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

Leaf Galls Common on Trees May 25, 2000 Strange bumps or growths often appear on leaves of trees and shrubs. These are usually leaf galls. Although they may look like a serious problem, most are harmless to the overall health of the tree or shrub. Leaf galls are fairly common on trees and shrubs. A gall is actually plant tissue that has developed as the result of feeding or other activity of insects or mites. Plant hormones are involved when the pest interferes with leaf development in the spring. 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. 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. These are caused by a very small wasp. 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. Leaves of hackberry trees often have the hackberry nipplegall, caused by an insect called a psyllid. Elms often get galls such as the cockscombgall, caused by an aphid. This irregular gall looks like rooster's combs on the leaves. Other shade tree, shrub, fruit crop, and even perennial flower foliage 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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