Abnormal growths, or lumps, on tree branches and stems are usually galls. Some trees are more susceptible to certain types of galls than others, and some galls occur only in certain regions of the country. Not all galls are caused by disease. Insect activity is a major source of galls on tree branches and foliage. Galls come in a staggering array of sizes, shapes and colors, so identifying the type of gall is essential to prevention and treatment. Most galls are benign and, other than potentially damaging insects they may harbor, do not harm the tree. Some galls, such as black knot gall on plums and cherries, may eventually kill the tree. Fusiform rust galls on pine trees can cause the tree to break off at the site of the gall.
Bacteria and Fungi
Fungal infections and bacterial diseases may be carried to the tree on the wind, injected into the tree by insects feeding on the tree, or from the ground. In the Southeast, loblolly and slash pines may suffer from fusiform rust (Cronartium quorum f. sp. fusiforme), a two-stage infection that affects both oak and pine trees. The galls develop on main stems and branches and produce bright orange spores in spring, and often kill the trees. Black knot is a fungal disease not only of cherry and plum trees, but also apricots and peaches. The spindle-shaped galls usually develop on young branches in late spring and turn dark as they elongate and slowly encircle the branch. Black knot can range in length from half an inch to more than a foot and older galls may generate new, smaller ones. Bacterial crown galls generally appear at the base of the trunk, near the top of the roots, but can appear higher on the trunk, on the branches and the limbs. The galls are round and spongy at first, then turn hard inside with a cork-like exterior.
Many species of small wasps cause galls on tree branches, twigs and limbs. Oak trees are especially susceptible to galls. Woody galls with small, spiky projections are called horned oak galls. Similar galls, but without the "horns," are called gouty oak galls. Oak bullet galls are small, brown growths that appear in long clusters, like a row of bullets. Inside the galls, which can girdle and kill branches, are female wasps that will eventually emerge to lay eggs and begin the cycle anew.
Use a fungicide if the tree's galls are caused by bacteria or fungi. First, identify the specific infection, then choose a fungicide formulated specifically to treat the problem. Follow label directions for when to apply the product as well as how much and how often. Always sanitize pruning shears and other gardening tools in between uses and in between plants to avoid spreading infection from one tree to another. If the galls on your tree present a significant danger to the tree, you may be able to excise the gall, cutting down to the wood under the gall and an inch or two beyond the gall's margin. Prune branches, when practical, to get rid of the galls, but be sure to prune at least 2 inches below the gall margin to make sure all of the fungus is removed. Burn or bury the cuttings away from the tree to prevent reinfection. Remove and destroy horned oak galls as soon as you spot them on the tree.
Plant disease-resistant tree varieties whenever possible. Keep trees healthy with proper watering and fertilizing. Test the soil to be sure the pH level is correct and the tree is not starved for iron, phosphorous and other essential nutrients. When necessary, prune to improve air circulation. Remove plant debris and weeds from the ground around the tree and make sure the soil has the proper drainage. Some fungicides work better at prevention than control. Most should be applied in early spring, before budbreak, to prevent infection.
- Iowa State University; Insect Galls on Trees and Shrubs; Mark Shour, et al.; September 2005
- Purdue University Extension; Landscape & Ornamentals; Clifford S. Sadof, et al.; January 2010
- University of Illinois Extension; Trees with a Lot of Gall; Sandra Mason; May 2006
- Michigan Technical University: Galls
- Pennsylvania State University; Galls on Oak; Gregory A. Hoover; March 2004
- North Dakota State University; Deciduous Tree Diseases; November 2005
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