Armillaria ostoyae (synonym Armillaria solidipes) is a species of plant-pathogenic fungus in the family Physalacriaceae. It is the most common variant, in the western United States, of the group of species under the name Armillaria mellea. Armillaria ostoyae is common on both hardwood and conifer wood in forests west of the Cascade Range in Oregon, United States. It has decurrent gills and the stipe has a ring. 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”). In most areas of North America, Armillaria ostoyae can be separated from other species by its physical features: cream-brown colors, prominent cap scales, and well-developed stem ring distinguish it from any Armillaria.
It is known as having grown possibly the largest living organism by area – estimated by scientists as a contiguous specimen found in the Oregon Malheur National Forest covering 3.7 square miles (2,400 acres; 9.6 km2) – and colloquially called the “Humongous fungus”. Armillaria ostoyae grows and spreads primarily underground and the bulk of the organism lies in the ground, out of sight, making it invisible from the surface. In the autumn, this organism blooms “honey mushrooms” as surface fruits of the underground organism. Low competition for land and nutrients have allowed this fungus to grow to huge proportions, as it possibly covers more geographical area than any other living organism.
A spatial genetic analysis estimated that a specimen of Armillaria ostoyae growing over 91 acres (37 ha) in northern Michigan, United States weighs 440 tons (4 x 105 kg). Approximations of the land area of the Oregon “humongous fungus” are 3.5 square miles (9.1 km2) (2,240 acres (910 ha), possibly weighing as much as 35,000 tons as the world’s most massive living organism.