Leaf Gall Treatment: Managing Abnormal Growth on Plants

Galls are peculiar growths that can appear on various plants, caused by the feeding of living organisms such as bacteria, fungi, nematodes, mites, and insects. While galls may disfigure twigs and foliage, they typically do not harm the overall health of the plant. However, it’s important to note that certain types of galls, such as horned and gouty oak galls, can cause significant damage to oak trees.

General Management of Gall Problems: Be Patient and Observe

Insects and mites that create galls spend the majority of their lives protected within a ball of plant tissue. As a result, it can be challenging to time pesticide applications effectively. Additionally, reaching gall-making insects with insecticides on tall trees is a daunting task. However, nature has a way of taking care of these pests. Natural enemies are attracted to the conspicuous galls and can help keep infestations in check. Patience is key in managing gall problems, as most galls will not kill a tree and tend to cycle through periods of abundance and scarcity.

Chemical Control: Carbaryl and Insecticidal Soap

Carbaryl (Sevin) is an insecticide labeled for use on galls caused by mites and adelgids. When applied correctly, it can be effective in controlling spruce galls. However, insecticidal soap can also prove to be equally effective against certain galls, such as those caused by ash flower gall. It’s worth noting that for some galls, the use of insecticides has been found to prolong the problem, particularly when dealing with ash flower gall.

Dormant Oil Sprays: A Preventative Measure

Many gall-making insects and mites overwinter in bud scales or bark crevices. By applying a horticultural oil spray just before bud break, when gall makers become active, you can minimize gall problems. Thorough coverage is essential as the oil works by suffocating and smothering pests. Keep in mind that using oils may temporarily alter the appearance of certain coniferous plants that have a blue hue due to wax. Always follow the label instructions and take precautions to avoid harming sensitive plant species.

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Physical Removal and Biological Control: A Balanced Approach

When removing galls from infested trees, it’s essential to consider that natural enemies often reside within these galls. Removing every single gall may inadvertently remove beneficial insects that play a role in keeping new infestations at bay. Leaving a few galls in the vicinity can actually contribute to the long-term stability of your gall management program.

Tackling Specific Gall Problems: Species-Specific Solutions

Horned Oak Galls: A Threat to Oak Trees

Horned oak galls are caused by tiny wasps that induce the growth of woody galls around the stems of pin and willow oak trees. These galls can girdle branches and lead to significant dieback. Female wasps emerge from the galls in early spring and lay eggs in swelling leaf buds. The hatched larvae cause blister-like galls along the veins. In summer, the adults fly from leaf galls and lay eggs in twigs. The resulting galls become visible the following spring and complete development within about three years. Dogwood borers may also feed on young woody galls.

To control horned oak galls, remove young expanding twig galls as soon as they become visible in the spring. However, cutting off old dried galls is unnecessary. Insecticides can kill leaf galls, but they do not reduce the number of new stem galls produced.

Ash Flower Gall: Taking Aim at Unsightly Galls

Green ash trees are often attacked by small mites that feed on male flowers in the spring, resulting in the formation of groups of galls surrounded by disfigured leaves. These galls dry up into unsightly brown formations that persist on trees throughout the winter. The mites migrate to new flower buds as the galls dry. To control ash flower gall, dormant applications of oil are most effective when the mites become active in spring before bud break. It’s also recommended to prune galls from the plant before the period of mite activity in the spring.

Maple Bladder Gall: A Cosmetic Issue

Silver or soft maple trees can fall victim to tiny mites that cause small, wart-like growths on the foliage. These growths change color from red to green and eventually black. Severe infestations can lead to crinkled, deformed leaves that drop prematurely. Unfortunately, once formed, these galls cannot be removed from the leaves as they are an integral part of the leaf structure. However, it’s important to note that maple bladder galls do not cause permanent injury to the tree’s health and vigor. Despite this, they do detract from the aesthetic beauty of the foliage.

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Maple bladder gall mites overwinter in cracks and crevices of the bark. As the buds swell in early spring, the mites migrate onto the bud scales before the buds open. This is the most vulnerable time for the mites and when dormant applications of oil spray are most effective. Once the buds open, the mites feed on the new leaves, causing the formation of hollow galls. The mites then reside and reproduce inside these galls until fall when they return to the bark to hide during the winter.

Hackberry Leaf Galls and Witches Brooms: Aesthetic Concerns

Many galls found on hackberry tree leaves are caused by jumping plant lice. In late summer or fall, the small winged adults emerge from the galls and fly about, often invading homes and becoming household pests. Additionally, hackberry trees can experience excessive growth of twigs, known as “witches brooms,” due to mites or a fungus carried by mites. While this condition may lead to disfiguration, it has minimal effect on tree health.

To control hackberry galls, remove and destroy old galls before the eggs hatch in the spring. Dormant oil applications can also help reduce gall problems. It is advisable to leave some galled hackberry leaves at the base of the tree to preserve natural enemies that serve as biological control agents against the remaining gall makers.

Hickory Pouch Gall: Affecting Foliage Health

Most hickory galls are caused by phylloxera, aphid-like insects that create pouch-like growths on twigs and leaves. Leaves heavily infested with these insects may turn yellow-brown and fall from the tree. The pouches open in early summer, allowing the phylloxera to continue their life cycle. During winter, the insects overwinter in the egg stage, usually in crevices within old galls.

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Control measures for hickory pouch gall involve removing and destroying old galls before the eggs hatch in the spring.

Cooley and Eastern Spruce Galls: Unique Conifer Challenges

Cooley spruce galls are cone-like formations found on new growth in certain types of spruce trees. These galls are caused by adelgids, aphid-like insects with waxy tails. Initially green or purplish in color, the galls resemble small pineapples in their early development. During mid-summer, the galls open, releasing the aphids that have developed inside. Eventually, they turn brown and resemble small pine cones. On Douglas-firs, this same gall maker twist and discolor needles in early May.

Eastern spruce galls, on the other hand, appear as pineapple-shaped formations at the base of new growth on spruce trees. These galls are also created by adelgids. Some spruce trees show resistance to this species of adelgid, while others are more susceptible. The galls can measure between 1/2 to 1 inch in length.

To control Cooley spruce galls, it is recommended to avoid interplanting spruce and Douglas fir, as they act as alternate hosts for the gall-making adelgid. Propagating individual trees that have shown resistance can aid in long-term control of Eastern spruce gall adelgids. Pruning out old galls may improve the appearance of an area, but it does not effectively control the issue. Insecticides can be applied in spring to kill adelgid immatures. Summer rates of horticultural oil, insecticidal soap, carbaryl (Sevin 50WP), or imidacloprid (Merit 75 WSP) have been found to be effective.


Horned oak gall on stem
Horned oak gall on stem. (Photo Credit: Cliff Sadof, Purdue University)

Horned oak gall maker on leaf
Horned oak gall maker on leaf. (Photo Credit: Cliff Sadof, Purdue University)


In conclusion, diagnosing and managing gall problems in plants requires careful observation and a targeted approach. While galls can be unsightly, most do not pose a significant threat to the overall health of the affected plant. By understanding the specific characteristics and life cycles of gall-making organisms, as well as employing a combination of control measures such as physical removal, biological control, and targeted insecticide applications, gardeners and arborists can effectively manage gall infestations and ensure the long-term health and beauty of their plants.

For more information on plant health and pest management, visit Ames Farm Center.