The Agaricales, or euagarics clade, is a monophyletic group of approximately 8500 mushroom species. As such it forms the largest clade of Agaricomycetes (=Homobasidiomycetes sensu Hibbett and Thorn 2001; Binder et al. 2005) and together with the Boletales and Atheliales it forms the Agaricomycetidae. Species of Agaricales are widespread and diverse on land ranging from desert, grassland, forests, tundra, and shorelines in tropical, temperate, and arctic-alpine habitats. While many Agaricales acquire sustenance by decomposing dead organic matter (as saprotrophs), others parasitize plants or other fungi, and a few capture or parasitize vertebrates or invertebrates. Many groups of Agaricales engage in mutualistic symbioses with the roots of vascular plants where they form ectomycorrhizas (EM), or more rarely associate with unicellular green algae or cyanobacteria as lichens. A few colonize bryophytes. Orchids and mycotrophic plants depend on some Agaricales (Armillaria, Tricholoma), in addition to other groups of Agaricomycetes, for their nutrition. Several clades of Agaricales include ant or termite symbionts or even nematode predators. Species of Armillaria, Athelia pro parte, Moniliophthora, and Mycena are economically devastating plant pathogens, whereas others in the genera Agaricus, Lentinula, Termitomyces, Tricholoma, and Volvariella are highly valuable commercial or collected foods. Psychotropic or hallucinogenic mushrooms of Psilocybe and other genera also occur in the Agaricales, and others, as in some species of Amanita, produce toxins lethal to humans.

A sample of different nutritional modes among Agaricales. Left: Ectomycorrizal (EM) habitat in Karri (Eucalyptus diversicolor) forest in the south-west botanical district of Western Australia. © Brandon Matheny. Center: Attack and consumption of a nematode by Pleurotus (Oyster Mushroom, Pleurotaceae). © Greg Thorn. Right. Termitomyces reticulatus (Lyophyllaceae) fruiting from underground fungus gardens of the termite Odontotermes badius. © Duur Aanen.

Molecular systematic studies of the Agaricales have radically transformed our interpretations of the evolution and classification of gilled mushrooms and their relatives (Hibbett et al. 1997; Moncalvo et al. 2000, 2002; Matheny et al. 2006). The overwhelming majority of species produces fruit bodies with gills (lamellae), but the evolution of gills has arisen numerous times in the Agaricomycetes (Hibbet et al. 1997). Likewise, multiple lineages of “gasteromycetes” (puffballs, bird’s nest fungi, false truffles), species that produce spores in an enclosed fruiting structure, have evolved independently among the Agaricales (Peintner et al. 2001). Studies by Bodensteiner et al. (2004), Larsson et al. (2004), and Binder et al. (2005) have shown that some non-gilled fungi, including reduced or cup-like (cyphelloid) forms and crustose or resupinate forms, share their evolutionary histories with numerous lineages of Agaricales, including some lineages that evolved in aquatic or marine environments (Binder et al. 2001; Hibbett and Binder 2001; Binder et al. 2006). In short, the gross morphology of mushroom fruit bodies is highly plastic and often a poor phylogenetic indicator. These and other studies demonstrated that a broad concept of Agaricales (Singer 1986), including boletes, some polypores, and the genera Russula, Lactarius and their allies, does not form a monophyletic group. Thus, the clade containing predominantly genera and families from the suborder Agaricineae (Singer 1986) was labeled the euagarics clade and represents what we currently regard as the Agaricales (Moncalvo et al. 2002). Most family-level relationships based on morphological characters are artificial, but progress is being made to delimit higher-level monophyletic groups with multiple gene data sets (Matheny 2005; Aime et al. 2005; Hofstetter et al. 2002; Binder et al. 2006). Remarkably, new species and genera continue to be described or placed in the order by molecular phylogenetic analyses.