Armillaria gallica

Armillaria gallica (synonymous with A. bulbosa and A. lutea) is a species of honey mushroom in the family Physalacriaceae of the order Agaricales. 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 is found in temperate regions of Asia, North America, and Europe. The species 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. Recent studies have revised the fungus's age to 2,500 years and its size to about 440 tons, four times the original estimate. 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. Confusion has surrounded the nomenclature and taxonomy of the species now known as Armillaria gallica, paralleling that surrounding the genus Armillaria. The type species, Armillaria mellea, was until the 1970s believed to be a pleiomorphic species with a wide distribution, variable pathogenicity, and one of the broadest host ranges known for the fungi. In 1973, Veikko Hintikka reported a technique to distinguish between Armillaria species by growing them together as single spore isolates on petri dishes and observing changes in the morphology of the cultures. 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); NABS VII was demonstrated shortly after to be the same species as EBS E. Because several research groups had worked with this widely distributed species, it was assigned several different names. The species that Korhonen called EBS B was named A. bulbosa by Helga Marxmüller in 1982, as it was thought to be equivalent to Armillaria mellea var. bulbosa, first described by Jean Baptiste Barla (Joseph Barla) in 1887, and later raised to species status by Josef Velenovský in 1927. In 1973, the French mycologist Henri Romagnesi, unaware of Velenovský's publication, published a description of the species he called Armillariella bulbosa, based on specimens he had found near Compiègne and Saint-Sauveur-le-Vicomte in France. These specimens were later demonstrated to be the same species as the EBS E of Korhonen; EBS B was later determined to be A. cepistipes. Therefore, the name A. bulbosa was a misapplied name for EBS E. In 1987 Romagnesi and Marxmüller renamed EBS E to Armillaria gallica. Another synonym, A. lutea, had originally been described by Claude Casimir Gillet in 1874, and proposed as a name for EBS E. Although the name had priority due to its early publication date, it was rejected as a nomen ambiguum because of a lack of supporting evidence to identify the fungus, including a specimen, type locality, and incomplete collection notes. A. inflata (Velenovský, 1920) may represent another synonym, but the type specimens were not preserved, so it is considered a dubious name (nomen dubium). As of 2010, both the Index Fungorum and MycoBank consider Armillaria gallica Marxm. & Romagn. to be the current name, with A. bulbosa and A. lutea as synonyms. Phylogenetic analysis of North American Armillaria species based on analysis of amplified fragment length polymorphism data suggests that A. gallica is most closely related to A. sinapina, A. cepistipes, and A. calvescens. These results are similar to those reported in 1992 that compared sequences of nuclear ribosomal DNA. The specific epithet gallica is botanical Latin for 'French' (from Gallia, 'Gaul'), and refers to the type locality. The prior name bulbosa is Latin for 'bulb-bearing, bulbous' (from bulbus and the suffix -osa). Armillaria is derived from the Latin armilla, or 'bracelet'. 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.