[1] The mycelium invades the sapwood and is able to disseminate over great distances under the bark or between trees in the form of black rhizomorphs ("shoestrings").

[2] In most areas of North America, A. ostoyae can be separated from other species by its physical features: cream-brown colors, prominent cap scales, and a well-developed stem ring distinguish it from other Armillaria.

[2] 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.

[2][3][4] A spatial genetic analysis estimated that an individual specimen of A. ostoyae growing over 91 acres (37 ha) in northern Michigan, United States, weighs 440 tons (4 x 105 kg).

[2] Approximations of the land area occupied by the "Humongous fungus" are 3.5 square miles (9.1 km2) or 2,240 acres (910 ha), and it possibly weighs as much as 35,000 tons (about 31,500 tonnes), making it the world's most massive living organism.

When two isolates of the same species but different mating types fuse together, they soon form coalesced colonies which become dark brown and flat.

[12] 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 x 105 kg), making it the equivalent in weight of 3 blue whales.

[5][6] 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][5][6] (2,240 acres (910 ha) which may weigh as much as 35,000 tons.

A mushroom of this type in the Malheur National Forest in the Strawberry Mountains of eastern Oregon, was found to be the largest fungal colony in the world, spanning an area of 3.5 square miles (2,200 acres; 9.1 km2).

[5][17] 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.

[7] 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.

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

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.

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.