Armillaria mellea is an edible basidiomycete fungus in the honey fungus 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 mycelium is capable of producing light via bioluminescence.
The mushroom is widely distributed in temperate regions of the Northern Hemisphere, usually in heavy soils with temperatures under 79 °F (26 °C). It typically grows on hardwoods but may be found around and on 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: +Still-current subtaxa under A. mellea per MycoBankBased on MycoBank search results for "Armillaria mellea", which should list all taxa containing this string in the name. The search results also list a "current name", which reflects MycoBank's current taxonomic opinion (last accessed May 5, 2025). The search function may be accessed via https://www.mycobank.org/Simple%20names%20search.NameAuthority Yearvar. camerunensis Henn. 1895var. flava Peck 1897var. glabra Gillet 1874var. javanica Henn. 1900var. maxima Barla 1887var. radicata Peck 1891 var. viridiflava Barla 1887subsp. nipponica J.Y.Cha & Igarashi1995f. roseaCalonge & M.Seq.2003f. sabulicolaA. Ortega & G. MorenoMycoBank #584201 2010
MycoBank also lists f. mellea, subsp. mellea, and var. mellea as current with a "(?)".
Reclassified species Armillaria mellea once included a range of species with similar features that have since been reclassified. The following are reassigned subtaxa, mostly variety-level entries from the 19th century:
+Reassigned subtaxa of A. melleaNameAuthority YearCurrent namevar. minor Barla 1887 A. mellea (variety simply discarded)var. bulbosa Barla 1887 A. luteavar. exannulata Peck 1893 Desarmillaria tabescensvar. laricina (Bolton) Barla 1887 A. laricinavar. obscura Gillet 1874 A. ostoyaevar. sulphurea (Weinm.) Fr.1879 Agaricus sulphureus Weinm. (?)Question mark from MycoBank. Not to be confused with Agaricus sulphureus Bull,. which is a synonym for Tricholoma sulphureum. Some sources instead just assign to A. mellea, discarding the variety part.var. tabescens(Scop.) Rea & Ramsb.1917 Desarmillaria tabescensvar. versicolor (With.) W.G.Sm.1908 A. versicolor Withering 1801
Common names It is commonly known as honey fungus, stump mushroom, stumpie, honey mushroom, pipinky or pinky.
Description
The basidiocarp of each has a smooth cap 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 flesh of the cap is whitish and has a sweetish odour and flavour with a tinge of bitterness.
The stipe is long and 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.
Under the microscope, the spores are approximately elliptical, 6–9 by 5–7 μm, inamyloid with prominent apiculi (short, pointed projections) at the base. The spore print is white. 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. This phenomenon results from a blockage that inhibits the synthesis of hispidin, the precursor to luciferin, as well as 3-hydroxyhispidin hydroxylase in the mushrooms.
Similar species In addition to a number of reclassified species, Armillaria gemina lacks the yellow cap background and A. nabsnona has a darker hue.
Distribution and habitat Armillaria mellea is widespread in northern temperate zones. It has been found throughout North America, and in Europe and northern Asia. It has been introduced to South Africa. It grows parasitically on a large number of broadleaf trees. It fruits in dense clusters at the base of trunks or stumps.
Economics Armillaria mellea poses a significant threat to managed forests, tree plantations, orchards, vineyards, and gardens, which can lead to substantial economic losses due to Armillaria root rot.
Ecology Armillaria mellea prefers moist soil and lower soil temperatures, as soil temperatures exceeding 79 °F (26 °C) can inhibit its growth. It can withstand extreme temperatures, such as forest fires, due to the protection of the soil. It is found in many kinds of landscapes, including gardens, parks, vineyards, tree production areas, and natural landscapes.
Armillaria mellea typically is symbiotic with hardwood trees and conifers, including orchards, planted forests, vineyards, and a few herbaceous plants. It infects new hosts through rhizomorphs and basidiospores. It is rare for basidiospores to be successful in infecting new hosts and often colonize woody debris instead, but rhizomorphs, however, can grow up to ten feet long in order to find a new host.
There are few signs, and they are often difficult to observe. The most prominent sign is honey-coloured mushrooms at the base of the infected plant. Additional signs include white, fan-shaped mycelia and black rhizomorphs with diameters between . These usually are not as noticeable because they occur beneath the bark and in the soil, respectively. Rhizomorphs and mycelial fans are more reliable indicators of Armillaria because the mushrooms can be easily mistaken for those of other species. Additionally, the mushrooms are not consistently present and, when they do occur, remain for only short durations. The symptoms are much more numerous, including slower growth, dieback of branches, yellowing foliage, rotted wood at base and/or roots, external cankers, cracking bark, bleeding stem, leaf wilting, defoliation, and rapid death. Leaf wilting, defoliation, and dieback occur after the destruction of the cambium. Symptoms typically develop gradually, resulting in increased dieback and a reduction in yield as the disease progresses.
It is one of the most common causes of death in trees and shrubs in both natural and cultivated habitats, and cause steady and substantial losses.
"A.mellea" has been recorded as food of animals such as the adults of the pleasing fungus beetles Tritoma biguttata biguttata, Tritoma atriventris and T.humeralis (which may be conspecific), and T.mimetica, which occasionally eat it.Goodrich, Michael A. & Skelley, Paul E. (1994): Fungal host records for species of Tritoma (Coleoptera: Erotylidae) of America North of Mexico. Entomol. News 105: 289-294. PDF fulltextGoodrich, Michael A. and Springer, Charles A. (1999): The Pleasing Fungus Beetles (Coleoptera: Erotylidae) of Nebraska. Transactions of the Nebraska Academy of Sciences 25: 53-71.Fulltext However, it is not quite clear whether such records in older sources refer to this honey mushroom species in its present more restricted delimitation, or to some other species of Armillaria or even related genera such as Desarmillaria.
Disease cycle Armillaria mellea infects both through basidiospore and penetration of host species by rhizomorphs which can grow up to long per year to find new, living tissue to infect. However, infection of living host tissue through basidiospores is quite rare. Two basidiospores must germinate and fuse to be viable and produce mycelium. In the late summer and autumn, Armillaria mellea produces mushrooms with notched gills, a ring near the cap base, and a white to golden color. They do not always appear, but when they do they can be found on both living and dead trees near the ground. These mushrooms produce and release the sexually created basidiospore which is dispersed by the wind. This is the only spore-bearing phase. The fungus overwinters as either rhizomorphs or vegetative mycelium. Infected wood is weakened through decay in roots and tree base after destruction of the vascular cambium and underlying wood.
Trees become infected 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. When rhizomorphs encounter a new, healthy host, mycelium grows over the surface of the roots. The host may remain asymptomatic for a period; however, under stress conditions, the fungus rapidly penetrates the bark and induces disease. 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, which often become encrusted with resin, soil, and occasionally fungal tissue. Whereas broad-leaved trees sometimes develop sunken cankers covered with loose bark that can contain gum or other exudates. 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.