Armillaria Root Disease

by Bob Tiplady
Both aspects of Armillaria root disease; cambial death and localized wood decay.

Armillaria root disease, also known as shoestring root rot, is probably the most damaging disease of forest, shade and ornamental trees and shrubs around the world.  For the most part, Armillaria fungi grow as saprophytes and are beneficial as  digesters of downed and dead timber and root systems. In the western US, Armillaria species are often aggressive pathogens.  In the eastern US, Armillaria species are predominantly pathogenic on stressed trees. Old age, prolonged drought, root damage, shading by taller trees, air pollution, insect defoliation, prior root infection by other fungi, wood-boring insects or other stresses allow Armillaria to infect trees and shrubs.  Armillaria invades the bark and cambial region of roots and root collars, girdling and then killing trees and shrubs of all sizes.  It can also cause root decay and will not necessarily kill infected trees.

Mycelial fans, rhizomorphs and honey mushrooms are diagnostic signs of Armillaria root disease.  The fungus forms mats of white mycelium called mycelial fans in the inner bark and between the bark and sapwood of the infected roots and they may extend up the trunk of the tree. They almost look like they?re painted on.   These fans, some of which glow in the dark (foxfire), are unmistakable signs of the fungus. Rhizomorphs, thick strands of hyphae that resemble shoestrings, are commonly produced by some Armillaria species.  Rhizomorphs are brown to black in color and they grow along the surfaces of living or dead roots, outward into soil or up between the bark and wood of dead trees.  Rhizomorphs under the bark are flattened and interconnected, but those found in the soil, litter or decayed wood are round and more dispersed. The mushroom stage of the fungus develops annually in autumn.  Mushrooms are honey colored to brown and sometimes dotted with brown scales.  They appear on or near wood that is undergoing decay, often arising from rhizomorphs connected to decaying wood.  In some strains, the mushrooms are luminescent, too.  Spores that disperse from these mushrooms can germinate and establish colonies in recently wounded sapwood, especially in stumps.

Stressed trees may become colonized by rhizomorphs growing from a nearly infected root system or by reactivation of a quiescent lesion on its own root system.  If the stress is abated and tree vigor is restored, colonization does not continue.  Localized wood decay caused by this infection does, however, continue.  Trees and shrubs attempt to resist Armillaria by producing phenolic and other compounds and walling off this fungus with protective callus tissue.

The crown of an infected tree may die either gradually, one limb at a time, or rather suddenly, depending on the extent of injury to the roots and the abundance of water.  If there is sufficient water, the crown of a tree may remain green and apparently alive for a year or two after practically all its roots have been killed by the fungus, but it is likely to die suddenly when the water supply becomes insufficient.  This is the  reason that periods of drought accentuate the damage done by the fungus.

Large trees often fight a seesaw battle with Armillaria  and are able to confine the fungus within cankers on roots or the root collar for years. When thus restricted, Armillaria may still decay the wood beneath the lesion but is usually compartmentalized (walled off with protective wood).  In many conifers infection stimulates heavy flow of resin which may saturate the bark and wood near points of attack, accumulate in pockets between bark and wood and seep out onto the surface and into surrounding soil and litter.  Symptoms above ground are gradual growth reduction, yellowish or undersized foliage, premature leaf drop, branch dieback in the upper crown, or rapid browning and death during summer.  Growth reduction becomes noticeable after more than half of the root system is dead.

Armillaria can also persist in dead stumps and dead root systems.  Where broad-leaved trees, especially oaks, were harvested, the stumps and roots are food sources for the fungus.  This allows Armillaria to buildup and cause infection and mortality among planted saplings for ten to fifteen years, or until the stumps are finally rotted away.  Young pines affected by Armillaria root disease stop growing, turn color to an off-green and the entire sapling seems to die off suddenly.

Promoting tree and shrub vigor, minimizing stress and denying the fungus large food bases by removing stumps and roots from soil can be important and effective Armillaria root disease prevention strategies.