Hello Thomas,

Those fuzzy balls are galls formed on the leaves, usually of white oak, (Quercus alba) when the larva of tiny gall wasps feed on the leaves and the tree responds by producing hormones which produce the galls.

Oak trees support many different gall wasps. They usually are not harmful to the trees and it is difficult to control them with pesticide which would have to be applied in the spring when the wasps are so tiny that they cannot be readily seen. Attached is an article about the gall wasp.

https://extension.psu.edu/galls-on-oak#:~:text=Gall%2Dforming%20wasps%20usually%20overwinter,eggs%20in%20expanding%20plant%20tissue.

And here: https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/other/galls/galls-on-trees

I hope this answers your question.

Thank you for contacting Ask Extension.