Armillaria is a genus of fungi that includes the A. mellea species ('honey fungus') that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as A. mellea. Armillaria sp. are long-lived and form the largest living fungi in the world. The largest known specimen (an A. ostoyae) covers more than in Oregon and is estimated to be 2,500 years old. Some species of Armillaria display bioluminescence.

Armillaria can be a destructive forest pathogen. It causes "white rot" root disease. As it feeds on dead plant material, it can kill its host with little negative effect to itself.

Taxonomy

The name Armillaria was defined in 1821 by Elias Magnus Fries, but at that time most gilled mushrooms were considered to belong to genus Agaricus and Armillaria was only a subgenus (a "tribe"). In 1857, Friedrich Staude established the independent genus.

Armillaria species morphologically similar to A. mallea were not effectively distinguished until the development of "biological species" defined as intersterile groups by Veikko Hintikka in 1973. Using a similar technique, Kari Korhonen showed in 1978 that the European Armillaria mellea species complex could be separated into five reproductively isolated species, which he named "European Biological Species" (EBS) A through E. About the same time, the North American A. mellea was shown to be ten different species (North American Biological Species, or NABS I through X).

{|class=wikitable

|+Intersterile biological species of Armillaria defined in Europe and North America

|-

! Code !! Taxonomic treatment !! Compatibility !! Known range

|-

| EBS A || Armillaria borealis || || Finland

|-

| EBS B || Armillaria cepistipes || NABS III ( + ), NABS IV (part) || Finland, Germany || EBS D (part)

|-

| NABS VIII || || EBS D (part + rg) and China (16 as of 2024).

Further data from molecular diagnostic tools have removed much uncertainty for mycologists and forest pathologists. New questions remain unanswered regarding the phylogeny of North American Armillaria species and their relationships to their European counterparts, particularly within the "Armillaria mellea complex". Some data suggest that North American and European A. gallica isolates are not monophyletic. Although North American and European isolates of A. gallica may be interfertile, some North American isolates of A. gallica are more closely related to the North American taxon A. calvescens than to European isolates of A. gallica. The increase in genetic divergence has not necessarily barred inter-sterility between isolated populations of A. gallica. Although the relationships among some groups in the genus seem clearer, the investigation of geographically diverse isolates has revealed that the relationship between some North American species is still unclear (Hughes et al. 2003). A similar situation happens in China, where some molecular phylogenetic data appear to line up with intersterile "biological species" and others do not.

Description

thumb|180px|right|[[Armillaria mellea]]

thumb|[[Armillaria hinnulea]]

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. The stipe (stalk) may or may not have a ring. All Armillaria species have a white spore print and none have a volva (cup at base).

Armillaria species are long-lived and form the largest living fungi in the world. The largest known organism (of the species A. ostoyae) covers more than in Oregon's Malheur National Forest and is estimated to be 2,500 years old. Some species display bioluminescence, known as foxfire.

Similar species

Similar species include Pholiota spp. which also grow in cespitose (mat-like) clusters on wood and fruit in the fall. Pholiota spp. are separated from Armillaria by its yellowish to greenish-yellow tone and a dark brown to grey-brown spore print. Mushroom hunters need to be wary of Galerina spp. which can grow side by side with Armillaria spp. on wood. Galerina have a dark brown spore print and are deadly poisonous (alpha-amanitin).

The white spore print and lack of volva can be compared to Amanita. Armillaria root rot enters hosts through the roots. In Alberta, 75% of trap logs (Mallett and Hiratsuka 1985) inserted into the soil between planted spruce became infected with the distinctive white mycelium of Armillaria within one year. Of the infestations, 12% were A. ostoyae, and 88% were A. sinapina (Blenis et al. 1995). Reviews of the biology, diversity, pathology, and control of Armillaria in Fox (2000) are useful.

Edibility

Honey fungus are regarded in Ukraine, Russia, Poland, Germany and other European countries as one of the best wild mushrooms. They are commonly ranked above morels and chanterelles and only the cep/porcini is more highly prized. However, honey fungus must be thoroughly cooked as they are mildly poisonous raw. Honey mushrooms are one of four UK species that can cause sickness when ingested with alcohol. For those unfamiliar with the species, it is advisable not to drink alcohol for 12 hours before and 24 hours 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 distinctive mushroomy and nutty flavour. Reference texts 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.

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

See also

  • List of Armillaria species

References

  • Images of Armillaria species