Armillaria gallica (synonymous with Armillaria bulbosa and Armillaria lutea) is a species of honey mushroom in the Physalacriaceae family of the Agaricales order. The species is a common and ecologically important wood-decay fungus that can live as a saprobe, or as an opportunistic parasite in weakened tree hosts to cause root or butt rot. It has a widespread distribution, being found in temperate regions of Asia, North America, and Europe, and forms fruit bodies singly or in groups in soil or rotting wood. The fungus has been inadvertently introduced to South Africa. Armillaria gallica has had a confusing taxonomy, due in part to historical difficulties encountered in distinguishing between similar Armillaria species. The fungus received international attention in the early 1990s when an individual colony living in a Michigan forest was reported to cover an area of 15 hectares (37 acres), weigh at least 9,500 kilograms (21,000 lb), and be 1,500 years old. This individual is popularly known as the "humungous fungus", and is a tourist attraction and inspiration for an annual mushroom-themed festival in Crystal Falls.
Armillaria gallica is a largely subterranean fungus, and it produces fruit bodies that are up to about 10 cm (3.9 in) in diameter, yellow-brown, and covered with small scales. On the underside of the caps are gills that are white to creamy or pale orange. The stem may be up to 10 cm (3.9 in) long, with a white cobwebby ring that divides the color of the stem into pale orange to brown above, and lighter-colored below. The fungus can develop an extensive system of underground root-like structures, called rhizomorphs, that help it to efficiently decompose dead wood in temperate broadleaf and mixed forests. It has been the subject of considerable scientific research due to its importance as a plant pathogen, its ability to bioluminesce, its unusual life cycle, and its ability to form large and long-lived colonies.
The fruit bodies of Armillaria gallica have caps that are 2.5–9.5 cm (1.0–3.7 in) broad, and depending on their age, may range in shape from conical to convex to flattened. The caps are brownish-yellow to brown when moist, often with a darker-colored center; the color tends to fade upon drying. The cap surface is covered with slender fibers (same color as the cap) that are erect, or sloping upwards.
When the fruit bodies are young, the underside of the caps have a cottony layer of tissue stretching from the edge of the cap to the stem—a partial veil—which serves to protect the developing gills. As the cap grows in size the membrane is eventually pulled away from the cap to expose the gills. The gills have an adnate (squarely attached) to somewhat decurrent (extending down the length of the stem) attachment to the stem. They are initially white, but age to a creamy or pale orange covered with rust-colored spots. The stem is 4–10 cm (1.6–3.9 in) long and 0.6–1.8 cm (0.24–0.71 in) thick, and almost club-shaped with the base up to 1.3–2.7 cm (0.5–1.1 in) thick. Above the level of the ring, the stem is pale orange to brown, while below it is whitish or pale pink, becoming grayish-brown at the base. The ring is positioned about 0.4–0.9 cm (0.16–0.35 in) below the level of the cap, and may be covered with yellowish to pale-brownish woolly cottony mycelia. The base of the stem is attached to rhizomorphs, black root-like structures 1–3 mm in diameter. While the primary function of the below-ground mycelia is to absorb nutrients from the soil, the rhizomorphs serve a more exploratory function, to locate new food bases.
When the spores are seen in deposit, such as with a spore print, they appear whitish. They have an ellipsoid or oblong shape, usually contain an oil droplet, and have dimensions of 7–8.5 by 5–6 µm. The spore-bearing cells, the basidia, are club-shaped, four-spored (rarely two-spored), and measure 32–43 by 7–8.7 µm. Other cells present in the fertile hymenium include the cheilocystidia (cystidia present on the edge of a gill), which are club-shaped, roughly cylindrical and 15–25 by 5.0–12 µm. Cystidia are also present on the stem (called caulocystidia), and are broadly club-shaped, measuring 20–55 by 11–23 µm. The cap cuticle is made of hyphae that are irregularly interwoven and project upward to form the scales seen on the surface. The hyphae that make up the surface scales typically measure 26–88 µm long by 11–27 µm thick and can be covered with a crust of pigment. Clamp connections are present in the hyphae of most tissues.
Like all Armillaria species, Armillaria gallica is considered edible. Thorough cooking is usually recommended, as the raw mushroom tastes acrid when fresh or undercooked. One author advises to consume only a small portion initially, as some people may experience an upset stomach. The taste is described as "mild to bitter", and the odor "sweet", or reminiscent of camembert cheese.
Armillaria calvescens is rather similar in appearance, and can only be reliably distinguished from Armillaria gallica by observing microscopic characteristics. Armillaria calvescens has a more northern distribution, and in North America, is rarely found south of the Great Lakes. Armillaria mellea has a thinner stem than Armillaria gallica, but can be more definitively distinguished by the absence of clamps at the base of the basidia. Similarly, Armillaria cepistipes and Armillaria gallica are virtually identical in appearance (especially older fruit bodies), and are identified by differences in geographical distribution, host range, and microscopic characteristics. Molecular methods have been developed to discriminate between the two species by comparing DNA sequences in the gene coding translation elongation factor 1-alpha.
