What is this Brown, Lumpy Thing I Found on My Oak Tree?
Sherrie Smith, plant diagnostician for the Plant Health Clinic in the University of Arkansas Division of Agriculture, replies:
What you have there is an oak apple gall. Galls are abnormal growths of plant tissue. Many things can cause galls, including bacteria, fungi, parasites and insects. In this case, the gall was caused by a species of small wasp, which laid its eggs on a leaf stem on the oak. This creates a chemical reaction, which causes the plant tissue to swell. The resulting gall is a safe place for the larvae to eat and grow. When the larvae reach the adult stage, they poke a hole in the gall and leave it behind.
The oak apple gall will not harm the tree.