|Ash tree defoliated by anthracnose|
J. O'Brien USDA Forest Service, Bugwood.org
Emerald Ash Borer
|Thin canopy of an EAB infested tree|
Jeff Hahn, UMN Extension
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.
Help Diagnosing the Problem
Concerned homeowners have many resources available to help them determine which problem is occurring in their ash tree. What's wrong with my ash? Is an online diagnostic tool that helps gardeners diagnose ash problems through a series of simple questions about what symptoms they are seeing. Gardeners should also visit the UMN Extension EAB resource webpage. This page includes many useful tools including a step by step guide to determine if EAB is a possibility in your tree, a publication on insects that may be confused with emerald ash borer, and maps showing where EAB has been found in MN. Those without internet access can call Forest Resources Extension at 612-624-3020 for further assistance.
What is Ash Anthracnose?
|Leaf symptoms of ash anthracnose|
M. Grabowski, UMN Extension
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.
What to Do About Ash Anthracnose?
Ash anthracnose is considered a minor stress to the health of a tree. A mature vigorously growing tree can tolerate complete leaf loss for 2-5 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.
Although there are fungicides that will prevent ash anthracnose, it is too late this year for fungicide applications to be effective. Fungicides must be applied early in the growing season to protect young emerging leaves and shoots. Homeowners should not apply fungicides to mature shade trees. The spray equipment available to homeowners will not provide effective coverage in a mature tree, and serious safety risks can occur to homeowners that attempt fungicide application with inappropriate equipment. Fungicide applications are only recommended in trees that have suffered severe leaf loss due to ash anthracnose several years in a row. In this situation a tree care company with a certified pesticide applicator (licensed by the MN Department of Agriculture) should be contracted to safely apply the fungicide at the appropriate time of year.