Armillaria is long lived and form some of the largest living organisms in the world. The largest single organism (of the species Armillaria ostoyae) covers more than 3.4 square miles (8.9 km²) and is thousands of years old. Some species of Armillaria are bioluminescent and may be responsible for the phenomena known as foxfire and perhaps will o' the wisp.
As a forest pathogen, Armillaria can be very destructive. It is responsible for the "white rot" root disease (see below) of forests and is distinguished from Tricholoma (mycorrhizal) by this parasitic nature. Its high destructiveness comes from the fact that, unlike most parasites, it doesn't need to moderate its growth in order to avoid killing its host, since it will continue to thrive on the dead material.
The fruiting bodies of the fungus are mushrooms that grow on wood, typically in cestipose clusters. The cap is 3–15 cm in diameter, typically has a honey yellow-brown color, and is covered with small dark scales. The stem may or may not have a ring. All Armillaria species have a white spore print and none have a volva (see Amanita).
Honey mushrooms are edible and are easy to identify. Grossly similar species include Pholiota sp. which also grow in cestipose clusters on wood and fruit in the fall. However Pholiota sp. have a yellowish to greenish yellow cast and a dark brown to grey-brown spore print. Mushroom hunters need to be especially wary of Galerina sp. which can grow side by side with Armillaria sp. also on wood. Galerina has a dark brown spore print and is deadly poisonous (alpha-amanitin) – see: Mushroom poisoning. There are some reports of temporary stomach problems, especially when eaten raw.
Honey fungus spreads both from living trees, dead and live roots and stumps by means of reddish-brown to black root-like rhizomorphs ('bootlaces') at the rate of around 1 m a year, although infection by root contact is also possible. Infection by spores is rare. Rhizomorphs grow relatively close to the soil surface (in the top 20 cm) and invade new roots, or the root collar (where the roots meet the stem) of woody plants. An infected tree will die once the fungus has girdled it, or when extensive root death has occurred. This can happen rapidly, or may take several years. Infected plants will deteriorate, although may exhibit prolific [[flower] or fruit production shortly before death.
Initial symptoms of honey fungus infection include the dying back of leafy branches or failure of leaves to appear in spring. Black bootlace-like strands appear under the bark and around the tree, and fruiting bodies grow in clusters from the infected plant in autumn and die back after the first frost. However these signs do not necessarily mean that the pathogenic (disease causing) strains of honey fungus are a cause of plant decline or death, so other identification methods are advised before a diagnosis is made. The presence of thin sheets of cream coloured mycelium, giving off a strong smell of mushrooms, beneath the bark at the base of the trunk or stem, sometimes extending upwards, or a gum or resin exuding from cracks in the bark of conifers, indicates that honey fungus is a likely cause of problems. If further confirmation is required, it is advisable to seek the advice of a qualified tree surgeon (ISA Certified Arborist).