Armillaria ostoyae

Armillaria ostoyae (synonym Armillaria solidipes) is a species of fungus (mushroom), pathogenic to trees, in the family Physalacriaceae. In the western United States, it is the most common variant of the group of species under the name Armillaria mellea. A. 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 a well-developed stem ring distinguish it from other Armillaria. Like several other Armillaria, the mycelium of Armillaria ostoyae can display bioluminescence, resulting in foxfire.

Armillaria ostoyae 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 of A. ostoyae growing over 91 acres (37 ha) in northern Michigan, United States, weighs 440 tons (4 x 10⁵ kg).

Another specimen in northeastern Oregon's Malheur National Forest is possibly the largest living organism on Earth by mass, area, and volume – this contiguous specimen covers 3.7 square miles (2,400 acres; 9.6 km²) and is colloquially called the "Humongous fungus". Approximations of the land area occupied by the "Humongous fungus" are 3.5 square miles (9.1 km²) (2,240 acres (910 ha)), and it possibly weighs as much as 35,000 tons (approximately 31,500 tonnes), making it the world's most massive living organism.