Those are galls from insects. They don't harm the tree much and tend to come and go with different years. It is not practical to try and spray for them. If you keep the tree well watered in the heat of summer and fertilize the lawn beneath the tree, that tree will be plenty vigorous and able to more than overcome the problem caused by the gall forming insects.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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