With the discovery of emerald ash borer (EAB) in Minnesota in 2009, many homeowners are keeping a close eye on the ash trees on their property. This has resulted in a great deal of concern as many ash trees began to drop their newly formed leaves early this spring. In most cases, however, the cause of this early leaf drop was a common fungal disease known as ash anthracnose.
If a tree is suffering from infestation with EAB, the tree's canopy will appear thin, with few to no leaves.
Eventually, dead branches will be noticed within the tree. Cracks and D-shaped exit holes can be found in the bark of infested trunks, and woodpeckers may be noticed frequently visiting infested trees to feed on EAB larvae.
The emerald ash borer itself is a slender, one-half inch long, iridescent green beetle. It is active anytime from late May into August. In contrast, trees infected with ash anthracnose will have dark brown to black water soaked blotches on leaves and young shoots. These leaves are often distorted and curled around the infected area of the leaf. Infection on petioles and young shoots can result in leaf drop.
In some years, leaf drop can be quite severe, resulting in almost complete loss of the first flush of leaves. Infection is often most severe in the lower and inner branches of the tree, where high humidity favors fungal growth. Ash anthracnose is very common in cool, wet spring weather, but does not persist in warm, dry summer weather.
Ash anthracnose is caused by the fungus Discula fraxinea, and is a common problem on Minnesota ash trees early in the growing season. Whereas Emerald Ash Borer is a serious threat to ash trees, ash anthracnose is closer in severity to a common cold.
It's not pretty, it can be stressful to the tree, but it is rarely life threatening. The ash anthracnose fungus can infect most species of ash trees (Fraxinus spp.). In the Great Lakes states, green ash (F. pennsylvanica) is often most severely infected.
The ash anthracnose fungus overwinters in last year's infected leaves and twigs. New fungal spores are produced in early spring in these old infections. This coincides with the opening of tree buds and the emergence of new leaves and shoots. These young undeveloped plant parts are highly susceptible to the ash anthracnose fungus. If cool wet weather persists, ash anthracnose can be quite severe.
The good news is that mature ash leaves are relatively resistant to the ash anthracnose fungus (although infection can occasionally be seen associated with insect feeding or other wounds). As the weather turns warm and dry and tree leaves mature, ash anthracnose is no longer able to spread rapidly throughout the tree canopy.
Trees that lost their first flush of leaves, replace them. By midsummer, symptoms of the disease are often difficult to find.
Ash anthracnose is considered a minor stress to the health of a tree. A mature vigorously growing tree can tolerate complete leaf loss for two to five years. Reducing other stresses on the ash tree throughout the growing season can help the tree recover.
Simple activities like watering trees during periods of drought, mulching the soil at the base of the tree to reduce competition with turf grass, and avoiding wounding trees with lawn equipment will help the tree recover its strength.
Raking up and removing infected leaves at the end of the growing season will help to reduce the amount of fungi that survive from one season to the next, although some fungi often survive in infections within the tree canopy.
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