Swelling on a Tree Branch

Cause

• The swollen area that appears on tree branch is referred to as a gall. The deformity is primarily caused by an infestation of a live organism in the tree. These organisms include fungi, viruses, bacteria, insects, mites and nematodes. Some galls also occur as a result of mechanical injury to plant. Of the insects and mites alone, there are more than 1,441 species likely to create galls on trees, cites the University of Illinois Extension website.

Growth Process

• The growth of galls on tree is in response to the feeding or egg laying of the causal organism in the plant tissue. By creating this raised area with additional cell multiplication in the plant tissue, the tree seeks to provide protection to these eggs or the microorganism from other predators, weather and parasites. Many causal agents grow within these galls and emerge at maturity. Because the production of galls occurs in live plant tissue, galls do not form on tree areas that are no longer growing.

Types

• Gall-creating organisms have specific host plants. These organisms also have to time their attack on a certain tree precisely as trees will not start to create galls at all times of the year. Some galls common to shade trees include the ash midrib gall, witchhazel gall, willow shoot gall, oak tree gall and honeylocust pod gall.

Management

• The growth of galls on trees is not serious enough to affect tree health. In cases of severe gall development on foliage, there is a chance of premature defoliation and deformed stems. However galls are rarely the cause of plant death. The use of chemicals to control galls is not recommended as it is a nearly impossible task. Unless the chemical is used before the formation of gall, it will not be effective. Even if the gall maker is eliminated by the pesticide, the process of gall growth initiated by the tree will not be interrupted.