Armillaria
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''Armillaria'' is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
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 Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...
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 Elias Magnus Fries (15 August 1794 – 8 February 1878) was a Swedish mycologist and botanist. He is sometimes called the Mycology, "Linnaeus of Mycology". In his works he described and assigned botanical names to hundreds of fungus and li ...
, 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 In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...
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). Similar lists of biological species have been constructed by mycologists working in Japan (10 as of 1998) 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).Hughes, M. B.; Weir, A.; Rogers, S. O. (2003). "Phylogenetic reconstruction of North American ''Armillaria'' species and related European taxa based on nuclear ribosomal DNA internal transcribed spacers". p. 32 in Laflamme, G.; Bérubé, J. A.; Bussières, G. (eds.), ''Root and Butt Rots of Forest Trees. Proceedings of the 10th International Conference of Root and Butt Rots'', IUFRO Working Party 7.02.01, Quebec, September 2001. Natural Resources Canada, Canadian Forest Service, Information Report LAU-X-126. 450 p. A similar situation happens in China, where some molecular phylogenetic data appear to line up with intersterile "biological species" and others do not. ''Armillaria'' root rot occurs in the
Northwest Territories The Northwest Territories is a federal Provinces and territories of Canada, territory of Canada. At a land area of approximately and a 2021 census population of 41,070, it is the second-largest and the most populous of Provinces and territorie ...
, and was identified on white spruce at Pine Point on Great Slave Lake prior to NABS findings.


''Rhizomorpha''

In 1791
Albrecht Wilhelm Roth Albrecht Wilhelm Roth (6 January 1757 – 16 October 1834) was a physician and botanist born in Dötlingen, Germany. He studied medicine at the Universities of University of Halle, Halle and University of Erlangen, Erlangen, where he received his ...
described the species ''Rhizomorpha fragilis'' for a collection which consisted entirely of rhizomorphs. The genus '' Rhizomorpha'' thus became established and later these mycelial cords were shown to belong to ''Armillaria mellea''. According to the code of nomenclature that means that the two genus names are synonyms and since genus ''Armillaria'' was not defined until 1857, the name ''Rhizomorpha'' takes precedence and should replace the name ''Armillaria''. To avoid this, a 2021 paper by Stalpers et al. proposes that the name ''Armillaria'' should be protected. The proposal awaits decisions by the Nomenclature Committee for Fungi and eventually by an International Botanical Congress.


Description

The basidiocarp (reproductive structure) of the fungus is a
mushroom A mushroom or toadstool is the fleshy, spore-bearing Sporocarp (fungi), fruiting body of a fungus, typically produced above ground on soil or another food source. ''Toadstool'' generally refers to a poisonous mushroom. The standard for the n ...
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 Convex or convexity may refer to: Science and technology * Convex lens, in optics Mathematics * Convex set, containing the whole line segment that joins points ** Convex polygon, a polygon which encloses a convex set of points ** Convex polytop ...
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 Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...
'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 The genus ''Amanita'' contains about 600 species of agarics, including some of the most toxic known mushrooms found worldwide, as well as some well-regarded Edible mushroom, edible species (and many species of unknown edibility). The genus is re ...
''. As a white rot, it is distinguished from '' Tricholoma'', a
mycorrhiza A mycorrhiza (; , mycorrhiza, or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, the plant root system and its surroundings. Mycorrhizae play ...
l (non-parasitic) genus.


Pathology

Honey fungus is a white-rot fungus, which is a
pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
ic organism that affects trees,
shrub A shrub or bush is a small to medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees by their multiple ...
s, woody climbers and rarely, woody herbaceous
perennial In horticulture, the term perennial ('' per-'' + '' -ennial'', "through the year") is used to differentiate a plant from shorter-lived annuals and biennials. It has thus been defined as a plant that lives more than 2 years. The term is also ...
plants. Honey fungus can grow on living, decaying, and also dead plant material (being a facultative saprophyte). This means it can kill its host with little consequence, unlike
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
s that must moderate their growth to avoid host death. Honey fungus spreads from living trees, dead and live roots and stumps by means of reddish-brown to black rhizomorphs (root-like structures) at the rate of approximately a year, but infection by root contact is possible. Infection by spores is rare. Rhizomorphs grow close to the
soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
surface (in the top ) and invade new roots, or the root collar (where the roots meet the stem) of plants. An infected tree will die once the fungus has girdled it, or when significant root damage has occurred. Additionally, Infections can occur through the activity of wood boring beetles. For example, '' Agrilus biguttatus'' most commonly girdle in oak trees, which allows fungi to grow in the stems of these infected trees. 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 dieback or shortage of leaves in spring. 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. It will give off a strong mushroom scent and the mushrooms sometimes extend upward. On conifers honey fungus often exudes a gum or
resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...
from cracks in the bark.


Hosts

Potential hosts include conifers and various monocotyledonous and
dicotyledon The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, ...
ous trees,
shrub A shrub or bush is a small to medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees by their multiple ...
s, and herbaceous species, ranging from asparagus and strawberry to large forest trees (Patton and Vasquez Bravo 1967).Patton, R. F.; Vasquez Bravo, R. 1967. "Armillaria root rot ''Armillaria mellea'' (Vahl ex Fr.) Kummer". p. 37–38 in Davidson, A. G.; Prentice, R. M. (compilers and eds.). ''Important forest insects and diseases of mutual concern to Canada, the United States, and Mexico''. Canadian Department for Rural Development, Ottawa, Ontario, Pub. 1180. ''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).Blenis, P.; Titus, S.; Mallet, K. 1995. "Impact of Armillaria root rot in intensively managed white spruce/asspen stands". Natural Resources Canada/Alberta Land and Forest Service, Edmonton, Alberta, Canada/Alberta Forest Resource Development Agreement (FRDA), Project A5023-129 Rep. 5 p. Reviews of the biology, diversity, pathology, and control of ''Armillaria'' in Fox (2000)Fox, R. T. V. (2000). "''Armillaria'' Root Rot: Biology and Control of Honey Fungus". ''Intercept'', Andover, Hants., England. 222 p. 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


External links

*
Images of ''Armillaria'' species
{{Authority control Fungal tree pathogens and diseases Agaricales genera Bioluminescent fungi