Galls on plants are mostly harmless, expert says

It can be unnerving to find that the leaves of an oak or hackberry tree are as bumpy as a rock-covered seashore in late summer. Other trees’ leaves may break out in acne-like red spots or sport projections as bristly as a porcupine.

“Chances are, those strange growths are galls,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “Some galls look weird, but the overwhelming majority of them are doing the trees no harm.”

A gall is a plant’s response to an irritant in its tissue. Galls can occur on leaves, twigs, branches or roots. Although galls can be provoked by bacteria, fungi or nematodes, the irritant is most often eggs laid by tiny native wasps, mites or other insects.

The plant surrounds each egg with its cells to seal it off. Inside the gall, the egg hatches into a larva. The surrounding plant tissue provides protection and a food source as the insect grows and goes through its life cycle. Eventually, an adult insect burrows out of the gall and departs.

Many kinds of trees, such as oaks and hackberries, have hosted gall-provoking insects for thousands of years. “The trees and the insects have evolved together,” Yiesla said. “They’ve worked out how to get along.”

Only a few kinds of gall, often caused by fungi rather than insects, cause actual harm, making some branch tips die back. Most galls, especially leaf galls, are just an aesthetic issue.

Since trees usually take galls in stride, there is no need for homeowners to take action. “The Arboretum does not recommend pesticides to treat or prevent galls,” Yiesla said. In any case, by the time you notice galls, it’s too late for an insecticide to work. “Once a gall is formed, the plant’s own tissue will keep the chemical from reaching the insect inside,” she said.

If you find a gall on a branch unsightly, prune it off, Yiesla said. Leaf galls are usually so numerous that most pruning would be pointless. “A tree that has galls on any leaves probably has thousands of them,” she said.

Galls take a wide variety of forms — splotches, balls, knobs, lumps, warts, spines and fuzz. Each form is caused by a particular species of insect. For example, oak spangles are galls that look like little disks on the back of an oak leaf. The spangles, like sequins on a figure skater’s costume, form around the eggs of a small wasp. Hedgehog galls appear on oak leaves as clumps of bristly pink fur, while wood sower galls look like white or creamy cotton balls. The fluff is actually a group of wasp-induced galls.

“Some kinds of galls can be unsightly if they occur in large numbers, but the effect is temporary,” Yiesla said.

For example, jumping oak galls are caused by wasp eggs laid in the leaves of such trees as white oaks, bur oaks and swamp white oaks. The galls are little bumps, like beads. In autumn, the beads detach from the leaves and fall to the ground. The activity of the larvae inside the beads may cause them to hop around a bit like jumping beans before settling in to overwinter under the leaf litter.

On the leaf left behind, the site of the detached gall will be a brown spot. A severely affected tree may have many leaves covered in brown and may drop some of them prematurely. “The tree will usually recover on its own,” she said.

If you are concerned about a growth or abnormality on any part of a tree, get it clearly identified by an expert such as the Arboretum’s Plant Clinic before you take any action. “It may be a problem, or it may be nothing to worry about, like a leaf gall,” Yiesla said.

The best way to help trees live with galls is to keep them healthy. Make sure there is moisture for their roots to absorb by watering them when the weather is dry. Cover the root zone with a wide, even layer of mulch to insulate the roots, protect the trunk and improve the soil.

“Galls may look freakish,” Yiesla said, “but in most cases, they are a normal part of the life of a tree.”

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.