Galls on Plants and Trees

By Kelly Miller

Galls are abnormal growths of plant tissue caused by the presence of parasites. These growths take varied forms, such as tumor-like swells on tree trunks, ball-shaped growths on branches, or bumps on foliage.

In most cases, galls don’t significantly harm plants or trees. Treatment usually isn’t necessary or helpful (with a few exceptions, which we’ll discuss below).

Most gall damage is aesthetic. However, although these odd growths are often unsightly, they’re also fascinating to look at and learn about. In fact, there’s an entire branch of science—cecidology—devoted to their study. Many hobbyists enjoy seeking out, identifying, and photographing new, interesting galls.

How Do Galls Form?

Galls are most often caused by insects (especially wasps and aphids) or microscopic gall mites. Fungi, bacteria, viruses, and nematodes can also cause galls.

Gall formation begins when a parasite injures a patch of plant tissue, causing the plant to develop an abnormal growth. With mites and insects, this occurs when they chew on or suck sap from a plant part. Their saliva provokes hormonal activity in the plant, which leads to the growth of a gall.

After feeding on a plant, the parasite may lay eggs that the abnormal plant tissue growth will form around. Or, they may allow the gall to form around themselves, then lay eggs inside it. The gall then provides a source of both food and shelter. Some insects spend many months inside of galls, feeding on the plant tissue while progressing through several life cycle phases before finally emerging.

Is It a Gall, or Something Else?

Galls are diverse and appear on all sorts of plants and trees. However, they’re particularly common and widely-recognized on certain trees, such as oaks and maples.

Common tree galls include:

Oak apple galls

Round, red or green apple-like wasp galls on oak tree leaves.

Oak marble galls

Puffy green or brown galls on oak tree leaves caused by wasps.

Maple bladder galls

Small, red or green wart-like mite galls on maple leaves.

Maple spindle galls

Long, thin mite galls that stand up straight on the surface of maple or linden leaves.

You may also discover galls on houseplants, such as blisters, bumps, or other strange growths on foliage. Sometimes, it’s difficult to tell if you’re seeing a gall, a regular part of the plant, or the effects of a disease.

If you’re curious about a potential gall on one of your plants, visit the database at Gallformers.org.  There, you can search by plant species to view a list of galls, with photos, that are known to affect that plant.

What to Do About Galls

Most galls do not cause significant harm to trees or plants. Usually, it’s not necessary to treat galls or to eradicate the parasites that form them.

There are several reasons not to interfere with galls:

• Timing gall pest control is difficult—by the time you detect a growth, it can’t be removed without damaging the plant, and the parasite may have already moved on
• The process of cutting off a gall is often more harmful than the gall itself
• A gall may contain the parasite that formed it, along with its natural predators; thus, removing too many growths could reduce the population of the pest’s predators, making future infestations more likely

While most galls can be ignored (or even appreciated!) there are some types of tree galls that are harmful and may require treatment. These include horned oak gall, crown gall, and mite galls on fuchsia plants.

Horned Oak Gall and Gouty Oak Gall

Horned oak galls are caused by wasps and occur on oak twigs. These growths are woody, golf-ball-sized, and covered in small horns.

Gouty oak galls are similar, but don’t have horns. Horned and gouty oak galls affect many types of oak trees, especially pin oaks and black oaks.

Minor oak gall infestations are usually harmless. However, major outbreaks can cause branch dieback and may eventually be fatal for a tree.

Heavy oak gall infestations are difficult to combat. The most helpful controls are to prune off infested branches and to remove or destroy fallen leaves, which may contain the gall-forming parasites during various life stages.

Crown Gall

Crown gall is a bacterial disease that causes trees or plants to produce galls around their roots, trunks, stems, or branches. This disease commonly affects willows, poplars, fruit trees, and roses.

New crown galls are light-colored, smooth, and spongy. They become darker and drier as they age. Crown galls are irregular, tumor-shaped, and ugly, sometimes resembling a woody bit of cauliflower or potato.

Crown galls disrupt the flow of water and nutrients within a plant or tree, resulting in stunted growth and weakened health.

Unfortunately, there’s no effective treatment for the bacteria that causes crown galls. Even if galls are removed, new ones are likely to continue forming. Thus, it’s often necessary to remove and destroy infected plants to avoid spreading the parasite to other hosts.

Rose Galls

Galls on roses are abnormal growths that are caused by cynipid wasps. The wasps inject substances into the plant tissue that trigger abnormal cell growth responses. There are two types of galls that are common on roses. One is the spiny rose gall can be found on leaf surfaces. Hybrid roses are not likely to be affected by spiny rose galls; these galls are most often seen on wild roses.

The other is the mossy rose gall, and it can affect your garden roses. This gall forms a fibrous-looking mass on the rose stem that can support multiple larvae.

Like all galls, rose galls provide a place for wasps to lay eggs and provide a source of shelter and nourishment for larvae as they develop to adulthood. Galls affect the appearance of the rose plant, but usually don’t impact its health. In a severe case the affected branch can be pruned off and disposed of. Keeping your roses well fed and watered will bolster your roses’ resilience to galls.

Fuchsia Mite Galls

The fuchsia gall mite (Aculops fuchsiae) is a microscopic species that feeds on fuchsia plants. When these mites suck sap from the tips of fuchsia shoots, they release a chemical that poisons the plant. Where the resulting galls form, the plant’s growth becomes reddened, stunted, and distorted.

Fuchsia mite galls reduce the plant’s flower production. If the plant is heavily infested, it may completely stop growing.

The best way to avoid fuchsia mites is to grow resistant fuchsia varieties and practice regular pruning. In the early stages of infestation, chemical treatments or trimming are sometimes helpful. However, heavy infestations are often irreversible.

Appreciating Insect Life in Your Garden

Most galls aren’t problematic, as long as they aren’t completely overrunning a plant or tree. Nature is all about balance, including the balance between parasites, their hosts, and their predators.

One of the best ways to maintain balance in your little corner of the planet is to promote biodiversity, especially with lots of beneficial insects. Learn more here: 8 Beneficial Bugs for the Garden.