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Armillaria solidipes
Armillaria ostoyae.jpg
Scientific classification
Kingdom: Fungi
Division: Basidiomycota
Class: Agaricomycetes
Order: Agaricales
Family: Physalacriaceae
Genus: Armillaria
Species: A. solidipes
Binomial name
Armillaria solidipes
Peck (1900)

Agaricus congregatus Bolton 1791
Armillaria mellea var. obscura Gillet 1874
Armillariella ostoyae Romagn. 1970
Armillariella solidipes (Peck) T.J.Baroni 1981

Armillaria solidipes (formerly Armillaria ostoyae) is a species of fungus in the Physalacriaceae family. It is the most common variant in the western U.S., of the group of species that all used to share the name Armillaria mellea. Armillaria solidipes is quite common on both hardwood and conifer wood in forests west of the Cascade crest. The mycelium attacks the sapwood and is able to travel great distances under the bark or between trees in the form of black rhizomorphs ("shoestrings").

In most areas of North America, Armillaria solidipes can be separated from other species by its physical features. Its brown colors, fairly prominent scales featured on its cap, and the well-developed ring on its stem sets it apart from any Armillaria. (Herink, 1973)

It is known to be one of the largest living organisms, where scientists have estimated a single specimen found in Malheur National Forest in Oregon to have been growing for some 2,400 years, covering 3.4 square miles (8.4 km²) and colloquially named the "Humongous Fungus." Armillaria solidipes grows and spreads primarily underground and the bulk of the organism lies in the ground, out of sight. Therefore, the organism is not visible to anyone viewing from the surface. It is only in the autumn when this organism will bloom “honey mushrooms”, visible evidence of the organism lying beneath. Low competition for land and nutrients have allowed this organism to grow so huge; it possibly covers more geographical area than any other living organism.[1][2]


A cluster of large, thick-stemmed light brown colored gilled mushrooms growing at the base of a tree
Armillaria solidipes

The species was formerly known as Armillaria ostoyae Romagn., until a 2008 publication revealed that the species had been described under the name Armillaria solidipes by Charles Horton Peck in 1900,[3] long before Henri Romagnesi had described it in 1970.[4]

Life cycle[edit]

This fungus, like most parasitic fungi, reproduces sexually. The fungi begin their life as spores, released into the environment by a mature mushroom. Armillaria solidipes has a white spore print. There are two types of mating types for spores (not male and female but similar in effect). The spores can be dispersed by environment factors such as wind or they can be redeposited by an animal. Once the spores are in a resting state, the single spore must come in contact with a spore of an opposite mating type and of the same species. If the single spore isolates are from different species, the colonies will not fuse together and they will remain separate. When two isolates of the same species but different mating types fuse together, they soon form coalesced colonies which become dark brown and flat. With this particular fungus it will produce mycelial cords also known as rhizomorphs.[5] These rhizomorphs allow the fungus to obtain nutrients from long distances away. These are also the main factors to its pathogenicity. As the fruiting body continues to grow and obtain nutrients, it forms into a mature mushroom. Armillaria solidipes in particular grows a wide and thin sheet-like plates radiating from the stem which is known as its gills. The gills hold the spores of a mature mushroom. This is stained white when seen as a spore print. Once spore formation is complete, this signifies a mature mushroom and now is able to spread its spores to start a new generation.


The disease is of particular interest to forest managers, as the species is highly pathogenic to a number of commercial softwoods, notably Douglas-fir (Pseudotsuga menziesii), true firs (Abies spp.) and Western Hemlock (Tsuga heterophylla). A commonly prescribed treatment is the clear cutting of an infected stand followed by planting with more resistant species such as Western redcedar (Thuja plicata) or deciduous seedlings.

