Dealing with Leaf Galls

What Are Those Bumps on Your Tree's Leaves?

Leaf Galls
Photo: © Marie Iannotti

Have you ever picked up a leave that was dotted with bumps or had long protrusions dangling from it? Chances are these are leaf galls.

What are Leaf Galls

Leaf galls are a frightening sight, but are not usually as serious as they appear. These bumps and deformities are usually the result of feeding by insects or some other foreign organism such as bacteria, fungi, mites, nematodes, and even viruses.

These organisms are usually not still on the plant leaf. The gall itself is actually the plant’s response to the irritation. It’s not unlike the bump you get when an insect feeds on you, expect the leaf gall is not going to go away.

Despite appearances, the insect is not living in the gall. In fact, it is very likely that once you notice the galls the insects have moved on. Before they do, they can do a lot of cosmetic damage to many plants and in particular trees. Galls can also form on stems and flowers, however leaf galls seem to be the ost prominent and get the most notice.

It is the fresh new growth that are attacked by the insects and other organisms and then produce the galls. Mature leaves are rarely affected. However many common trees are susceptible to leaf galls, especially when first leafing out in the spring. Maple, oak, elm, hackberry and others each are favored by a different insect that causes unsightly and intimidating galls.

Damage will be greater following a mild winter, since more insects have survived and are hungry. Galls won’t usually kill a tree, but they may cause early leaf drop. A healthy tree will send out new growth and recover.

What Can You Do About Leaf Galls?

Since the damage occurred before the gall formed, treatment is rarely recommended.

If you have a serous reoccurring insect problems, you can spray your tree in early spring, to lessen the severity of the damage. Contact your local extension office for specific guidelines and recommendations in your area. But if you're patient, nature may take care of the problem for you. Gall making insects tend to attract their own predators.

One way leaf galls can cause a serious problem for trees is that their formation requires energy and nutrients from the tree, which can stress and weaken the tree itself, as it is coming out of dormancy in the spring. This can happen when there is an unusually high concentration of galls on the plant or when the plant is attacked and galls are produced several years in a row. For this reason, you should not ignore the presence of galls entirely. If this is the case, you should consider finding out what organism is causing the galls and treat for it the following spring, to prevent further stress and damage.