Good morning; Thanks for submitting your garden and landscaping questions to UMd Ask Extension.

Your elm tree foliage is being invaded by an insect pest that causes leaf galls to form. See UMd Extension publication   https://extension.umd.edu/resource/shade-tree-galls. Also the Extension publication from Ohio State University:   https://bygl.osu.edu/node/1586

Once galls form there is nothing that can be done to remediate the condition. See UMd Extension publication   https://extension.umd.edu/resource/aphids-trees-and-shrubs for controlling aphids in winter/spring.  

Consider adding flowering plants near your elm tree. Flowering plants attract many insects including beneficial insects like lady bug or ladybird insects that are huge predators of aphid insects. Rake up and dispose of any elm leaves in the fall. Leaf galls will not permanently harm your elm trees.  

Good luck with your landscape and gardens. Christopher