Pathogenicity is seen to differ among trees of varying age and location. Younger conifer trees at age 10 and below are more susceptible to infection leading to mortality, with an increased chance of survival against the fungus where mortality can become rare by age 20.[6] While mortality among older conifers is less likely to occur, this does happen, however, in forests with dryer climates.[7]

The pathogenicity of Armillaria solidipes appears to be more common in interior stands, but its virulence is seen to be greater in coastal conifers. Although conifers along the coastal regions show a lower rate of mortality against the root disease, infections can be much worse. Despite differences in how infections occur between these two regions, infections are generally established by rhizomorph strands, and pathogenicity is correlated to rhizomorph production.


Armillaria solidipes is mostly common in the cooler regions of the northern hemisphere. In North America, this fungus can be found on host coniferous trees located in the northwestern forests of the continent in British Columbia and the Pacific Northwest. While Armillaria solidipes is distributed throughout the different biogeoclimatic zones of British Columbia, the root disease causes the greatest amount of problem in the interior parts of the region in the Interior Cedar Hemlock (ICH) biogeoclimatic zone.[8] It is both present in the interior where it is more common as well as along the coastal lines.

A mushroom of this type in the Malheur National Forest in the Strawberry Mountains of eastern Oregon, U.S. was found to be the largest fungal colony in the world, spanning 8.9 square kilometres (2,200 acres) of area. This organism is estimated to be 2,400 years old. The fungus was written about in the April 2003 issue of the Canadian Journal of Forest Research. While an accurate estimate has not been made, the total mass of the colony may be as much as 605 tons. If this colony is considered a single organism, then it is the largest known organism in the world by area, and rivals the aspen grove "Pando" as the known organism with the highest living biomass. In 1992, a relative of the Strawberry Mountains clone was discovered in southwest Washington state. It covers about 6 square kilometres (1,500 acres). Another "humongous fungus" is a specimen of Armillaria gallica found at a site near Crystal Falls, Michigan, which covered 0.15 square kilometres (37 acres).


A tree is diagnosed with this parasitic fungus once the following characteristics are identified:

  • Resin flow from tree base
  • Crown thinning or changing color to yellow or red
  • Distress crop of cones
  • White mycelial fan under bark
  • Black rhizomorphs penetrating root surfaces
  • Honey-colored mushrooms near base of tree in fall
  • Affected trees often in groups or patches on the east side of the Cascades; usually killed singly on the west side.

A. solidipes may be confused with Mottled rot (Pholiota limonella). It has similar mushrooms, but only if mycelial fans are not present. Dead and diseased trees usually occur in disease centers, which appear as openings in the canopy. GPS tracking can aid in the monitoring of these areas. However, sometimes distinct centers will be absent and diseased trees are scattered throughout the stand. [9]


Armillaria can remain viable in stumps for 50 years. Chemical treatments do not eradicate the fungus entirely, and they are not cost-effective. The most frequent and effective approach to managing root disease problems is to attempt to control them at final harvest by replanting site-suited tree species that are disease tolerant. In eastern Washington that typically means replacing Douglas-fir or true fir stands with ponderosa pine, western larch, western white pine, lodgepole pine, western red cedar, alder, or spruce. Species susceptibility varies somewhat from location to location. All trees in the disease center as well as uninfected trees within 50 feet should be cut. No tree from a highly susceptible species should be planted within 100 feet of a disease center.

The use of another fungus, Hypholoma fasciculare has been shown in early experiments to competitively exclude Armillaria solidipes in both field and laboratory conditions, but further experimentation is required to establish the efficacy of this treatment.

Another more expensive alternative to changing species is to remove diseased stumps and trees from the site by pushing them out with a bulldozer. The air will dry and kill the fungus. And any small roots left underground will decay before they can reinfect the new seedlings, so it is not necessary to burn the stumps. After stump removal, any species may be planted. The removal of stumps (stumping) has been used to prevent contact between infected stumps and newer growth resulting in lower infection rates. However, it is unknown if the lower infection rates will persist as roots of young trees extend closer to the original inoculate from the preceding stand.

