Jeff, Thanks for the question and photo. Those are leaf galls, not a fungus.  Very common! I have a hackberry tree in my yard covered with them. They are not harmful. The U does not make any suggestion for dealing with them. They are caused by an insect biting the leaf and then the trees growth response. Here is a link with more detailed information on galls: https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls

Paul -Tree Care Advisor volunteer