Image of Armillaria


Armillaria root rot is a general name for a group of diseases caused by fungi of the genus Armillaria. There are many species of Armillaria (many of which have only recently been described), but in general they are pathogens of the roots and lower stems of both hardwoods and conifers and are important decomposers of wood. Armillaria root rot is also called shoestring rot because the fungus produces black stringy rhizomorphs below the bark of infected trees. Armillaria fungi are commonly referred to as honey mushrooms because they produce large, golden-colored mushrooms around infected trees and stumps. Armillaria root rot is most often a secondary disease of stressed trees, but occasionally the pathogen can attack healthy trees as well. Armillaria ostoyae holds the record as the world’s largest organism. 


The symptoms of Armillaria root rot often resemble many other diseases and disorders of trees such as drought, decline, Hypoxylon canker, Annosus root rot, and Phytophthora root rot. Growth reduction, chlorotic or scorched leaves, early fall coloration and/or premature leaf drop, branch dieback, wind-throw, and tree death are common above-ground symptoms. Conifers may produce large crops of undersized cones during decline.

Trees are often affected in groups.

Armillaria causes cankers (lesions) on the inner-bark and outer-sapwood on the root-crown and lower stem. Cankers may expand slowly and eventually kill large roots; entire stems are not usually completely girdled, but large lesions may cause dieback or death. In conifers, cankers are often pitch-soaked, and resin may ooze and dry on the canker surface. Some cankers do not expand at all if the tree’s defense responses are adequate; healthy trees may eventually compartmentalize infections. Callus / wound wood or scars may be visible at the site of old cankers for several years. After a tree dies, the fungus colonizes and causes decay in sapwood.

White mycelial fans (sheets of white fungal tissue) are often visible beneath the bark of cankers of rotted wood. Black or brown branched rhizomorphs (also fungal tissue) that resemble fine roots or shoe strings may also be visible beneath bark, on root surfaces, and may even extend into the soil.

Rhizomorphs may be flattened when found beneath bark, but are cylindrical (< 1/32 inches in diameter) when found on the bark surface or in the soil. Golden-yellow mushrooms may be produced around dead or diseased trees in the fall. Many species of Armillaria are bioluminescent. Presence of Armillaria signs does not necessarily mean the fungus is the cause of death or disease; it is a common wood rotter. 


Prevention is difficult; no practical treatment options are available. It is critical to maintain proper tree health. Select the proper tree species for the site; provide adequate water and fertilization if necessary. Avoid mechanical damage and soil compaction. Remove diseased trees and infected root systems if possible. 

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