Ash trees are widely planted as landscape trees throughout the United States. They are desirable because of their rapid growth rate and spreading habit that creates dense shade in the summer. However, because of their short life, 30-40 years, and profusion of small tender growth in the spring, they are prone to several diseases that can adversely affect the health of the tree.
Ash yellows appears as a yellowing of leaves and a slowing of growth over a five year period. It is not noticeable until the tree has completely declined because the eventual slowing of growth is not usually noticed until it is too late. A healthy ash tree should put on about six inches of new growth a year. A tree infected with ash yellows puts on half that and then stops growing altogether after several years. The leaves may turn yellow and drop prematurely in the fall. There is no known cure for ash yellows.
Verticillium wilt is a fungus that lives in the soil and infects many types of trees and shrubs including ash trees. When an ash tree is infected, the tree closes off the cellular structures in the infected limbs to protect itself from the disease. This in turn causes the area of the tree that is infected to die back due to lack of water and nutrients. Verticillium is easy to identify because only a portion of the tree is affected at first. Eventually, it can affect and kill the entire tree. For control, you can cut out the dead portions of the tree, but the disease is usually fatal in the long run. Occasionally a tree can recover with fertilization and extra water added during the driest part of the growing season.
Ash decline is a term used to describe an overall decline in the health of an Ash tree. It is usually caused by a combination of diseases. It appears as the stunted growth or yellowing of leaves that occurs with ash yellows and the die back of limbs common with verticillium wilt. The tree declines over several years and eventually dies. There is no cure for Ash Decline. After the tree becomes unattractive from the shedding of small and dying limbs over several years, it is best to remove the tree from the landscape.