We get lots of questions each year about abnormal growths on oak and other trees. These abnormal growths, called galls, can be very disturbing to the people whose plants are affected. Fortunately, most galls affect only the appearance of the trees and are not detrimental to plant health.
Galls are a plant's response to insects, mites, bacteria, fungi, or nematodes. Galls are actually created by the plants themselves in response to some stimulus from the invading organism. They plant may create the gall in response to the invaders feeding or the critter might "trick" the plant into forming a home for it to live in.
Each gall-inducing insect or mite chooses a specific plant to associate with. Identification of the gall maker is based on the gall it produces. Insects on trees produce over 2,000 types of galls. The majority of these insect galls are produced by wasps, and over 700 of these wasp-produced galls occur on oak trees.
I've seen three types of wasp-produced stem galls on oaks in my yard. A white oak tree in my yard had the very interesting wool sower gall. It is tan, spongy, and pong-pong ball sized. It is fun to pull the gall apart to look for the ant-sized adult wasp inside. Don't worry, they don't sting.
Oak apple gall is also caused by several species of gall wasps. It consists of large, dry galls attached to the midrib or petiole of a leaf. As the galls mature they become papery. The single larva in each "apple" is inside a small and very hard seedlike cell. No, they are not edible!
Although leaf galls may be unsightly, most do little or no damage to the host plant. However, the horned and gouty oak galls can be debilitating, even killing younger trees. It too is caused by a wasp, but this one causes woody masses on stems that reach two inches or more. Over it's one to three year life cycle, this wasp's gall can choke off and kill the ends branches and twigs. The pin oak is a primary host for the horned oak gall.
There is no effective chemical control for galls. The gouty and horned oak galls can be removed by pruning, and the debris should be destroyed. The other galls mentioned do not remain on the tree from year to year so don't have to be pruned away. They may or may not return the following year.
A factsheet on oak galls is available online at http://web.extension.illinois.edu/fmpt/downloads/42439.pdf.
Enjoy your oak trees!