The most important control measure after planting is to manage for reduced tree stress. This includes regulating species composition, maintaining biological diversity, and reducing the chances for insect pest buildup. Mixed-species forests are more resistant to insect defoliation, and also slow the spread of species-specific pests such as dwarf mistletoe, which are both predisposing agents for Armillaria.[10]


Armillaria solidipes are considered edibles.[11]

See also[edit]


  1. ^ "Biggest Living Thing". Extreme Science. 1 Dec 2010. 
  2. ^ Puiu, Tibi (February 6, 2015). "The largest organism in the world". ZME Science. Retrieved February 20, 2015. 
  3. ^ Peck, C. H. (1900). "New species of Fungi". Bulletin of the Torrey Botanical Club 27 (12): 609–613. doi:10.2307/2477998. JSTOR 2477998. 
  4. ^ Burdsall, H. H.; Volk, T. J. (2008). "Armillaria solidipes, an older name for the fungus called Armillaria ostoyae" (pdf). North American Fungi 3 (7): 261–267. doi:10.2509/naf2008.003.00717. 
  5. ^ Korhonen, K. (1978). "Interfertility and Clonal Size in Armillaria mellea Complex". Karstenia 18: 31–42. 
  6. ^ Morrison, D. J.; Pellow, K. W. (2002). "Variation in Virulence Among Isolates of Armillaria ostoyae". Forest Pathology 32 (2): 99–107. doi:10.1046/j.1439-0329.2002.00275.x. 
  7. ^ Harington, T. C.; Wingfield, M. J. (2000). "19. Diseases and the Ecology of Indigenous and Exotic Pines". In Richardson, D. M. Ecology and Biogeography of Pinus. Cambridge University Press. p. 386. ISBN 9780521789103. 
  8. ^ Cruickshank, M. G.; Morrison, D. J.; Lalumiere, A. (2009). "The Interaction Between Competition in Douglas-fir Plantation and Disease Caused by Armillaria ostoyae in British Columbia". Forest Ecology and Management 257 (2): 443–452. doi:10.1016/j.foreco.2008.09.013. 
  9. ^ "CTD-Root Disease: Armillaria Root Disease". Forestry Development. 19 March 2009. Retrieved 7 Dec 2010. 
  10. ^ "Armillaria Root Rot, Shoestring Root Rot, Honey Mushroom". Forest Health Notes. Washington State University - Department of Natural Resource Science Extension. Retrieved 2013-06-11. 
  11. ^ Oregon's monster mushroom is world's biggest living thing

External links[edit]

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Armillaria_solidipes — Please support Wikipedia.
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The Conversation UK

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Thu, 19 Mar 2015 10:36:10 -0700

The main body of the fungus exists largely out of sight as a colony of white thread-like hyphae growing through a food source such as a piece of wood or leaf litter. In some instances fungal colonies can be old and very large. A colony of Armillaria ...

BBC News

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Wed, 19 Nov 2014 02:41:15 -0800

Tests showed all but four of the trees had been infected with the honey fungus Armillaria solidipes (previously known as Armillaria ostoyae). When mycelia from genetically identical A. solidipes meet, they can fuse to form one individual. The ...
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View image of Armillaria solidipes fruiting bodies in Belgium (Credit: Arterra Picture Library / Alamy). We only see mushrooms breach the surface when the fungus reproduces sexually. If a fungus never has sex, we might not know it's there. Scientists ...
Napa Valley Register
Fri, 09 Jan 2015 14:38:08 -0800

There is a mycelium structure of an Armillaria solidipes in the Malheur National Forest in Oregon that is estimated to be more than 2,400 years old. Mushrooms have been used for many purposes over the centuries. We are all aware of the yummy edible ...

The Guardian

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Sat, 29 Nov 2014 05:41:03 -0800

Shortly after this humongous fungus was discovered lurking beneath the forest on the foothills of Mount Adams in southwestern Washington state, another, larger specimen of Armillaria solidipes (formerly Armillaria ostoyae) or “honey mushroom” fungus ...

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Wed, 18 Feb 2015 22:00:18 -0800

as tall as a minnow is small; as high as a kite in the sky; as high as an elephant's eye; as tall as the Alamo's wall. Astoria, Oregon 7. Oregon is home to an example of Armillaria solidipes that is thought to be the largest organism in the world. What ...


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