What You Should Know
Agaricus abruptibulbus is a species of mushroom in the genus Agaricus. This bulbous-stemmed edible species smells slightly of anise and turns yellow when bruised or cut. The mushroom is medium-sized, with a white, yellow-staining cap on a slender stipe that has a wide, flat-bulb on the base.
Historically, "Agaricus abruptibulbus" is a hot mess, beginning with the original author of the species, Charles Peck (1892, 1900, 1905), who conflated three separate species in his concept and his type collection, according to Kerrigan. In the years since, the name has been applied inconsistently, and sometimes subordinated to synonymy with Agaricus sylvicola (Kerrigan himself did this in his 1986 treatment of the genus in California). But DNA sequencing and micromorphology have clarified things somewhat—at least, for the time being.
This mushroom is known to bioaccumulate the toxic element cadmium—in other words, it absorbs cadmium faster than it loses it—so specimens collected in the wild often have higher concentrations of this element than the soil in which they are found. It is believed that the cadmium-binding ability comes from a low molecular weight metal-binding protein named cadmium-mycophosphatin.
Other names: Abruptly-Bulbous Agaricus, The Flat-Bulb Mushroom, Woodland Agaricus.
Agaricus abruptibulbus Mushroom Identification
Saprobic; growing gregariously in hardwood forests or in urban locations under ornamental Norway spruce; fall; documented by Kerrigan (2016) from Pennsylvania, New York, and Washington.
5–9 cm; convex to bell-shaped or a little blocky at first, becoming broadly convex or nearly flat; dry; minutely radially fibrillose or nearly bald; white when young, becoming yellowish with age; the margin not lined, yellowing slightly when rubbed repeatedly.
Free from the stem; crowded; short-gills frequent; white when young, becoming grayish and eventually dark brown; when in the button stage covered with a whitish partial veil that develops yellowish stains.
5–10 cm long; 0.5–1 cm thick; equal above an abruptly bulbous base; bald; with a white to yellowish ring; whitish; bruising yellowish toward the base when rubbed; basal mycelium white.
White; unchanging when sliced.
Odor and Taste
Strong; reminiscent of almonds.
Cap and stem dull orangish-yellow.
KOH yellow on cap surface.
Spores: 6–7 x 3.5–4.5 µm; ellipsoid; smooth; thick-walled; brown in KOH; brown in Melzer's. Basidia 4-sterigmate. Cheilocystidia 20–35 x 4–7 µm; multiseptate and catenulate; terminal elements subglobose to clavate or obpyriform; smooth; thin-walled; hyaline in KOH; soon collapsing. Pleurocystidia not found. Pileipellis a cutis; elements 2.5–5 µm wide, smooth, hyaline in KOH.
Agaricus abruptibulbus Look-Alikes
Is very similar in appearance and also grows in woodlands, but it may be distinguished by the lack of an abruptly bulbous base.
Has a more robust stature, lacks the bulbous base and grows in grassy open areas like meadows and fields. It has larger spores than A. abruptibulbus, typically 7.0–9.2 by 4.4–5.5 µm.
Agaricus abruptibulbus Taxonomy
The species was originally named Agaricus abruptus by American mycologist Charles Horton Peck in 1900. In his 1904 Report of the State Botanist publication, he changed the name to Agaricus abruptibulbus. He explained that Elias Magnus Fries had earlier named a species in the subgenus Flammula, which he called Agaricus abruptus; the subgenus was later raised to the rank of genus, and the species was given the name Flammula abruptus. Under the transitioning nomenclatural conventions of the time, it was unclear if Agaricus abruptus would remain available for use, so he changed the name.
Agaricus abruptibulbus belongs in the Arvenses clade of the genus Agaricus (along with species A. silvicola, A. arvensis, and A. semotus).
Some American authors consider this species to be synonymous with A. silvicola, while some in Europe have synonymized it with the similar species A. essettei. American mycologists Steve Trudell and Joseph Ammirati noted in a 2009 field guide: "The name A. abruptibulbus has been applied to forms with bulbous stipe bases, but variation in stipe shape is so great that the use of this name has been largely abandoned."
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