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The Genus Agaricus  

[ Basidiomycota > Agaricales > Agaricaceae > Agaricus . . . ]

by Michael Kuo

The mushrooms in Agaricus are terrestrial saprobes, and have caps that are not brightly colored. At maturity the gills are free or almost free from the stem, and are dark brown. The stem breaks away cleanly from the cap—a fact known to anyone who has cleaned commercial "button mushrooms" from the store (Agaricus bisporus). Agaricus species have a partial veil which often forms a ring on the stem. The spore print, like the mature gills, is dark brown.

In 2016 North American Agaricus expert Richard Kerrigan published his life's work (so far) on our continent's members of the genus, providing some much needed clarity. Kerrigan's book, Agaricus of North America is a must for anyone seriously interested in the genus; it represents decades spent collecting across North America (especially in California, Colorado, and Pennsylvania), studying herbarium specimens, and studying collections sent to him by others. Even with so much experience informing the work, however, it is not the "final word" on North America's Agaricus species. (If it were, it would represent fundamentalism, not science.) Kerrigan is a very careful taxonomist, leaving dozens of potential new species described but unnamed, preferring to apply names when he has data from multiple collections. And, Kerrigan points out, new Agaricus species are continually being discovered!

Thus, you should not expect to be able to identify every Agaricus collection you make. As Kerrigan says, "[t]here may be cases where the very best solution presently available will be to say that 'this specimen is very close to . . .' and if you learn to live with that, I expect that you will be happier while studying Agaricus." That said, identification of Agaricus species ranges from fairly easy to very difficult. Robust collections with mushrooms in immature and mature stages of development are sometimes essential. Not infrequently, microscopic features must be consulted. Below I have listed some identification characters especially important in Agaricus.

In situ: Most species of Agaricus are found in woodland settings or in grass without any trees nearby—or, frustratingly, in grassy urban settings near planted trees with which they appear to have some sort of association (though not a mycorrhizal relationship). Thus, careful observation of the trees within striking distance of an Agaricus collection can be important in the identification process.

Standard morphological features. Dimensions and shapes, colors (usually in the white-to-gray-to-brown range in Agaricus), and textures are informative in Agaricus. As far as I know you can safely skip gill attachment (always free from the stem or nearly so) and spacing (close or crowded), as well as the color of the spore print (although you may have needed it to get to Agaricus in the first place).

Bruising and staining. Rubbing the mushroom's cap repeatedly along the margin with your thumb may cause the surface to change color—usually to a shade of yellow or red. The same may occur on the stem, especially near its base. And, when a species of Agaricus is sliced in half with a knife, the white to brownish flesh may change color (usually fairly promptly); this is often seen in the kitchen with commercially produced Agaricus bisporus, which blush reddish to pink when sliced. Be sure to slice open and observe the flesh in the very base of the stem; in some cases this is the only place where a change (to yellow, in these cases) can be detected.

Odors. If there were an HTML code for the *rolls eyes* emoji, I would type it here. Yes, the odor of an Agaricus is often an important feature in the identification process. The trouble is, some of us have defective sniffers. I can't smell all of the odors that Richard Kerrigan can. The main distinctive Agaricus odors are "phenolic" and "almond-like": I can usually (but not always) detect the latter, and almost never the former. I don't know what's up with that because, when I open a dropper bottle of lactophenol for microscope work in my herbarium, the phenol odor fills the room quickly—so it's not like I can't detect the odor in general. At any rate, the best way to assess the odor of an Agaricus is to crush the flesh in the very base of the stem.

Chemical reactions. Various chemicals, applied to the mushroom's surfaces, can produce distinctive color changes in Agaricus, but the most comprehensively used chemical is potassium hydroxide (KOH) in a 2% or stronger solution. Apply KOH to the cap surface and to the sliced flesh; reactions are fairly prompt and include no change (negative), yellow, and, in a few cases, nearly orange.

Dried specimens. Many species of Agaricus in section Arvenses acquire a fairly distinctive appearance when properly preserved by drying, becoming orangish yellow (see the illustration) instead of a shade of white or brown. This fact can sometimes serve as a shortcut in Agaricus identification.

Spores. For the most part, microscopic work in Agaricus identification consists of basic spore morphology; sporal dimensions should be measured with a fair degree of precision, since relatively small differences (for example, the difference between 6.5–8 µm and 8–8.5 µm) can indicate species differences. A 2% KOH mount of spores from a spore print is ideal, since the spores will by definition be mature, but a crush mount of mature gill tissue will also serve. Mature Agaricus spores are brown in KOH; be wary of measuring yellowish spores, which are immature and may not have developed fully.

