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Decay of a large, structural root by Armillaria fungi

Armillaria root rot is a fungal disease that can attack trees and shrubs.   While oaks are commonly attacked, many tree and shrub species can be colonized by Armillaria fungi.  These include most of the trees inhabiting our New England woodlands including maples, beech and hemlocks.  As the name implies, the fungal organisms responsible for this disease decay the supporting roots and the base of trees and shrubs.  While a decayed root system of a shrub will cause decline and death, a decayed root system of a large shade tree becomes a much more serious concern.

There are actually a number of fungal species in the genus Armillaria that can cause this type of effect.  Some are more aggressive pathogens than others and their pathogenicity can vary based on tree species.  The fungus can exists for years in soils living off decaying roots and stumps.  Healthy trees are usually not impacted. It is when trees become stressed or injured that thetheir defense mechanisms may become impaired to a point the fungus can colonize the tree. It starts by colonizing the main supporting roots of trees and works its way toward the base of the tree.  Usually by the time evidence of the fungus is apparent at the base of trees the supporting root system is seriously impaired.  This is important in the fact that you usually have little idea that major structural damage has occurred because it is unseen.  As the root system of a tree starts to decay and fail there is often a beginning of decline in the upper canopy of a tree.  This can often, however, be misleading based on the amount of actual damag below ground relative to what is showing above ground.

Armillaria rhizomorphs atacking the base of a tree

Armillaria colonizes wood through vegetative fungal structures called rhizomorphs.  These appear “shoestring-like” in appearance- in fact the common name of this disease is shoestring root rot. These thick, dark-colored masses of fungal hyphae look a lot like roots and can often be overlooked. Armillaria fungi can be commonly found in soils.  Even though you may see these rhizomorphs on a root or at the base of a tree they may not be actively colonizing and causing decay.  This is where the trees health and defense mechanisms, along with the pathogenicity of the species of Armillaria fungus determines its potential impact.

In advanced cases of attack, the base of trees starts to become decayed and both rhizomorphs and white-colored fungal tissue may be seen underneath the bark.  In the fall, honey-colored mushrooms in clumps arising underneath trees may be seen in advanced cases but not always.  The mushrooms will often have a ring on the upper stem, have gills on the underside and produce a white spore print.

Due to root and basal decay tree failure is a serious concern on trees with advanced decay.  Decay of large structural roots reduces tree stability. When Armillaria attacks the base of trees they can decline and die outright due to loss of functional cambium and conductive tissue.  Other times the fungus can also attack the inner, supportive wood (heartwood) on the lower several feet of a tree.  This leads to decay and a hollowing of the lower tree trunk and making it more

Armillaria mushrooms at the base of a Katsuratree in fall

susceptible to breakage.  How a tree that is being attacked by Armillaria will look above ground depends on the degree of conductive tissue loss at the roots and basal portion of the tree.  Often times a slow, general decline occurs over time and trees suffering a large amount of damage can still maintain a relatively acceptable appearance.  Sometimes the decay moves into the heartwood on the lower part of the tree and causes loss of structural support here while maintaining enough conductive tissue to support a fair amount of above ground growth. It is during this time hazard potential is high due to the fact homeowners may not be aware of the structural damage that has occurred.  Inspection of trees that are in decline by a certified arborist can identify trees that have an increasing hazard potential.

White fungal mycelia (on top of root) and the white wood decay (underside of root) caused by Armillaria

There are no “cures” for Armillaria root rot.  Maintaining the health of trees by avoiding tree stressing agents such as insect defoliaters, maintaing proper tree nutrition and avoiding drought stress are some best practices.  If you lose a tree due to Armillaria be aware the fungus can exist for many years by feeding on a decaying stump.  Grinding stumps can reduce the woody tissue needed by the fungus to exist in the soil.


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