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.

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 9.5 tonnes (9,500 kg; 21,000 lb), and be 1,500 years old.

Recent studies have revised the fungus's age to 2,500 years and its size to about 400 tonnes (400,000 kg; 880,000 lb), four times the original estimate.

[1] 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.

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.

[4] 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.

The species that Korhonen called EBS B was named A. bulbosa by Helga Marxmüller in 1982,[9] as it was thought to be equivalent to Armillaria mellea var.

[11] 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.

[13] Another synonym, A. lutea, had originally been described by Claude Casimir Gillet in 1874,[14] and proposed as a name for EBS E.[15][16] 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.

[23] 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.

[30] Similarly, A. cepistipes and A. 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.

[27] Armillaria gallica can produce cyclobutane-containing metabolites such as arnamiol,[31] a natural product that is classified as a sesquiterpenoid aryl ester.

[32] 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.

Using molecular genetics, they determined that the underground mycelia of one individual fungal colony covered 15 ha (37 acres), weighing over 9,500 kilograms (21,000 lb), with an estimated age of 1,500 years.

In their conclusion, the authors noted: "This is the first report estimating the minimum size, mass, and age of an unambiguously defined fungal individual.

Although the number of observations for plants and animals is much greater, members of the fungal kingdom should now be recognized as among the oldest and largest organisms on earth.

"[40] After the Nature paper was published, major media outlets from around the world visited the site where the specimens were found; as a result of this publicity, the individual acquired the common name "humongous fungus".

[39] There was afterward some scholarly debate as to whether the fungus qualified to be considered in the same category as other large organisms such as the blue whale or the giant redwood.

[45] These regular and repeating haploidization events result in increased genetic diversity, which helps the fungus to adapt to unfavorable changes in environmental conditions, such as drought.

[66] 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.

[72] The fungus has also been shown to infect Daylily in South Carolina,[73] Northern highbush blueberry (Vaccinium corymbosum) in Italy[74][75] and vineyards (Vitis species) of Rías Baixas in northwestern Spain.

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.

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".

The whitish-gray malformed fruit bodies that may result are due to the E. abortivum hyphae penetrating the mushroom and disrupting its normal development.