Armillaria ostoyae is a species of plant-pathogenic fungus in the Physalacriaceae family. It is the most common variant, in the western United States. 'Armillaria ostoyae' is common on both hardwood and conifer wood in forests west of the Cascade Range in Oregon, United States.

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 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).

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.

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

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