Armillaria gallica can produce cyclobutane-containing metabolites such as arnamiol, a natural product that is classified as a sesquiterpenoid aryl ester. Although the specific function of arnamiol is not definitively known, similar chemicals present in other Armillaria species are thought to play a role in inhibiting the growth of antagonistic bacteria or fungi, or in killing cells of the host plant prior to infection.
The mycelia (but not the fruit bodies) of Armillaria gallica are known to be bioluminescent. Experiments have shown that the intensity of the luminescence is enhanced when the mycelia are disturbed during growth or when they are exposed to fluorescent light. Bioluminescence is caused by the action of luciferases, enzymes that produce light by the oxidation of a luciferin (a pigment). The biological purpose of bioluminescence in fungi is not definitively known, although several hypotheses have been suggested: it may help attract insects to help with spore dispersal, it may be a by-product of other biochemical functions, or it may help deter heterotrophs that might consume the fungus.
Armillaria gallica can normally be found on the ground, but sometimes on stumps and logs. Mushrooms that appear to be terrestrial are attached to plant roots underneath the surface. It is widely distributed and has been collected in North America, Europe, and Asia (China, Iran, and Japan). The species has also been found in the Western Cape Province of South Africa, where it is thought to have been introduced from potted plants imported from Europe during the early colonization of Cape Town. In Scandinavia, it is absent in areas with very cold climates, like Finland or Norway, but it is found in southern Sweden. It is thought to be the most prevalent low altitude species of Armillaria in Great Britain and France. The upper limits of its altitude vary by region. In the French Massif Central, it is found up to 1,100 m (3,600 ft), while in Bavaria, which has a more continental climate, the upper limit of distribution reaches 600 m (2,000 ft). In Serbian forests, it is the most common Armillaria between elevations of 70 to 1,450 m (230 to 4,760 ft).[60] Field studies suggest that Armillaria gallica prefers sites that are low in organic matter and have high soil pHs.
Armillaria gallica can develop an extensive subterranean system of rhizomorphs, which helps it to compete with other fungi for resources or to attack trees weakened by other fungi. A field study in an ancient broadleaved woodland in England showed that of five Armillaria species present in the woods, A. gallica was consistently the first to colonize tree stumps that had been coppiced the previous year. Fractal geometry has been used to model the branching patterns of the hyphae of various Armillaria species. Compared to a strongly pathogenic species like A. solidipes, A. gallica has a relatively sparse branching pattern that is thought to be "consistent with a foraging strategy in which acceptable food bases may be encountered at any distance, and which favours broad and divisive distribution of potential inoculum." Because the rhizomorphs form regular networks, mathematical concepts of graph theory have been employed to describe fungal growth and interpret ecological strategies, suggesting that the specific patterns of network attachments allow the fungus "to respond opportunistically to spatially and temporally changing environments".
Armillaria gallica may itself be parasitized by other soil flora. Several species of the fungus Trichoderma, including Trichoderma polysporum, T. harzianum and T. viride, are able to attack and penetrate and the outer tissue of A. gallica rhizomorphs and parasitize the internal hyphae. The infected rhizomorphs become devoid of living hyphae about one week after the initial infection. Entoloma abortivum is another fungus that can live parasitically upon A. gallica. The whitish-gray malformed fruit bodies that may result are due to the E. abortivum hyphae penetrating the mushroom and disrupting its normal development.
In North America, it is common east of the Rocky Mountains, but rare in the Pacific Northwest. In California, where it is widely distributed, the fungus is found in a variety of plant communities, including aspen, coastal oak woodland, Douglas Fir, Klamath mixed conifer, montane hardwood, montane hardwood-conifer, montane riparian, Redwood, Sierran mixed conifer, valley oak woodland, valley-foothill riparian, and White Fir. It was found to be the most common Armillaria species in hardwood and mixed oak forests in western Massachusetts.
A Chinese study published in 2001 used the molecular biological technique restriction fragment length polymorphism to analyze the differences in DNA sequence between 23 A. gallica specimens collected from the Northern Hemisphere. The results suggest that based on the restriction fragment length polymorphism patterns observed, there are four global A. gallica subpopulations: the Chinese, European, North American–Chinese, and North American–European geographical lineages. A 2007 study on the northeastern and southwestern Chinese distribution of Armillaria, using fruit body and pure culture morphology, concluded that there are several unnamed species (Chinese biological species C, F, H, J and L) that are similar to the common A. gallica.