Gall Tree Diseases

A gall is the swelling of plant tissue caused by the infestation of a living organism. Galls can also be caused by injury, but they are most often caused by insects, mites, nematodes, bacteria, fungi or viruses. Galls can form on any part of a tree and come in many shapes, sizes, colors and textures. Galls that are caused by insects or mites form in reaction to the laying of eggs or the feeding of the insect that causes the gall. Most insect galls don't cause serious or long-term damage to an established tree or shrub. Galls can cause leaves to prematurely fall. In immature trees, twig dieback can kill the tree. Twig dieback is a condition that occurs when new growth dies, in this case as the result of gall infestation.

Wasps are the most common gall-causing insects and can cause galls on rose bushes and oak trees. The galls vary in appearance, depending on the host plant and the species of wasp causing the galls. Galls on the leaves cause little or no permanent damage to the tree, but twig or branch galls can cause harm or kill a heavily-infested tree. The wasps lay their eggs in the growing tissues of the tree in the spring. The wasps secrete growth-regulating chemicals that interact with chemicals in the plant to create the abnormal growth. The gall produced provides food for the larvae that live in the gall.

Mites (microscopic, wormlike organisms with two legs) are responsible for many galls. Mites are spread primarily by the wind. They cause flower galls on ash and poplar trees and leaf galls on elm, maple, linden, basswood, birch, poplar and cherry trees. Each type of tree has its own mite, which has evolved with the tree. Galls produced by mites are caused by a chemical the mite injects into the tree while it is feeding. The mites produce several generations a year.

The most common aphid-caused galls are stem and petiole galls on poplar and cottonwood trees. Aphids can also infect conifers such as Norway spruce, Douglas fir and hemlock trees. The galls are caused by the laying of eggs. The larva feeds on the gall that is formed as the egg hatches and the larva grows. The galls open and release winged adults, which lay eggs for the overwintering of the next generation.

In most cases, gall infestations don't need to be controlled; however, damaged leaves should be removed. If you want to get rid of galls, you need to start by determining the insect that causes the gall and work toward eliminating the insects.