around them, some of them with the branches intertwined and I saw a none of these on anything but the plum trees.
Cass County Minnesota
They are galls and are made by the plant in response to insects.
Resembles "Spindle gall Mites". There are no effective chemical treatments at this stage from what I know of. Other than being unsightly, this infestation is not commonly known to hurt the tree. Dormant oil can be applied during the dormant season to try to smother over wintering eggs.