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Amanita bisporigera

[ Basidiomycetes > Agaricales > Amanitaceae > Amanita . . . ]

by Michael Kuo

The distinguishing features of Amanita bisporigera are:

  • The white "sack" around the base, known as a "volva," resulting from the universal veil.

  • The pure white gills, which are usually free from the stem.

  • The stark white color of the entire mushroom.

  • The smooth cap, which almost always lacks patches or warts.

There is ongoing debate about how many closely related species actually occur in North America. Since I am by nature a "lumper" (someone who would rather define species in their broadest possible sense; a "splitter," by contrast, prefers to separate many species), I will follow the lead of several contemporary mycologists and present only two: Amanita bisporigera and the western species Amanita ocreata. For the gory details, see the comments below.


Ecology: Mycorrhizal with hardwoods; summer and fall; probably widely distributed in North America.

Cap: 5-12 cm; almost oval, becoming convex, then broadly convex to nearly flat in age; smooth; dry; stark white, sometimes discoloring towards the center in age; the margin (usually) not lined.

Gills: Attached or free from the stem; white; close.

Stem: 7.5-20 cm long; 0.5-2 cm thick; more or less equal, or more frequently tapering somewhat to apex and flaring to an enlarged base; smooth or shaggy; white; with a persistent skirtlike ring that almost always remains into maturity; without a rim at the base; with a white, sacklike volva encasing the base, which may be underground or broken up.

Flesh: White throughout.

Odor: Distinctive in older specimens--rather foul.

Spore Print: White.

Chemical Reactions: KOH on cap surface producing a yellow reaction (but see below).

Microscopic Features: Spores 7-10 x 6.5-8.5 µ smooth; broadly elliptical or nearly round; amyloid. Basidia 2-spored or 4-spored (possibly 2-spored early in the season and 4-spored as the season progresses).

REFERENCES: Atkinson, 1906. (Kauffman, 1918; Smith, 1949; Smith, 1975; Smith, Smith & Weber, 1979; Weber & Smith, 1985; Arora, 1986; Jenkins, 1986; Phillips, 1991/2005; Lincoff, 1992; Metzler & Metzler, 1992; Barron, 1999; Roody, 2003; Tulloss, 2003; McNeil, 2006; Miller & Miller, 2006.) Herb. Kuo 06269504, 07080706.

Until recently Amanita bisporigera, described above, was accompanied not only by Amanita ocreata but also by Amanita verna and Amanita virosa in North American accounts. The latter two species were defined in part by the fact that their basidia bore four spores, rather than the two spores on Amanita bisporigera basidia; they were further separated on the basis of spore shape, distribution, reaction to KOH, and the texture of the stems (smooth or shaggy).

Several mycologists, including Amanita expert Rod Tulloss, have suggested that the documentation of Amanita verna and Amanita virosa may be based in part on misidentified collections of a white form of Amanita phalloides. Additionally, there is evidence that Amanita bisporigera--named for its two-spored basidia--may actually develop four-spored basidia with some regularity. Tulloss (1999; link below) notes that "[t]here seems to be a tendency to shift from 2-spored to 4-spored basidia as the season progresses."

Another point of confusion involves the reaction of the mushroom's cap to KOH. As best as I can determine, Amanita bisporigera, Amanita ocreata, and "Amanita virosa" were said to turn yellow when a drop of KOH was applied to the cap; "Amanita verna" lacked a color change. However, the reaction to KOH was inconsistently recorded, and rather depended on which source one was consulting. Tulloss (1999, 2003) suggests that Amanita bisporigera turns yellow in KOH--and that collections which do not turn yellow may represent white forms of Amanita phalloides.

It is probable that Amanita verna is a European species, and that the name has been misapplied to North American collections of Amanita bisporigera. Moreover, it may well be the case that the "Amanita virosa" described by many North American authors is actually Amanita bisporigera bearing four spores instead of two. However, the western Amanita ocreata is fairly clearly distinct; it is stouter, has somewhat larger spores, is found only on the west coast and in the southwest, and has inflated cells with thin walls in its flesh.

My experience with dozens of collections in Illinois and Michigan over the years indicates that they all turn yellow in KOH, though some do so more eagerly than others; that they are both 2-spored and 4-spored, and that the number of spores does not correlate to any other data; that attempting to assess the "shagginess" of the stem is an exercise in futility; and that the spores all fall within the 7-10 x 6.5-8.5 µ range and are broadly elliptical to nearly round, demonstrating small differences without correlation to other details. However, please do not confuse an amateur's collecting experience with true scientific evidence.

As far as I know, the mycological world still awaits a DNA study that will put to rest the speciation question for these mushrooms. Mating studies, I am told, are out of the question, since amanitas do not mate in culture like the rest of us.

Further Online Information:

Amanita bisporigera at Tulloss's Studies in Amanita
Species of Section Phalloideae at Tulloss's Amanita Site
Death Angels at Tom Volk's Fungi
Amanita ocreata at MykoWeb
Amanita bisporigera at Roger's Mushrooms
Amanita virosa at Roger's Mushrooms
Amanita virosa at Fungi of Poland


Amanita virosa

Amanita virosa

Amanita bisporigera

Amanita virosa

Amanita bisporigera

Amanita bisporigera

Amanita bisporigera

Amanita ocreata:

Amanita ocreata

Differs from Amanita bisporigera as follows:

  • Range: Limited to the West Coast and the Southwest.

  • Stature: Often (but not always!) stouter; stem 1.5-3 cm wide.

  • Spores: Slightly larger; 9-12 x 6-8.5 µ.

  • Inflated Cells: In context and universal veil, inflated cells with thin walls (up to .5 µ).

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  • Cite this page as:

    Kuo, M. (2003, October). Amanita bisporigera. Retrieved from the MushroomExpert.Com Web site: