Most Leaf Galls Don't Hurt Trees

Each year in late spring and early summer, homeowners begin noticing strange growths on the leaves or stems and are concerned for the health of their trees.

Several different types of galls are common in Nebraska and are abnormal growths caused by the feeding or egg laying activities of an insect or mite. Either the physical damage done by the insect or its salivary secretions as it feeds causes the plant to increase its production of plant hormones. These plant hormones cause abnormally increased plant growth in small, localized areas and result in the formation of a gall. 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.

Maple spindle gall is commonly found on sugar maple. This gall appears as a thin, almost worm-like structure that grows up from the upper surface of the leaf. The galls rarely distort the leaf but considerable number of galls can make the leaves unsightly. Although the galls are ugly, maple leaf galls seldom, if ever, cause permanent damage to trees. Therefore chemical control is not recommended.

Ash flower gall appears as clusters of black structures attached to the tips of branches and is caused by several generations of tiny eriophyid mites that feed in the male flower clusters. The black clusters persist until the year following their development then fall from the tree. They are unsightly, but do not affect tree health.

Hackberry Nipple Gall, which resembles a round green pouch, is very commonly found on hackberry. An insect called a psyllid causes this gall, which forms on the underside of leaves.

Elms often get galls such as the cockscomb gall, caused by an aphid. This irregular gall looks like rooster's combs on the leaves.

Oaks are susceptible to many different types of galls most of which are not serious. Translucent oak gall, oak flake gall and oak blister gall are just a few of the leaf galls that can occur in oaks. However, oaks can also develop stem galls, such as oak bullet gall, which in high numbers may reduce plant growth. Oak bullet gall, which affects primarily bur and swamp white oaks, develops as circular woody growths that often occur in masses along the stems. There appears to be some genetic differences between trees within a species, so that about 20% of bur and swamp white oak trees can have very high levels of infestation. The growth rate of heavily infected trees may be reduced.

Homeowners often become quite alarmed when they discover that their tree is infected with a gall. They fear that the tree is going to die unless something is done quickly. This is not the case. Leaf galls seldom, if ever, cause permanent injury to a tree, although they do detract from the trees' beauty.

Other shade trees, shrubs and even fruit crop or perennial flowers may also develop a variety of galls. Treatment is rarely suggested and would have been needed prior to the gall forming. 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.

More information

• Galls on Shade Trees and Shrubs, Purdue University
• Galls, Ohio State University

Photo Credits:

• Hackberry nipple gall - Sarah Browning, UNL Extension in Lancaster County
• Maple bladder gall - John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org
• Ash flower gall - Steven Katovich, Bugwood.org.
• Oak bullet gall - Ryan Armbrust, Kansas Forest Service, Bugwood.org.