Armillaria mellea

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***Armillaria mellea***, commonly known as **honey fungus**, is a basidiomycete fungus in the genus *Armillaria*. It is a plant pathogen and part of a cryptic species complex of closely related and morphologically similar species. It causes Armillaria root rot in many plant species and produces mushrooms around the base of trees it has infected. The symptoms of infection appear in the crowns of infected trees as discoloured foliage, reduced growth, dieback of the branches and death. The mushrooms are edible but some people may be intolerant to them. This species is capable of producing light via bioluminescence in its mycelium.

*Armillaria mellea* is widely distributed in temperate regions of the Northern Hemisphere. The fruit body or mushroom, commonly known as stump mushroom, stumpie, honey mushroom, pipinky or pinky, grows typically on hardwoods but may be found around and on other living and dead wood or in open areas.

## Taxonomy

The species was originally named *Agaricus melleus* by Danish-Norwegian botanist Martin Vahl in 1790; it was transferred to the genus *Armillaria* in 1871 by Paul Kummer. Numerous subtaxa have been described:

## Description

 File:Coloured Figures of English Fungi or Mushrooms - t. 101.jpg
The basidiocarp of each has a smooth cap Template:Convert in diameter, convex at first but becoming flattened with age often with a central raised umbo, later becoming somewhat dish-shaped. The margins of the cap are often arched at maturity and the surface is sticky when wet. Though typically honey-coloured, this fungus is rather variable in appearance and sometimes has a few dark, hairy scales near the centre somewhat radially arranged. The gills are white at first, sometimes becoming pinkish-yellow or discoloured with age, broad and fairly distant, attached to the stipe at right angles or are slightly decurrent. The spore print is white. The stipe is of variable length, up to about Template:Convert long and Template:Convert in diameter. It is fibrillose and of a firm spongy consistency at first but later becomes hollow. It is cylindrical and tapers to a point at its base where it is fused to the stipes of other mushrooms in the clump. It is whitish at the upper end and brownish-yellow below, often with a very dark-coloured base. There is a broad persistent skin-like ring attached to the upper part of the stipe. This has a velvety margin and yellowish fluff underneath and extends outwards as a white partial veil protecting the gills when young. The flesh of the cap is whitish and has a sweetish odour and flavour with a tinge of bitterness. Under the microscope, the spores are approximately elliptical, 7–9 by 6–7 µm, inamyloid with prominent apiculi (short, pointed projections) at the base. The basidia (spore-producing structures) lack basal clamps.

The main part of the fungus is underground where a mat of mycelial threads may extend for great distances. They are bundled together in rhizomorphs that are black in this species. The fungal body is not bioluminescent but its mycelia are luminous when in active growth.

### Similar species

*Armillaria mellea* once included a range of species with similar features that have since been reclassified.

## Distribution

*Armillaria mellea* is widespread in northern temperate zones. It has been found in North America, Europe and northern Asia, and It has been introduced to South Africa. The fungus grows parasitically on a large number of broadleaf trees. It fruits in dense clusters at the base of trunks or stumps.

## Ecology

Trees become infected by *Armillaria mellea* when rhizomorphs growing through the soil encounter uninfected roots. Alternatively, when infected roots come into contact with uninfected ones the fungal mycelium may grow across. The rhizomorphs invade the trunk, growing between the bark and the wood and causing wood decay, growth reduction and mortality. Trees that are already under stress are more likely to be attacked but healthy trees may also be parasitized. The foliage becomes sparse and discoloured, twig growth slows down and branches may die back. When they are attacked, the Douglas-fir, western larch and some other conifers often produce an extra large crop of cones shortly before dying. Coniferous trees also tend to ooze resin from infected areas whereas broad-leaved trees sometimes develop sunken cankers. A growth of fruiting bodies near the base of the trunk confirms the suspicion of Armillaria root rot.

In 1893, the American mycologist Charles Horton Peck reported finding *Armillaria* fruiting bodies that were "aborted", in a similar way to specimens of *Entoloma abortivum*. It was not until 1974 that Roy Watling showed that the aborted specimens included cells of both *Armillaria mellea* and *Entoloma abortivum*. He thought that the *Armillaria* was parasitizing the *Entoloma*, a plausible hypothesis given its pathogenic behaviour. However, a 2001 study by Czederpiltz, Volk and Burdsall showed that the *Entoloma* was in fact the microparasite. The whitish-grey malformed fruit bodies known as carpophoroids were the result of *E. abortivum* hyphae penetrating the *Armillaria* and disrupting its normal development.

