These are suspected to be galls, and their causal agent is unknown thus far. Numerous insects and other organisms can create galls, and they rarely harm the overall health of the plant. No insecticide will treat them, nor is one needed. Most of the leaf is healthy and still able to feed the plant, so if you wish to remove any as eyesores, remove as few as possible. Otherwise, you don't need to take any action. Cleaning-up and disposing of the foliage in winter may help reduce the population of the gall-formers so they aren't as noticeable in future years.

Here's a page with this same symptom reported from elsewhere in MD, as it happens; the ID experts in that group were unable to determine the organism's exact identity: https://bugguide.net/node/view/1184815

Here is a general introduction to plant galls: https://extension.umd.edu/resource/galls-flowers

Miri