Suillus americanus is a species of mushroom-forming fungus in the family Suillaceae. Commonly known as the chicken fat mushroom and the American suillus, it produces bright yellow, often slimy caps marked with red to reddish-brown scales, a pore surface of large yellow angular pores that can stain brownish when bruised, and a slender yellow stipe dotted with darker glandular spots. The species was first described in the late 19th century by the American mycologist Charles Horton Peck and later transferred to Suillus. It closely resembles S. sibiricus, and DNA-based studies have led some authors to treat the two names as referring to a single, wide-ranging species complex, with several regional variants recognized at infraspecific rank. Suillus americanus is an ectomycorrhizal fungus that forms symbiotic associations with pines, best known in North America from eastern white pine (Pinus strobus). Fruiting bodies appear on the ground, singly or in clusters, most often from late summer into autumn, and records based on collections and DNA sequencing indicate a broader Northern Hemisphere distribution than was once assumed, including parts of East Asia and the Balkan Peninsula. Greenhouse experiments suggest strong host filtering during spore germination, and competition studies indicate it can colonize pine seedlings readily and dominate some other pine-associated Suillus species. The mushroom is edible but often considered mediocre because of its slimy texture, and some people develop contact dermatitis after handling it; laboratory work has also identified a beta glucan with anti-inflammatory activity. Although assessed as Least Concern globally, it has been treated as threatened or legally protected in parts of southeastern Europe where it occurs with scattered high-elevation five-needle pine forests.
Taxonomy
Suillus americanus was first described scientifically by American mycologist Charles Horton Peck in 1888, based on specimens he had originally collected as far back as 1869, in New York state, near Sand Lake, Albany, and Port Jefferson. In his 1888 publication he indicated that he had originally listed these collections as Boletus flavidus (now known as Suillus flavidus) in his 1869 Report of the State Botanist (published in 1872). However, as was pointed out nearly a century later in 1986, the 1869 report does not actually mention the species; rather, Peck's field notes that year (which served as the basis for the report) reference a collection at Sand Lake upon which the original (1888) description was most likely based. Because Peck failed to designate a type specimen, one of the Sand Lake specimens was lectotypified in 1986. In 1931, French mycologist Édouard-Jean Gilbert transferred the species to the genus Ixocomus, a now-defunct taxon that has since been subsumed into Suillus. In 1959, Walter H. Snell, collaborating with Rolf Singer and Esther A. Dick, transferred the species to Suillus. In his 1986 version of the authoritative monograph The Agaricales in Modern Taxonomy, Singer included the species in the subsection Latiporini of genus Suillus, an infrageneric grouping (below the taxonomic level of genus) characterized by a cinnamon-colored spore print without an olive tinge, and wide pores, typically greater than 1 mm when mature. Because several Suillus species have overlapping macroscopic and microscopic characters, later authors have increasingly relied on DNA barcoding to verify identifications. In a re-examination of Korean herbarium material using internal transcribed spacer (ITS) DNA sequences alongside morphology, Min and colleagues found that misidentifications were common in Suillus collections, and that microscopic measurements (spores, basidia, and cystidia) were generally not diagnostic among the Korean species they assessed. Common names for the species include the American slipperycap, the American suillus, or the chicken-fat mushroom. The latter name is a reference to its yellow color. The specific epithet americanus means "of America". Studies using molecular phylogenetics have altered how some authors define the limits of Suillus americanus. In a global analysis of ITS barcode sequences (the standard DNA marker used for identifying fungi), Nguyen and colleagues found that collections identified as S. americanus and S. sibiricus form a single, widely distributed lineage across North America and Eurasia, with little internal genetic structure; they therefore supported treating S. sibiricus as the same species and retaining the older name S. americanus. They also placed the recently described Himalayan taxon S. himalayensis within the genetic and morphological variation of S. americanus. Nguyen and colleagues also noted that several infraspecific names have been applied to this species (two varieties and three forms), which they interpreted as expected morphological variants in a taxon with a very wide range; these variants were not readily separable using ITS sequences alone. In a world-wide identification key of Suillus, Wolfgang Klofac treated S. sibiricus as an infraspecific taxon of S. americanus, publishing the combinations S. americanus f. sibiricus and S. americanus f. helveticus and noting that reported macroscopic and microscopic differences between S. americanus and S. sibiricus are slight. He also cautioned that North American collections filed under the name "S. sibiricus" required further study, because the name has been applied inconsistently and distribution records can be distorted by misidentifications. Subsequent nomenclatural work has treated several regional variants as infraspecific forms within S. americanus, including western North American material long reported as "S. sibiricus". The 2025 IUCN Red List assessment follows this interpretation and summarises the two-name history. Subsequent nomenclatural work formalized some of this variation by describing Suillus americanus f. pseudosibiricus for western North American material previously misidentified as "S. sibiricus", based on both morphology and placement within a distinct ITS subclade of the S. americanus complex.
Description
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