Most Leaf Galls Don't Hurt Trees
NEBLINE Newsletter Article by Mary Jane Frogge, Extension Associate
Warmer weather has helped plants leaf out and also get us out into yards and gardens. Looking at new leaves on trees and shrubs may cause panic to some when bumps or distorted growth is noticed. These are usually leaf galls.
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 by 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 tiny 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 cause 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 affect twigs, such as the gouty oak gall, and actually cause some dieback. Most leaf galls on oak are not damaging.
Leaves of hackberry trees often have the hackberry nipple gall, caused by an insect called a psyllid. Elms often get galls such as the cockscomb gall, 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.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office