Wintertime is a great time to look for galls on trees

With no leaves to block our view, winter allows us to see trunk and branch structure more clearly. We also sometimes see unusual growths, or galls, on tree trunks and branches. There are a variety of different causes, depending on the tree species.

Galls are abnormal growths that occur on leaves, twigs, or branches. They may be simple lumps or complicated structures, plain brown or brightly colored. There are 1500 species of gall producers, the majority of which are insects and mites. Some galls form where insects or mites feed or lay eggs. They may also develop as a response to infections by several kinds of fungi, bacteria, and viruses. Galls affecting leaves are seldom if ever a serious problem. Galls affecting twigs, such as the gouty oak gall and horned oak gall, can be more serious.

Symptoms and Diagnosis

In most cases, galls are unsightly but not damaging to the tree. Small plants may be stunted because the water and nutrient circulatory system of the plant may be damaged. Infested leaves, which can be twisted and curled, are usually able to carry out photosynthesis at near normal levels. Less striking and non-apparent galls that can occur on twigs, small branches, and roots can, over time, kill and weaken portions of a tree or cause a general decline in plant vigor. Galls can be confused with normal parts of the tree, for example, seed-bearing structures or insects such as scale.

Life Cycle

Galls result from the interaction between a chemical stimuli produced by the pest organism and the plant’s hormones. These pest organisms are masters in the art of compelling the host plant to provide food and shelter for the larvae which resides inside. The walls of the gall are structurally strong, rich in protein, and provide the larvae inside with protection and food.

A specific gall producer can often be identified by the unique color, shape, and structure of the gall that develops around it or by the host plant it is on. In our area most gall-causing insects have only one generation a year making control after symptoms appear of little or no value. Many gall producers have not been extensively studied and their life histories are poorly understood. In the absence of such basic information few protocols for control exist. Since the vast majority of galls produce only cosmetic blemishes, economic incentives for funding further study are lacking.

Integrated Pest Management Strategies

1. Do nothing; live with the problem. Despite the unattractive appearance of galls, their presence is usually not harmful to the host plant. Gall producers are usually kept in control by their natural enemies. The only galls that may warrant control in St. Louis are gouty oak gall and horned oak gall.

2. Use cultural controls. Some gall producers overwinter in leaf litter beneath the tree. Therefore, some control may be achieved by raking and destroying fallen leaves. If needed, control can be helped by pruning off and destroying gall-infested twigs and branches. Prevent stress on the plant by watering during dry periods and fertilizing if needed.

3. If necessary, use chemical sprays. If a tree is small, in poor health, or defoliated several years in a row, using chemical sprays may be warranted. Sprays will also kill beneficial insects that usually keep galls and other insect pests under control so spraying may make your tree vulnerable to other pests. Also, spraying will not reduce the number of galls this year. To prevent galls, sprays must be used as a preventive before the galls form. In the rare cases where control of galls is necessary, a qualified arborist will be needed to spray the tree. Effective control is difficult, but an arborist would be best able to identify the gall-producing organism and recommend and carry out control measures.

In some cases, such as the golf-ball sized galls on hickory caused by Phomopsis on hickory (above), the branches can die as the gall encircles the branch. Burls on oaks are often caused by fungi or bacteria, and the trees can live with them for their entire lives. While these galls may lessen the value of timber for lumber purposes, they are mostly a curiosity that we notice more often in winter. If you see swellings or galls on branches of your landscape trees, identifying the cause may be important to determine if pruning them away is recommended.

Many galls can be easily pruned. Check out this information on pruning deciduous trees. Typically, you want to use sanitized pruners, and you’ll want to cut about 8 inches down on the branch when removing galls.

Article by missouribotanicalgarden.org

Photo By Diana Alfuth