[1] The largest known organism (of the species Armillaria ostoyae) covers more than 3.4 square miles (8.8 km2) in Oregon's Malheur National Forest and is estimated to be 2,500 years old.

It causes "white rot" root disease (see Plant pathology section) of forests, which distinguishes it from Tricholoma, a mycorrhizal (non-parasitic) genus.

[3] The basidiocarp (reproductive structure) of the fungus is a mushroom that grows on wood, typically in small dense clumps or tufts.

Their caps (mushroom tops) are typically yellow-brown, somewhat sticky to touch when moist, and, depending on age, may range in shape from conical to convex to depressed in the center.

Rhizomorphs (also called mycelial cords) appear under the bark and around the tree, and mushrooms grow in clusters from the infected plant in autumn and die back after the first frost.

However these symptoms and signs do not necessarily mean that the pathogenic strains of honey fungus are the cause, so other identification methods are advised before diagnosis.

Thin sheets of cream colored mycelium, beneath the bark at the base of the trunk or stem indicated that honey fungus is likely the pathogen.

[5] The linkage of morphological, genetic, and molecular characters of Armillaria over the past few decades has led to the recognition of intersterile groups designated as "biological species".

[8] Armillaria root rot occurs in the Northwest Territories,[citation needed] and was identified on white spruce at Pine Point on Great Slave Lake prior to NABS findings.

[citation needed] In 1791 Albrecht Wilhelm Roth described the species Rhizomorpha fragilis for a collection which consisted entirely of rhizomorphs.

[9][10][11] Honey fungus are regarded in Ukraine, Russia, Poland, Germany and other European countries as one of the best wild mushrooms.

In Norway, some mycologists have started to regard honey fungus as poisonous, as the Norwegian health ministry is moving away from the parboiling practice.

In Alberta, 75% of trap logs (Mallett and Hiratsuka 1985)[15] inserted into the soil between planted spruce became infected with the distinctive white mycelium of Armillaria within one year.