Cheilocystidia. Occasionally one must enter the circle of Dante's Hell that should be named "Cheilocystidia in Agaricus." A Roman aqueduct section is required, in order to orient the gill edges accurately. Then the Devil gets out his whips and chains. To start, the gill edge is inevitably a BFM (M stands for "mess") in Agaricus, and most of the cheilocystidia are collapsed, even in young specimens. Crack! Then there's the "Is it a cheilocystidium or basidiole?" dilemma, because why should that be an easy distinction? Crack! And, finally, let's try to decide, before accepting our eternal damnation, whether the cheilocystidia are catenulate, indicating section Arvenses, or just a little swollen at the septa. Crack!

The identification key below is based on Kerrigan's 2016 treatment, along with other sources for Agaricus (see the references listed at the bottom of the page) and my experience collecting and studying about a fourth of the 100 species treated in the key. I have de-emphasized odors (see the discussion above) and, when possible, microscopic features. The key is divided into three main sections—red stainers, yellow stainers, and non-stainers—so I have provided "quick links" to bypass unneeded parts of the key.


 

Agaricus leptocaulis

Agaricus augustus

Agaricus bernardi

Agaricus brunneofibrillosus

Agaricus spore print
Spore print

Agaricus reducibulbus
Dried specimens in section Arvenses

Agaricus leptocaulis
KOH

Agaricus abruptibulbus
Spores

Agaricus auricolor
Cheilocystidia



Key to 100 Agaricus species, subspecies, and varieties in North America  


quick links: red staining | yellow staining | not staining


1.Flesh when sliced turning pink to red under normal conditions (in very wet weather some species appear to stain pink when they would not normally do so).
2

1.Flesh not staining pink to red under normal conditions.
48


2.Associated with Monterey cypress.
3

2.Not associated with Monterey cypress.
12


3.Cap white or nearly so.
4

3.Cap tan to brown.
5


4.Red staining prompt and intense; stem shaggy below the ring.
Agaricus benesii

4.Red staining slow and faint to moderate; stem not shaggy.


5.Cap conical to broadly conical; underside of ring dark brown.
Agaricus fuscovelatus

5.Cap convex to planoconvex; underside of ring whitish to brownish or yellow but not dark brown.
6


6.Large mushrooms (caps 6–25 cm across); flesh purplish pink when sliced; underside of ring often developing yellow stains and patches; refrigerated specimens developing orange, red, and purple shades.
Agaricus lilaceps

6.Small to medium sized mushrooms (caps 4–15 cm across); flesh pink to red when sliced; underside of ring not yellowing; refrigerated specimens not developing notable colors.
7


7.Mature cap small (2–3 cm across), brown and finely fibrillose; rare.
Agaricus rubroanus

7.Mature cap larger than 2–3 cm across, variously colored and textured; rare or common.
8


8.Stem shorter than, or about equal to, the width of the cap.
9

8.Stem longer than the width of the cap.
11


9.Mushroom usually stocky; cap developing fine brown scales.

9.Mushroom with the stature of store-bought mushrooms ("button mushrooms," "portabellas"); cap bald or with appressed fibrils.
10


10.Red staining faint to moderate; basidia 2-spored.

10.Red staining intense; basidia 4-spored.
Agaricus arginferus


11.Cap and stem pale tan to buff; ring thin and fragile; stem becoming hollow, without ringlets near the base.
Agaricus cupressophilus

11.Cap medium to dark brown, stem white; stem not hollowing, often featuring one to several brownish ringlets near base.


12.Found in Rocky Mountain forests (including pinyon-juniper forests).
13

12.Found elsewhere (including non-forest Rocky Mountain locations).
21


13.Associated with Rocky Mountain juniper in semi-arid, pinyon-juniper forests; staining strongly red; cap with overlapping scales and edge adorned with hanging tissue.
Agaricus hupohanae

13.Associated with spruces, firs, or pines at higher elevations; red staining strong or weak; cap lacking overlapping scales; cap edge with hanging tissue or not.
14


14.Cap white or nearly so.
15

14.Cap brown.
18


15.Red staining faint.
16

15.Red staining moderate to pronounced.
17


16.Flesh very firm and hard; odor not distinctive; truly woodland, associated with spruces; spores broadly ellipsoid.
Agaricus devoniensis
subsp. bridghamii

