Galls on trees can be formed by a variety of different diseases insects, mites, bacteria, fungi, viruses or injury. Most often galls are caused by insects or mites. If gall attacks the leaves, usually no harm or damage will come to your tree. But if your tree becomes heavily infested with gall on its branches then it can harm or kill your tree.

Galls attack all parts of the tree, but most often it attacks the leaves of the tree. Galls also vary in size. They can be small like ¼ inch, or they can grow to over 12 inches. It is important to remember to disinfect any tools that you used for the job. This keeps the disease from spreading to other trees. Combine a mixture that consists of 20 percent commercial bleach with 80 percent water. Either dip the tools into this solution, or wash the tools with a rag dipped in this solution. Wipe the tools off with a dry towel to keep them from rusting. Although there are hundreds of different types of galls, this article will cover some of the more frequent ones that may attack your tree.

Galls from Wasps
When a gall is caused by a wasp laying her eggs in the tissues of the tree, they have some distinctive features. The wasps generally attack the leaves or twigs of the oak trees or roses. The gall will be green or red in color and round or irregular in shape. The larvae eat the food inside the gall to survive and grow. The galls will deform the leaves of the trees. If you notice gall on the roses, you can prune them away. If the gall is on the leaves, you can either leave them or use carbaryl in the spring. Carbaryl is toxic and in some countries it is illegal to use. For an available alternative to where you live, talk to your local county extension agent. They can give you a list of safer solutions to use.

Cedar Apple Rust

Cedar apple rust is also called Gymnosporangium juniperi-Virginian. This gall starts out small, but when exposed to the spring rains, they swell and grow bigger. The cedar apple rust usually forms at the tips of the cedar trees and they are easily recognized. They are dark orange in color and they look like a rubbery, gelatinous substance with many telial horns coming out of it. If left on the tree, the galls will die during the summer months. These should be pruned off the tree when you see them because they can cause the tip of the branches to die. By removing the galls as they appear, you will be breaking the fungus life cycle.

Crown Gall

Bacterium causes crown gall and generally cherry and apple trees are the most susceptible. This disease attacks the roots and stems of the tree. When the disease is young, the gall will appear soft and light tan in color. As the gall ages, it turns hard and black in color. This disease can remain in the ground for a year or longer. To get rid of crown gall, you'll need to find a special pesticide made for this type of gall.

Ash Flower Gall

This gall is caused by a tiny mite called the eriophyid mite. You'll need a magnifying glass in order to see this mite. During the winter, the mites live under the buds of the ash tree. They feed on the male flowers in the spring. The galls range from a half-inch to one inch. They do not harm your tree, but they can make it look bad with deformed flowers.