(Vahl) P. Kumm.
|c. 45 species|
|gills on hymenium|
|cap is convex|
|hymenium is adnate|
|stipe has a ring|
|spore print is white|
|ecology is parasitic|
Honey fungus, or Armillaria or оpenky (Ukrainian: опеньки), is a genus of parasitic fungi that live on trees and woody shrubs. It includes about 10 species formerly lumped together as A. mellea. Armillarias are long lived and form some of the largest living organisms in the world. The largest single organism (of the species Armillaria solidipes) covers more than 3.4 square miles (8.8 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.
In the Canadian Prairies (particularly Manitoba), the term "honey fungus" is unknown to many; due to the large presence of Ukrainian Canadians in this area, the fungus is often referred to as pidpenky (Ukrainian: підпеньки), from the Ukrainian term, "beneath the stump".
The fruit bodies of the fungus are mushrooms that grow on wood, typically in small dense clumps or tufts. Their caps 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. The stem may or may not have a ring. All Armillaria species have a white spore print and none have a volva (compare Amanita).
Grossly similar species include Pholiota spp. which also grow in cespitose clusters on wood and fruit in the fall. However Pholiota spp. 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 spp. which can grow side by side with Armillaria spp. also on wood. Galerina has a dark brown spore print and is deadly poisonous (alpha-amanitin) – see: mushroom poisoning.
Plant pathology 
Honey fungus, a "white rot" fungus, is a pathogenic organism that affects trees, shrubs, woody climbers and, rarely, woody herbaceous perennials. Honey fungus grows on living trees as well as on dead and decaying woody material.
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.
Edible - Choice. Honey Fungus or pidpenky (Ukrainian: підпеньки) are considered in Ukraine, Russia, Poland, Germany and other European countries to be one of the best wild mushrooms and highly prized. They are commonly ranked above morels and chanterelles and only the cep / porcini is more highly prized. However pidpenky must be thoroughly cooked as they are mildly poisonous raw. Additionally one of the four UK species identified can lead to sickness when ingested with alcohol. Therefore for the non expert mycologist it is advisable not to drink alcohol for 12 hours before and 24 after eating this mushroom to avoid any possible nausea and vomiting. However, if these rules are followed this variety of mushroom is a delicacy with a strong distinctive mushroomy and nutty flavour. Recommended reference text for identification are Collins Complete British Mushrooms and Toadstools for the variety of field pictures in it and Roger Philips Mushrooms for the quality of his out of field pictures and descriptions.
See also 
- John L. Ingraham (15 February 2010). March of the Microbes: Sighting the Unseen. Harvard University Press. p. 201. ISBN 978-0-674-03582-9.
- Pegler DN. (2000). "Taxonomy, nomenclature and description of Armillaria". In Fox RTV. Armillaria Root Rot: Biology and Control of Honey Fungus. Intercept. pp. 81–93. ISBN 1-898298-64-5.