16.Flesh not notably firm and hard; odor faintly briny; ecology poorly documented but likely appearing only in meadows near woodlands; spores subglobose or broadly ellipsoid.
Agaricus pilosporus


17.Cap 6–12 cm across; stature stocky (stem about as long as the cap is wide, or shorter); most spores over 5 µm long.
Agaricus subfloccosus

17.Cap 2.5–4 cm across; stature not stocky; most spores under 5.5 µm long.
Agaricus sylvaticus
subsp. occidentalis


18.Stature stocky (stem about as long as the cap is wide, or shorter); cap very pale brown.
Agaricus subfloccosus

18.Stature not stocky (mature stem usually longer than the width of the cap); cap pale to dark brown.
19


19.Cap 2.5–4 cm across, bald or only slightly appressed-fibrillose; most spores under 5.5 µm long.
Agaricus sylvaticus
subsp. occidentalis

19.Cap larger than above, appressed fibrillose to scaly; most spores more than 5 µm long.
20


20.Cap with small scales with uplifted tips; red staining usually faint.
Agaricus cordillerensis

20.Cap appressed fibrillose, sometimes in old age developing large, appressed scales; red staining usually pronounced, and often brownish red.


21.Growing in grass (lawns, meadows, pastures, etc.), disturbed-ground settings like ditches and roadbeds, compost heaps, landscaping areas, and so on.
22

21.Growing in woods.
32


22.Cap white or nearly so.
23

22.Cap brown.
28


23.Cap surface bruising red, then brown; often growing in clusters; known from California and Mexico.
Agaricus subsubensis

23.Cap surface, if bruising red, not then changing to brown; not growing in clusters; variously distributed.
24


24.Red staining primarily limited to base of stem; odor faintly briny; spores often subglobose.
Agaricus pilosporus

24.Red staining not restricted to stem base; odor briny or not; spores subglobose or ellipsoid.
25


25.Odor briny; appearing in sandy, coastal areas and in the Denver area.

25.Odor not briny; variously distributed.
26


26.Stem lacking a ring but featuring a basal volva; known from Maryland.
Agaricus pequinii

26.Stem with a ring; variously distributed.
27


27.Flesh tough; ring sheathing the stem; gills usually becoming brown without a pink stage; often growing in hard-packed soil and road beds.

27.Flesh not notably tough; ring not sheathing; gills becoming pink, then brown; often found in lawns and compost piles (and in grocery stores).


28.Cap bald; stem lacking a ring but featuring a basal volva; spores subglobose; known from Maryland.
Agaricus pequinii

28.Cap fibrillose to scaly; stem with a ring; spores ellipsoid; variously distributed.
29


29.Cap 2–5 cm across, broadly bell-shaped; spores longer than 7.5 µm; known from Ontario.
Agaricus erindalensis

29.Cap larger than above, usually convex; spores variously sized; variously distributed.
30


30.Cap distinctly scaly with fine, coppery brown scales; most spores under 6 µm long; known from Tennessee.
Agaricus tennesseensis

30.Cap bald to fibrillose but not finely scaly; most spores longer than 6 µm; variously distributed.
31


31.Cap reddish brown, fibrillose; spores longer than 8 µm; basidia 4-spored; known from Santa Cruz County, California.
Agaricus depauperatus

31.Cap tan to dark brown but not reddish brown, fibrillose or not; spores shorter than 8 µm; basidia 2-spored; potentially widely distributed (native populations are primarily western but escapees from cultivation occur in many places).


32.Found from the Rocky Mountains westward.
33

32.Found from the Great Plains eastward.
42


33.Cap white or very pale brown.
34

33.Cap brown to reddish brown.
36


34.Cap flattened in the middle, fairly small (about 4 cm across); known from the Seattle area.
Agaricus bivelatoides

34.Cap convex, usually larger than 4 cm; variously distributed.
35


35.Red staining pronounced; stem very long (4–13 cm); cap with small concolorous scales toward the edge; found in conifer forests in the Pacific Northwest; basidia 4-spored.
Agaricus albosanguineus

35.Red staining weak to moderate; stem not notably long; cap scaly or not; variously distributed; basidia 2-spored.


