These are galls. They are often caused by insects feeding and laying eggs on the leaves. Typically, no treatment is necessary for galls because natural predators and parasitoids (beneficial insects) will catch up to the pest and control them naturally. The galls can be unsightly, but they don't typically do major damage to a tree. You can read more about galls on our website:  https://extension.umd.edu/hgic/shade-tree-galls

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