Leaf and Twig Galls on Trees

Gall Bology and Their Types

Oak Apple Gall
Oak Apple Gall. Steven Katovich, USDA Forest Service, Bugwood.org

What are Galls?

Abnormal leaf and twig bodies, called "galls", can be simple bumps, fruit-like structures or complicated growths caused as a result of the activity of invading insects, mites, nematodes, fungi, bacteria, and viruses. Insects and mites generally cause tree galls by feeding and egg-laying. Fungi and bacteria cause galls from the plant's reaction to their infection.

One particularly common version of this rapid explosion of growth on a tree (see image) is called the common oak gall.

It is most noticeable on the leaf, stem and twig of an oak tree. Although these, as do many galls, may look like a serious problem but are harmless to the overall health of the tree. Twig galls are a bit more complicated and can cause minor damage in some cases.

Gall Biology

Galls are irregular plant growths stimulated by a reaction between plant hormones and a powerful growth regulating chemical produced by a feeding insect or mite. Galls can occur on leaves, bark, flowers, buds, acorns, or roots. The gall makers will first create the gall, then feed on the nutrients produced from the inner gall tissue.

Gall formation takes advantage of the accelerated growth period in late spring. The new leaves, emerging twig shoots and flowers react quickly to the growth regulators so you will see galls most on budding and flowering trees. Mature plant tissues are usually unaffected by gall-inducing organisms.

The gall-making insect or mite uses the inside of the gall to grow to maturity. By feeding within and on the inner gall wall, the insect/mite grows and is safe from natural enemies and chemical pesticides. Once gall formation is initiated, many galls will continue to form even if the insect dies.

Galls fully form very quickly but can remain on a tree for several years.

It is very possible that the gall will have served its purpose and the invader long gone.

Prevention

Gall makers must attack at a particular time of year to be successful (mid-spring). Missing this seasonal growth spurt, they will have little success in stimulating the plant to produce the tissue which forms the gall. Controlling the pest at that time would be required.

Since most galls do not adversely affect tree health, prevention is usually not suggested to "protect" the plant. Effective chemical sprays are expensive, can harm the environment and must be timed to coincide with initial insect/mite activity before gall formation begins. Once galls start to form, the organism is protected and it is too late for treatment.

Control

A few galls on a tree or plant seldom warrant control. They may be hand picked and discarded. Most galls are difficult to control since little is known about the gall makers and insect life cycles can vary so much. Check with your state Cooperative Extension office horticulturist or entomologist if you have a specific gall problem.

The Gall Types

Leaf galls - these are galls that appear on leaf blades or petioles and the most common galls. They may appear as leaf curls, blisters, nipples, or hairy felt-like growths.

They can be on the upper or lower leaf surface.

Stem and twig galls - these are deformed gall growth on stems and twigs.They can look like a large knotty growth but can be as simple as a slight swelling.

Bud/flower galls - These galls deform bud and flower structures enough to alter their shapes.