What Is the Tree Fungus Armillaria?


Armillaria is a fungal root disease that can be found in both temperate and tropical climates. It is caused by a soil-borne fungus. Armillaria is also known as shoestring root rot, honey mushroom, honey agaric, mushroom root rot or toadstool disease. There are more than 700 species of conifers, deciduous trees and some herbaceous plants that are susceptible to the disease. It is important to know the symptoms and life cycle of this fungal root disease.

About the Fungi

The fungus (Armillaria mellea and several closely related species) live in both living hosts (trees and plants) and on dead woody material, such as felled trees, tree stumps and dead tree roots. They are called parasites when they live on a living host tree or plant and saprophytes when living on dead material. According to the U.S. Department of Agriculture Forest Service, “… these fungi are a natural component of forests, where they live on the coarse roots and lower stems of conifers and broad-leaved trees.”


Armillaria mellea fungus infects living trees, causing a reduction in growth and wood decay. It can also kill the tree. Weakened trees are more susceptible to armillaria root disease, yet healthy trees can also be infected. The fungus can kill a tree or leave it in a weakened state, leaving it more susceptible to attack by other pests or disease. Some infected trees are easily blown over in a windstorm.


Symptoms are not always apparent since the fungus attacks the roots of a tree. Sometimes you may see mushrooms growing around the base of the trunk. There are also some crown symptoms to look for, such as thinning and discolored (yellow/brown) foliage, a reduction in growth and dieback of branches. Crown symptoms can develop rapidly on small, severely infected trees, or they can take several years before they develop on large, healthy, slightly infected trees. Conifers may show a large amount of resin on the lower stem--the reason the disease is called resin glut or resin flow. Deciduous trees may show signs of sunken cankers on the trunk. If the tree is infected, bark removal will expose white mycelial mats between the wood and the bark.

Life Cycle

The soil-borne fungus survives in dead/dying tree stumps and roots of infected trees. It is spread by: root-to-root contact--and by spores that are released by mushrooms that grow at the base of the trunk of an infected tree--to dead stumps and/or injured tree bark.


Susceptible tree species should not be planted in an area where Armillaria fungus is known to be present, or where trees have previously died from the disease. Instead choose a tree that is resistant to armillaria root rot, such as gingko, magnolia, sweet gum, incense cedar or dawn redwood. Infected roots and infected tree stumps should be removed and destroyed. Trees that are susceptible should be watered during drought conditions. Any dead, diseased or damaged branches should be pruned as soon as possible because damaged branches provide an entryway for disease. There are no known chemical treatments for this disease.

Keywords: armillaria root disease, fungus trunk roots, shoestring resin glut

About this Author

Paula M. Ezop’s inspirational column "Following the Spiritual Soul" appeared in "Oconee Today," a Scripps Howard publication. She has published her first book, "SPIRITUALITY for Mommies," and her children's chapter book, "The Adventures of Penelope Star," will be published by Wiggles Press. Ezop has a Bachelor of Arts degree from Northeastern Illinois University and has been writing for 10 years.