The main part of the fungus is underground where a mat of mycelial threads may extend for great distances. The rhizomorphs of A. mellea are initiated from mycelium into multicellular apices of rhizomorphs, which are multicellular vegetative organs that exclude soil from the interior of the rhizomorph tissues. The rhizomorphs spread through far greater distances through the ground than the mycelium. The rhizomorphs are black in this species. The fungal body is not bioluminescent but its mycelia and rhizomorphs are luminous when in active growth. A. mellea producing rhizomorphs is parasitic on woody plants of many species, including especially shrubs, hardwood and evergreen trees. In one example, A. mellea spread by rhizomorphs from an initially infected tree killed 600 trees in a prune orchard in 6 years. Each infected tree was immediately adjacent to an already infected one, the spread by rhizomorphs through the tree roots and soil. (Piper and Fletcher, 1903, Wash. Age. Exp. Sat. But., 59: 1-14); cited in Rhizomorph Development in A. mellea, Ph.D. thesis, by Philip Snider(1957), Farlow Herbarium Library Harvard Univ., 20 Divinity Ave., Cambridge, Mass.

## Edibility

 File:Honey fungus in basket 2018 G1.jpg
*Armillaria mellea* mushroom are considered good edibles, although some individuals have reported "allergic" reactions that result in stomach upsets. Some authors suggest not collecting mushrooms from the wood of various trees, including hemlock, buckeye, eucalyptus, and locust. The mushrooms have a taste that has been described as slightly sweet and nutty, with a texture ranging from chewy to crunchy, depending on the method of preparation. Parboiling mushrooms before consuming removes the bitter taste present in some specimens, and may reduce the amount of gastrointestinal irritants. Drying the mushrooms preserves and intensifies their flavour, although reconstituted mushrooms tend to be tough to eat. Can also be pickled and roasted.

## Chemistry

Several bioactive compounds have been isolated and identified from the fruit bodies. The triterpenes 3β-hydroxyglutin-5-ene, friedelane-2α,3β-diol, and friedelin were reported in 2011. Indole compounds include tryptamine, L-tryptophan and serotonin.

The fungus produces cytotoxic compounds known as melleolides. Melleolides are made from orsellinic acid and protoilludane sesquiterpene alcohols via esterification. A polyketide synthase gene, termed *ArmB*, was identified in the genome of the fungus, which was found expressed during melleolide production. The gene shares ca. 42% similarity with the orsellinic acid synthase gene (*OrsA*) in *Aspergillus nidulans*. Characterization of the gene proved it to catalyze orsillinic acid in vitro. It is a non-reducing iterative type 1 polyketide synthase. Co-incubation of free orsellinic acid with alcohols and ArmB showed cross-coupling activity. Therefore, the enzyme has transesterification activity. Also, there are other auxiliary factors suspected to control substrate specificity. Additionally, halogen modifications have been observed. Overexpression of annotated halogenases (termed *ArmH1-5*) and characterization of the subsequent enzymes revealed in all five enzymes the chlorination of mellolide F. In vitro reactions of free standing substrates showed that the enzymes do not require auxiliary carrier proteins for substrate delivery.

## See also

* Forest pathology
* List of *Armillaria* species
* List of bioluminescent fungi

## References

1. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `urlMycoBank: Armillaria mellea`
2. ↑ 2.0 2.1 2.2 2.3 2.4 Cite error: Invalid `<ref>` tag; no text was provided for refs named `Barla 1887`
3. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Hennings 1895`
4. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Peck 1893`
5. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Peck 1897`
6. ↑ 6.0 6.1 Cite error: Invalid `<ref>` tag; no text was provided for refs named `Gillet 1874`
7. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Hennings 1900`
8. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Peck 1891`
9. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Karsten 1879`
10. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Smith 1908`
11. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Cha 1995`
12. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Calonge 2003`
13. ↑ Template:Cite book
14. ↑ 14.0 14.1 14.2 Kuo, Michael (2004-10-01). *Armillaria mellea*: The Honey Mushroom. *MushroomExpert.Com*. Retrieved on October 18, 2013.
15. ↑ 15.0 15.1 Cite error: Invalid `<ref>` tag; no text was provided for refs named `Desjardin 2008`
16. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Ross-Davis 2012`
17. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Roberts 2011`
18. ↑ Armillaria Root Disease. *Forest Insect & Disease Leaflet 78*. US Department of Agriculture Forest Service (1989-04-01). Retrieved on October 17, 2013.
19. ↑ Kuo, Michael (2004-10-01). *Entoloma abortivum*. *MushroomExpert.Com*. Retrieved on October 19, 2013.
20. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Czederpiltz 2001`
21. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Kuo 2007`
22. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Guo 2011`
23. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Muszynska 2011`
24. ↑ Lackner et al., 2013
25. ↑ Wick et al., 2015

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