Armillaria Root Rot Disease in North American Forests

The Main Cause of Associated Oak Decline

Armillaria Root Rot
Armillaria Root Rot. USFS, FIDL Leaflet 78

The Armillaria sp. group of pathogens attack the roots and base of hardwoods and softwoods as well as shrubs, vines, and forbs. The two most common names of this very common tree disease are Armillaria root disease and shoestring root rot.  It is pervasive in North American forests, is commercially destructive and a major cause of oak decline.

The principle fungus called Armillaria mellea primarily attacks and kills trees that are already weakened by competition, other pests, or climatic factors.

The fungi can also infect healthy trees, either killing them outright or predisposing them to attacks by other fungi or insects.

One very interested fact about this pathogen is that its mycelium, the vegetative part which is a network of white filaments, is bioluminescent in active rotting wood. This night glow is an identifier and often called "foxfire".

Disease Identification and Symptoms

These fungi commonly inhabit roots which could make their detection difficult unless characteristic mushrooms are produced around the base of the tree or symptoms become obvious in the crown or on the lower stem. Fortunately, there is an easy way to definitively diagnose the disease by exposing roots and scraping bark.

Crown symptoms on conifers and broad-leaved trees vary somewhat but in general, the foliage thins and discolors, turning yellow and then brown. You will see some branches dying back and shoot and foliar growth is reduced.

However, these symptoms are common for a number of insects and diseases.

Basal symptoms are the most reliable way to diagnose a fungal attack. By scraping the bark covering a resinous conifer with resinosis (bleeding sap), the characteristic, white mycelial mats or the rhizomorphs that grow between the wood and the bark are obvious.

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The white mycelial mats (smelling like mushrooms) are marked by irregular, fanlike striations that are commonly called mycelial "fans." The thick mats decompose, leaving impressions on the resin-impregnated inner bark. The shoestring-like rhizomorphs are often attached to infected roots but can also be attached to uninfected wood. Fruiting mushrooms sometimes may be seen in summer and appear as clusters near the base of the tree.

Pathogenic Biology and Spread

Armillaria species reproduce by both sexual and asexual processes. Sexual reproduction occurs when the infected tree has hosted the disease over time. Under the right conditions, mushroom fruiting bodies attached to the tree ultimately develop special cells called basidia to complete the sexual cycle. Asexual spread occurs by growing as mycelium through root contacts or root grafts between two trees, or by growing through the soil as rhizomorphs.

This pathogen can live for decades in the existing coarse woody material that is its food source. This ability to survive over a long period is a major way the fungi can spread to a new living host(s). Spread then occurs when rhizomorphs, growing through the soil, contact uninfected roots or when uninfected roots contact infected ones.

With the ability to reproduce in multiple ways, it is hard to find a simple and practical way to reduce the disease spread without proper plant and forest management. There are only limited ways to use chemicals and other expensive agents on large areas where the disease is endemic. Thus, it is very important in determining the best solution by defining areas prone to the disease, understanding the host systems and following various approaches that may be effective at reducing disease incidence on a large scale.

Effect on Hardwoods Versus Conifers

Remember that both conifer and hardwood trees of different species and sizes may be killed individually throughout stands. This pattern often occurs in managed stands reforested with species unsuited to the site but may also occur in unmanaged stands.

Hardwoods - Hardwood trees infected with Armillaria can experience a "decline" in growth with rapid individual tree mortality. They will be heavily decayed (white rot) with root loss resulting in increased windthrow. Armillaria can infect non-wounded root systems of trees, and the fungus spreads from tree to tree through roots, and via rhizomorphs (strands of hyphae - tubular cell arrays resembling a fine root) which infect intact bark.

These fungi produce a honey-colored mushroom around the base of the tree in late summer or fall during moist periods. Damage caused by Armillaria mellea contribute to tree decline in association with other factors such as site, tree age, drought and insects.  

ConifersArmillaria kills trees, primarily conifers, in a pattern progressively expanding from the center of diseased spots. These spots can develop in both managed or unmanaged stands and can range from small areas affecting several trees to areas of up to 1 ,000 acres. Within the infected area and on the expanding margins, there will be trees in varying stages of decline normally present. One or all species and sizes of conifers may be affected.

Disease Management

Armillaria root disease "is among the most studied diseases of trees in the world" according to and "in many cases, we do not have an effective, practical means of reducing the disease." However, there are moderately effective methods used in the management of the disease.

It has been determined that the disease can only be effectively controlled by forest and orchard best management practices.

Here are those practices:

  • Tree Species Selection -  Resistant tree species should be favored. Correcting this occurs during planting, regeneration cuts, thinning, and partial harvests. 
  • Removing the Pathogen -  Its practicality is limited to certain silvicultural systems. It is most effective when combined with root raking to remove mid-sized roots that are not extracted with the stump.
  • Alternate Use - If you have a disease prone area, you might consider an alternate use including wildlife habitat, outdoor recreation or potential development.
  • Tree Thinning - Tree density in excess will cause stress and increasing stress causes disease. Thinning will theoretically reduce the disease but I have found no advocate that assures success.

  • Biological Control - There is a way to control the disease with an inoculation of an "antagonistic fungi" to colonize the target stumps. More study needs to be done here. 

  • Chemical Treatment - This is only for high-value, small area control. Fumigants, such as chloropicrin, carbon disulphide, and methyl bromide are sometimes used in orchard crops to help eradicate inoculum from soil, usually after stump removal and before planting. It is not applicable to forestry practices.