36.Associated with western red-cedar; cap reddish brown and fibrillose-scaly; stem with small brown scales below the ring.
Agaricus thujae

36.Not associated with western red cedar; cap variously colored and textured; stem without small brown scales.
37


37.Associated with Mexican cypress and possibly with other high-elevation cypresses in the southwestern United States; cap gray-brown, 2–8 cm across; stature slender.
Agaricus tlaxcalensis

37.Not associated with cypresses; cap and stature varying.
38


38.Associated with various conifers in the San Francisco Bay area; cap slightly conical; ring with a dark brown underside.
Agaricus fuscovelatus

38.Associations various; found in the Bay Area or not; cap convex; ring underside not dark brown.
39


39.Cap appressed-fibrillose; KOH yellow on cap surface; known from the San Francisco Bay area.
Agaricus arorae

39.Cap bald or fibrillose; KOH negative on cap surface (or very faintly yellow in one poorly documented species); variously distributed.
40


40.Cap reddish brown; most spores longer than 8 µm; known from the San Francisco Bay area.
Agaricus depauperatus

40.Cap brown but not reddish brown; most spores shorter than 8 µm; variously distributed.
41


41.Mushroom usually robust (more than 10 cm tall); most basidia 4-spored; in western North America known from the Denver area.
Agaricus subperonatus

41.Mushroom not typically robust; basidia 2-spored; variously distributed.


42.Cap center with brown scales but surface mostly pale elsewhere; stem generally longer than the width of the cap; woodland; known from the southern Appalachians; October.
Agaricus vinosobrunneofumidus

42.Cap brown overall; stature varying; habitat varying; range and season varying.
43


43.Cap broadly bell-shaped, 2–5 cm across; spores longer than 7.5 µm; known from Ontario.
Agaricus erindalensis

43.Cap convex, variously sized; spores variously sized; variously distributed.
44


44.Cap about 5 cm wide at maturity; stem with a brown, bracelet-like ring; known from Québec and New York.
Agaricus rubribrunnescens

44.Cap variously sized; ring usually not brown bracelet-like; variously distributed.
45


45.Stem base bulbous; cap with fine, coppery brown scales; red staining intense; known from Tennessee.
Agaricus tennesseensis

45.Stem base not bulbous; cap scales, if present, varying in color; red staining weak or intense; variously distributed.
46


46.Cap scaly; stature generally tall at maturity, with fairly slender stem; found in woods.

46.Cap bald to appressed-fibrillose, but not usually scaly; stature reminiscent of store-bought mushrooms; found in urban areas or rarely in woods.
47


47.Mushroom usually robust (more than 10 cm tall); most basidia 4-spored.
Agaricus subperonatus

47.Mushroom not typically robust; basidia 2-spored.


quick links: red staining | yellow staining | not staining


48.Cap or stem surface bruising yellow (rub cap repeatedly near margin), and/or flesh yellow when sliced (check especially the flesh in the base of the stem).
49

48.Neither surfaces nor flesh bruising or staining yellow; flesh in stem base not yellow.
106


49.Appearing in the tropics or in greenhouses, often in clusters; cap dark brown to nearly black, often splitting radially.
Agaricus endoxanthus

49.Not with the above combination of features.
50


50.Appearing in grass, woodchips, compost areas, or thin woods across North America; cap white to very pale brown, medium-sized to large but notably thin-fleshed; surfaces yellowing when bruised; flesh in stem base yellow; ring with a thick outer edge; spores under 6 µm long.

50.Not with the above combination of features.
51


51.Found from the Rocky Mountains westward.
52

51.Found from the Great Plains eastward.
83


52.Found in woods (including pinyon-juniper woods) or chaparral.
53

52.Found in grass, or in urban settings under planted trees.
74


53.Mature cap small—usually 1–4 cm across.
54

53.Mature cap not small—usually more than 4 cm across.
56


54.Mushroom fairly stocky (stem 1–3 cm long); ring bandlike; montane; appearing in spring after snowmelt.
Agaricus stevensii

54.Mushroom tall and slender (stem 2–7 cm long); ring fragile and skirtlike; appearing at lower elevations in fall and winter.
55


55.Cap convex to broadly convex, sometimes with a broad central hump; velar tissues, when mounted in KOH, with orange inclusions.
Agaricus diminutivus

55.Cap often with a pronounced central bump; velar tissues without orange inclusions in KOH.
Agaricus comptuloides


56.Mature cap brown overall—densely covered with brown, fibrillose scales; odor reminiscent of almonds.
57

56.Mature cap whitish to brown overall, or whitish with a brown center—bald to finely fibrillose or scaly (especially over the center) but not densely scaly overall; odor reminiscent of almonds or not.
59


57.Found in spruce-fir forests in the Rocky Mountains from New Mexico to Alaska.
Agaricus julius

57.Found elsewhere.
58


58.Mature cap 7–13 cm across; known from Arizona, under oaks (and from eastern North American locations).

