University of Illinois Extension
Bruce Spangeberg

These articles are written to apply to the northeastern corner of Illinois. Problems and timing may not apply outside of this area.

Stateline Yard & Garden

Leaf Galls Common on Tree Foliage

May 24, 2001

Noticed any unusual bumps or distorted growth on the leaves of your trees? Most likely these are galls. Although they may appear damaging, they rarely cause problems for trees and not much can be done about them.

Galls are actually distorted plant tissue developed as the result of insect or mite activity. These pests interfere with leaf development, and plant hormones may influence the distorted growth. There are also galls caused by fungi, bacteria, and other organisms.

Once the gall appears on the leaf, there is no way to control it. Preventing most leaf galls is extremely difficult, as the insect or mite would need to be controlled before it got into the plant. However, other than being unsightly, most leaf galls are not harming the tree or shrub.

Maple bladder gall is a common example of leaf galls. Small green bumps appear on the tops of silver and red maple leaves, turning bright red. This is due to eriophyid mites feeding on newly developing leaves. While it may look bad, in reality the health of the tree is not threatened. Control is not practical or necessary.

Galls frequently appear on oaks. They may small bumps or larger, more visible growths. For example, the oak-apple gall appears as fairly large, round, apple-like growths. A very small wasp causes these. Some may also affect twigs, such as the gouty oak gall, and actually cause some dieback. Most leaf galls on oak are not damaging, however.

Many other shade trees, shrubs, fruit crops, and even perennial flowers may have galls appearing. Treatment is rarely suggested, and would have been needed prior to the gall forming. This usually is not practical. Once the gall had formed, even if the pest is killed, the gall remains since it is actually plant tissue. Many gall makers also have natural predators or parasites that help keep populations in check.

 

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