I've recently gotten e-mail messages with images of a gnarly looking leaf gall appearing on pin oaks in Cincinnati, OH, and Lexington, KY. The unsightly, lumpy growths are produced by a gnat-like gall-midge (Macrodiplosis niveipila, family Cecidomyiidae) and have the descriptive common name of Gnarled Oak Leaf Midge Galls.
The galls appear as irregular, brain-like twisted masses of leaf tissue that may arise near the base of the leaf with normal leaf tissue extending beyond the gall, or the misshapen growths may be comprised of an entire leaf, or parts of a leaf. The affected leaf tissue is often thickened and darker green compared to normal tissue, and covered in sporadic patches of short, fuzzy, white hairs. The galls tend appear throughout the tree canopy in random collections, with several galls in close proximity to one another.
Although the galls may present a dramatic appearance, as with the vast majority of oak leaf galls, they have little to no impact on the overall health of their host tree. So, controls are not needed. Usually, only a very small percentage of the leaves on an entire tree are affected. Also, this gall-maker is highly sporadic from year to year. It may not reappear next season, or even many years later, so there's little chance for an accumulated impact on tree health.