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The first step for an effective diagnosis or evaluation of a pest or disease problem is to know the affected tree species.
The second step is to have some observations over time as the course of the disease or symptom expression can often lead to a proper diagnosis.
Lacking those two initial steps, I'll offer that that symptom is the result of a "leaf skeletonizer", a not very restrictive term, and caused by several very different sorts of insects with possibly very different control strategies.
All of that being said, I'll very tentatively suggest that given the scalloping of the leaf margin, you have an elm or closely related woody plant. The most prominent skeletonizer of elm that I know is the elm leaf beetle. The pattern of skeletonization can vary between and within pest species and affected hosts, but that is a place to start.
So that is just a suggestion based on inadequate information.