Question
We have a very large pin oak and this year the leaves have
developed unusual brown bumps on the leaves. They are large,
maybe 1/4-1/2" half spheres. On the back side of the leaf it
is brown under the bumps.  I really didn't notice them until
late summer/early Fall and then looked up and they are
everywhere on all the leaves.  Any ideas?  Thank you.

Answer
These are leaf galls formed by an insect -tiny wasp.  There are a variety of gall-forming species of small wasps that commonly infest oak, Quercus spp. Most leaf galls on oak cause little or no harm to the health of a tree.

Galls are abnormal plant growth or swellings comprised of plant tissue. Galls are usually found on foliage or twigs. These unusual deformities are caused by plant growth-regulating chemicals or stimuli produced by an insect or other arthropod pest species. The chemicals produced by these causal organisms interfere with normal plant cell growth.

general, most leaf galls on oak do not affect the health of the host tree. A few can cause leaves to drop prematurely, or distort them so that photosynthesis (the plant抯 food-making process) is interrupted. Galls generally are aesthetically objectionable to homeowners who find them unattractive and fear that galls will cause damage to the health of their oak trees.

Chemical control is seldom suggested for management of leaf galls on oak. Cultural methods of control may be effective in reducing the impact of these insects. Some fallen leaves may harbor various life stages of gall-producing pests. Therefore, it may be useful to collect and destroy all infested leaves. Some of these pests overwinter in twigs and branches of oak. Where such woody galls are detected, prune and destroy the infested plant material when the galls are small and have just started to develop.

Once a gall begins to develop, it is almost impossible to stop or reverse its development. Unless registered insecticides can be applied when gall wasps are flying, they offer little or no effective measure of control. Lack of serious plant damage from leaf galls and the difficulty in proper timing of insecticide applications pose a strong argument against the use of insecticides to reduce galls on oak.

In short, the galls will not kill the tree and usually natural controls will keep the population from repeating next year.