What's on my Elm leaves

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I suspect if you flip over and look at the underside of the leaves you will see sign of insect egg deposit. The curled parts are likely caused by larvae hatching out in the leaf. If that is the case I would remove the badly affected leaves and use an insecticidal soap on the rest of the plant for precautionary measures if further eggs remain.

Yeah, looks like leaf galls. Good thing is they won’t spread, so if you just take those leaves off and spray a little before this time next year the tree should be fine.