Figure 1. Fruit bodies of A. gallica. The largest fruit body is approximately 10 cm in length.

Figure 2. Mating in a 9 cm petri dish. Left and right are colonies of compatible, haploid strains of A. gallica. Center, a pairing of these two strains showing a compatible mating reaction. The strand-like structures are rhizomorphs.

Armillaria gallica Marxmüller & Romagnesi is one of many species of the genus collectively known as shoestring root-rot fungi or honey mushrooms. A. gallica is a nearly ubiquitous opportunistic pathogen and saprophyte of hardwood trees in northern temperate forests. In dead wood, the fungus produces a characteristic white rot. After colonizing woody substrates, the fungus produces strand-like aggregations of hyphae called rhizomorphs, which are similar to plant roots in appearance and ability to extend through the soil. On encountering newly available woody substrates, the rhizomrphs initiate new infections and further growth. Mating in populations of A. gallica is effectively random over continental scales, presumably due to the dispersal of air-born basdiospores. Specificity in mating is controlled by the bifactorial sexual incompatibility system typical of many basidiomycetes, but the predominant vegetative phase of A. gallica is diploid, not dikaryotic as in most other basidiomycetes. In nature, each individual of A. gallica originates in a unique mating event and the resultant diploid mycelium may colonize a territory encompassing many adjacent tree root systems. Many individuals are large, extending over many hectares of forest floor, and old, often in excess of 1000 years. Because of its large geographic distribution and large biomass per unit area of forest, and formidable capabilities as a wood degrader, A. gallica is likely to play an important role in global carbon cycling.