Landscape - Armillaria Root Rot

Drought places enormous stress on landscape trees. Drought can influence susceptibility of trees to root and butt rot caused by the fungus Armillaria mellea. The drought of 2007 in Delaware may have increased the susceptiblilty of some trees. The following is information on this disease.

Some of our most valuable shade and ornamental trees are susceptible to Armillaria root rot disease. This disease is also called shoestring root rot disease because of fungal rhizomorphs closely resembling shoestrings produced by the fungus underneath infected bark, over infected roots, or in the soil. Among shade and ornamental trees, oaks and maples appear to be the most commonly infected, although the disease is occasionally destructive on a wide range of other woody plants. It is likely that the disease may occur on almost any tree or shrub grown, if the necessary conditions for infection are present. The disease is associated with trees previously in poor vigor, usually caused by winter injury, drought, or even construction damage.

Symptoms

The aboveground symptoms cannot be differentiated from those produced by many other diseases or agents that cause root or trunk injuries. Probably the most striking external symptom is a decline in vigor of a part or the entire top of the tree. Where the progress of the disease is slow, branches die back from time to time over a period of several years.
Signs Clearly identifiable signs of the fungus causing this disease are found at the base of the trunk at or just below the soil line or in the main roots in the vicinity of the root collar.

● Fan-shaped, white wefts of fungal tissue closely appressed to the sapwood are visible when the bark is cut away or lifted. Scraping or lifting the white wefts of mycelium, which have a strong mushroom odor, will reveal water-soaked sapwood. Where the entire top has wilted and died back, the fungal tissue will be found completely around the trunk. Where a large branch has died back, or only one side of the tree shows poor vigor, the fungus will be found on one or two main roots or on one side of the trunk base.
● Where the tree has been dead for some time, dark brown to black “shoestrings,” also called rhizomorphs, may occur beneath the bark or in the soil near the infected parts.
● Clusters of light brown mushrooms, called honey mushrooms, may appear in the vicinity of the rotted wood in late autumn. In landscape circumstances, infected trees are often removed before the mushrooms have a chance to form.

Disease management Armillaria root rot is very difficult to control:

● Provide good growing conditions for the tree. Pay attention to the need for additional water during drought.
● An infected tree whose entire root system or trunk is diseased cannot be saved. When the tree dies, the large roots in the vicinity of the trunk as well as the trunk itself should be removed and destroyed. Soil in the immediate vicinity should also be removed.
● Avoid replanting the same species as the one removed. Oaks, maples, and other highly susceptible species should be avoided.
● The following trees are thought to be less susceptible to Armillaria root rot except when growing under extremely stressful conditions: Bald cypress, boxwood, callery pear, catalpa, Chinese elm, cork tree, crabapple, gingko, hackberry, holly, honey locust, Japanese maple, magnolia, mulberry, pine, smoke tree, sumac, sweetgum, sycamore, tree-of-heaven, tuliptree, white fir.

Information from "Armillaria Root Rot" by John Hartman, Extension Plant Pathologist, University of Kentucky. Adapted from Kentucky Pest News, Issue 1068, Aug. 8, 2005.