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Armillaria ostoyae (synonym A. solidipes) is a pathogenic species of fungus in the family Physalacriaceae.

It has decurrent gills and the stipe has a ring. The mycelium invades the sapwood of trees, and is able to disseminate over great distances under the bark or between trees in the form of black rhizomorphs ("shoestrings"). In most areas of North America, it can be distinguished from other Armillaria species by its cream-brown colors, prominent cap scales, and a well-developed ring. The species grows and spreads primarily underground, such that the bulk of the organism is not visible from the surface. In the autumn, the subterranean parts of the organism bloom "honey mushrooms" as surface fruits. Low competition for land and nutrients often allow this fungus to grow to huge proportions, and it possibly covers more total geographical area than any other single living organism. A spatial genetic analysis estimated that an individual specimen growing over 91 acres (37 ha) in northern Michigan weighs 440 tons (4 × 105 kg). Another specimen in northeastern Oregon's Malheur National Forest is possibly the largest living organism on Earth by mass, area, and volume; it covers 3.5 square miles (2,200 acres; 9.1 km2) and weighs as much as 35,000 tons (about 31,500 tonnes).

The species was long known as Armillaria ostoyae Romagn., until a 2008 publication revealed that the species had been described under the earlier name Armillaria solidipes by Charles Horton Peck in 1900, long before Henri Romagnesi had described it in 1970. Subsequently, a proposal to conserve the name Armillaria ostoyae was published in 2011 and has been approved by the Nomenclature Committee for Fungi.

Once the spores are in a resting state, the single spore must come in contact with a spore of a complementary 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. Armillaria ostoyae in particular grows wide and thin gills, radiating decurrently from the stem. Like several other Armillaria, the mycelium of A. ostoyae can display bioluminescence, resulting in foxfire.

Using genotyping and clonal analysis, scientists determined that a 2,500-year old specimen of Armillaria ostoyae in northern Michigan, United States originated from spores of a parent fungus in Ontario, Canada, then grew over millennia into the 21st century to a mass of 440 tons (4 × 105 kg), making it the equivalent in weight of 3 blue whales. By comparison of acreage, the Michigan A. ostoyae covers only 38% of the estimated land area of the Oregon "humongous fungus" at 3.5 square miles (9.1 km2), (2,240 acres (910 ha) which may weigh as much as 35,000 tons. The species possibly covers more total geographical area than any other single living organism.

Armillaria ostoyae is mostly common in the cooler regions of the northern hemisphere. While Armillaria ostoyae is distributed throughout the different biogeoclimatic zones of British Columbia, the root disease causes the greatest problem in the interior parts of the region in the Interior Cedar Hemlock biogeoclimatic zone. Another "humongous fungus" – a specimen of Armillaria gallica found at a site near Crystal Falls, Michigan – covers 91 acres (0.37 km2; 0.142 sq mi), was found to have originated from a parent fungus in Ontario.

Pathogenicity of Armillaria ostoyae appears to be more common in stands in interior regions, but its virulence is seen to be greater near the coast.

Armillaria can remain viable in stumps for 50 years. Another fungus, Hypholoma fasciculare, has been shown in early experiments to competitively exclude Armillaria ostoyae in both field and laboratory conditions; further experimentation is required to discover whether this treatment works.

Early Results from Field Trials Using Hypholoma fasciculare to Reduce Armillaria ostoyae Root Disease in Canadian Journal of Botany (2004)

Quick Facts

• The species was long known as Armillaria ostoyae Romagn., until a 2008 publication revealed that the species had been described under the earlier name Armillaria solidipes by Charles Horton Peck in 1900, long before Henri Romagnesi had described it in 1970.
• Low competition for land and nutrients often allow this fungus to grow to huge proportions, and it possibly covers more total geographical area than any other single living organism.
• In most areas of North America, it can be distinguished from other Armillaria species by its cream-brown colors, prominent cap scales, and a well-developed ring.
• Early Results from Field Trials Using Hypholoma fasciculare to Reduce Armillaria ostoyae Root Disease in Canadian Journal of Botany (2004)
• The mycelium invades the sapwood of trees, and is able to disseminate over great distances under the bark or between trees in the form of black rhizomorphs ("shoestrings").

Source material: Wikipedia - "Armillaria ostoyae". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.