Ask Mr. Smarty Plants

The fuzzy balls are galls formed by the tree leaf in response to the feeding of the larvae hatched from an egg deposited by a tiny gall wasp in the Family Cynipidae, Genus Andricus.  Here are photos of Andricus quercusflocci galls that form on leaves of Quercus alba (white oak) and Quercus prinus (chestnut oak).  Andricus pattoni is reported to be the species that lays eggs on postoaks, but I could find no photos for that species of gall wasp.  Here is a link to an article about other Oak Cynipid Galls from the Department of Entomology, University of Minnesota.  You can see other types of galls in this article from the University of Wisconsin-Milwaukee and here is more information and photos from the University of Pennsylvania.  Here is a link to an article on Randy Lemmon's Gardenline on KTRH radio about galls on oak trees in Houston in 2010.

In general, the galls are not harmful to the trees and there is really no insecticide that is effective since the wasps are tiny and not easily seen, plus the larvae are well-protected from pesticides within the galls.  Additionally, insecticides applied are very likely to be harmful to beneficial insects.