Utah Pests News Summer 2009

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Disease Spotlight: Armillaria Root Rot

Armillaria may be more widespread in Utah than we know.  Organisms of the phylum Basiodiomycota will form short-lived fruiting bodies only when enough moisture is present, which isn’t always the case in the arid West.  Without the obvious mushrooms, armillaria cannot be identified without invasive investigation.  Removing infected bark will expose a thick mat of white mycelium (shown in upper and lower image), and sometimes the black string-like rhizomorphs (shown lower), both of which are characteristic of this disease.  Rhizomorphs are tightly bound hyphal structures that “explore” for food in distances of up to 10 feet.  They penetrate healthy roots by a combination of mechanical pressure and enzyme action–wounds are not necessary.

Still not sure if it is armillaria? Look at the mycelial mats
at night and they will glow–they are bioluminescent.

This spring, the weather has been moist enough in northern Utah for a host of fungal fruiting bodies to emerge, including the mushrooms of the “honey fungus,” armillaria.  The mushrooms are only a small part of the organism–like apples on a tree.  The main body (thallus) lives underground, helping to decompose wood, or causing a “shoestring root rot” disease in a wide variety of woody plants.  Armillaria species are most commonly found in native forests, but Utah landscape hosts include almost all conifers, ash, birch, black locust, dogwood, elm, hackberry, maple, oak, planetree, poplar, sycamore, willow, and most fruit trees.


Armillaria invades the roots, causing symptoms of the crown that resemble a host of other problems.  Trees that are infected by a less aggressive strain will decline over a period of several years.  The canopy will be thin, with stunted growth, off-color foliage, branch dieback, and on conifers, an excessive crop of cones before death.  Aggressive forms of armillaria will kill trees within one season.

At the initial stages, decayed wood looks healthy in color, but water soaked.  Advanced decay is whitish in color, and spongy in hardwoods and stringy in conifers.  This type of decay is called “white rot.”


Armillaria has a tricky adaptation that allows it to become named the “humongous fungus.” (In 1992, an underground mat representing 37 acres of a continuous fungal clone in Michigan was found to be the heaviest in the world–weighing as much as a blue whale. )   Armillaria can live as a parasite, causing disease as described above, or it may live for decades as a saprophyte, happily feeding on coarse woody debris.  When the opportunity arises, it can easily transform its metabolic processes from absorbing dead tissue to absorbing living plant tissue.  Spread of the disease occurs when vigorous rhizomorphs grow through the soil and happen to contact healthy tree roots.  Root-to-root contact or root grafting can also spread armillaria from an infected to an uninfected host.


Like all wood-invading fungi, once it has become established, there is no chemical that can eradicate it.  Since armillaria is a root rot, pruning the canopy or removing a portion of the tree will have no effect.  Trees that are suspected of light infections can sometimes survive several years with optimal watering and fertilization.  Dead trees should be removed.  Grinding the stump will prevent armillaria from remaining in the site longer than one to three years.

-Marion Murray, IPM Project Leader