What is the common name of the fungus that forms the largest living organism?

The common name of the fungus associated with the largest living organism is honey fungus. The record-holding colony is an underground individual of Armillaria ostoyae, also called the dark honey fungus.

Most of this enormous organism is hidden in soil and wood rather than forming one giant visible mushroom. Its mycelium spreads through tree roots and its black, cord-like rhizomorphs—sometimes called shoestrings—can travel between trees. The mushrooms seen above ground are only its seasonal fruiting bodies.

A famous colony in Oregon’s Malheur National Forest covers roughly 3.5 square miles and has been estimated to weigh up to about 35,000 tons. Genetic evidence indicates that it may be thousands of years old. Another Armillaria colony studied in Michigan covered about 91 acres and was estimated at roughly 440 tons.

Honey fungus is not the same as a single huge mushroom. The “largest organism” claim refers to one genetically connected fungal colony. Morel and puffball are unrelated mushroom types, while shelf fungus usually describes woody bracket fungi.