Armillaria mellea

Shoestring Root Rot, Honey fungus, and Oak Root Rot Disease

Armillaria root rot disease affects many species of trees. Often found in forest environments, it can also occur in landscape plantings, orchards, and anywhere trees grow in temperate North America. The disease is caused by a fungal pathogen, Armillaria mellea, commonly called shoestring root rot.

Etiology

The fungus persists in the soil for many years, living on dead wood and roots. Rhizomorphs, long, thin runners of the fungus, grow outward seeking new sources of nourishment. The rhizomorphs are dark brown or black, about 0.3 to 1.8 millimeters thick, and resemble shoestrings in the soil. They grow along tree roots and dead wood, advancing when soil moisture levels are optimal.

The fungus usually grows along the surface of roots, seeking cracks in the bark, or other injuries and weak points. Healthy trees are seldom successfully attacked. Trees weakened by stress, such as drought or insect infestation, are most susceptible to the disease. Trees whose roots are damaged or injured by construction activity are also at risk of being infected. Healthy roots that come in contact with dead infected wood may also become infected.

Once the fungus penetrates the bark of the wood, it begins colonizing and spreading through the root. It kills the tissues as it progresses, gradually destroying large parts of the root system. As the disease advances, it eventually reaches the root crown at the base of the tree, where it girdles the trunk. At this time, symptoms appear to spread rapidly and the tree quickly declines. Decay of the structural support roots increases the possibility of uprooting and wind-throw.

Symptoms

The first noticeable symptoms on the tree are usually a general decline in vigor. Leaves are smaller than normal, and often are slightly chlorotic or yellowed. Twig growth is reduced, especially at the terminal twigs. The symptoms may appear only in one portion of the crown, or on a single scaffold limb. The symptoms will spread during the growing season, and over several years may become apparent throughout the canopy. Branches and limbs usually die within one year of first showing symptoms, but may persist for two or even three seasons before dying completely.

Armillaria may be suspected whenever decline symptoms are seen. Trees at high risk due to stress or injury should be carefully inspected. Root collar excavations will reveal the extent of infection and decay at the base of the tree. Portions of the root system several feet from the base of the tree should also be inspected for infection by Armillaria. The brown-black rhizomorphs are symptomatic of the disease, and are easily found on close examination.

Treatment

There are no chemical treatments currently available for control of Armillaria on infected trees. In cases where infection is limited to a small portion of the root collar or root system, excavation of that portion may slow or temporarily stop disease progression. The infected portions should be carefully exposed, and remain uncovered by soil or mulch for at least one year. This allows the exposed tissue to dry on the surface, inhibiting the spread of the fungus.

Any areas of dead bark on the roots or root collar should be excised, back to healthy, uninfected tissue. Cultural treatments, as recommended for prevention, should be applied to increase the vigor of the tree. In some cases, the tree can compartmentalize the infection, and eventually recover from the disease.

Prevention

Prevention of the first infection of the disease is most important, since there are no direct curative treatments available. Maintaining the overall health of the tree is the main consideration in a preventive program. Prevention of stress factors will greatly reduce the chances of infection by Armillaria, or other root diseases. Care should be taken to avoid any injury to the root system of the tree from trenching or construction damage. Proper mulching of the tree, and watering during dry periods, will reduce stress from drought and heat. Periodic fertilization, based on soil analysis, can help to maintain the vigor of the tree. Keeping the tree free of other pests is also important in reducing stresses.

If nearby trees are removed for any reason, the main stump and structural roots should be promptly removed. The dead wood left in the soil provides a source of nourishment to the fungus, and becomes the center from which it spreads.

Summary

Armillaria root rot disease is a fungus that is present in many areas, infecting many species of trees in the forest and in the landscape. It primarily infects injured or stressed trees with low resistance to the infections. Prevention of the disease is best accomplished by maintaining healthy and vigorous plants, and by preventing stress factors that weaken the trees or lessen their vigor.

While direct treatments are not available, in some cases excavation of the root collar and the surrounding structural support roots may reduce the rate of spread of the disease. Infected trees must be inspected regularly to assess the risk of uprooting or wind?throw as the root system declines and decays. Trees that have been extensively colonized by Armillaria, and with decay of the roots or root collar evident, should be removed if they are a hazard to nearby objects.

© Copyright 1996, Russell E. Carlson

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This page was last updated on June 11, 2003.