58.Mature cap 8–30 cm across; known from the West Coast.


59.Odor of sliced or crushed flesh reminiscent of almonds; most species whitish to yellowish overall (one species brown).
60

59.Odor not reminiscent of almonds (odor phenolic or not distinctive); most species brownish to brown, or whitish with a brown center (one species whitish without brown shades).
67


60.Cap 4–7 cm across, pinkish when young, becoming brown after a purplish brown stage; most spores 5.5µm long or shorter.
Agaricus kerriganii

60.Cap larger than above, whitish to yellowish; most spores 5.5 µm long or longer (one exception).
61


61.Cap surface bruising strongly yellow to orange when rubbed.
62

61.Cap surface bruising weakly yellow or not bruising.
64


62.Cap surface becoming patchy; known from coastal California and coastal Oregon; most spores 6.5 µm or shorter.
Agaricus albolutescens

62.Cap surface not becoming patchy; variously distributed; most spores 6.5 µm or longer.
63


63.Stature stocky; cap surface bald; montane.
Agaricus moronii

63.Stature not stocky; cap surface fibrillose; montane or coastal.
Agaricus summensis


64.Most spores under 6.5 µm long.
65

64.Most spores longer than 7 µm.
66


65.Cap generally bald or nearly so, white bruising weakly to moderately yellow; coastal.
Agaricus sylvicola

65.Cap often scurfy to scaly, whitish at first but soon yellowish to yellow; montane.
Agaricus mesocarpus


66.Cap finely appressed-scaly, developing a brownish center; often growing gregariously.
Agaricus didymus

66.Cap bald or finely fibrillose, evenly whitish to yellowish; often growing alone.
Agaricus gemellatus


67.Cap whitish, without brown or gray shades; montane; most spores 7.5 µm or longer.

67.Cap grayish to brownish or brown—or whitish with a brownish to brown center; ecology varying; most spores shorter than 7 µm long.
68


68.Cap grayish, bruising yellow promptly; coastal and central California.
Agaricus berryessae

68.Cap whitish to brownish or brown, or whitish with a brownish to brown center—bruising yellow or not; variously distributed.
69


69.Mature cap 8–18 cm across.
70

69.Mature cap 5–13 cm across.
72


70.Cap surface smooth, with innate, appressed fibrils; ring becoming detached from the stem.
Agaricus deardorffensis

70.Cap surface bald or fibrillose to fibrillose-scaly; ring not becoming detached from stem.
71


71.Cap usually dark brown, at least over the center; ring rubbery, with a thick, grooved outer edge; spores averaging 4 µm wide; coastal; fall and early winter.

71.Cap usually brownish to brown (occasionally dark brown); ring stiff, without a thick, grooved edge; spores averaging 3.5 µm wide; coastal and montane; early winter through spring.
Agaricus hondensis


72.Associated with conifers (pines, junipers, spruces) in the southern Rocky Mountains; most spores wider than 4.5 µm.

72.Associated with oaks or cypress from California to Arizona and Mexico; most spores under 4.5 µm wide.
73


73.Known from California and Mexico under oaks or cypress; cap (at least the center) becoming dark brown; flesh in stem not usually yellowing.
Agaricus californicus

73.Known from Arizona under oaks; cap becoming pale to medium brown; flesh in stem yellowing.
Agaricus arizonicus


74.Mature cap 2–5 cm across.
75

74.Mature cap usually larger than 5 cm across.
76


75.Cap brownish pink, darkening to pinkish brown with age.
Agaricus micromegethus

75.Cap whitish, becoming yellowish or slightly brownish.
Agaricus comtulus


76.Cap brown overall, fibrillose to scaly with brown fibrils and/or scales.
77

76.Cap whitish to brownish or yellowish overall—or whitish with a brown center; not markedly fibrillose or scaly.
81


77.Flesh in stem base yellowing strongly when sliced; odor strongly ammonia-like, iodine-like, or phenolic; known from New Mexico.
Agaricus iodosmus

77.Flesh in stem base not yellowing, or yellowing only slightly; odor reminiscent of almonds.
78


78.Fibrils and scales on cap surface medium to dark brown.
79

78.Fibrils and scales on cap surface tan to reddish brown, orangish brown, or yellow-brown.
80


79.Scales dark brown and well-separated (increasingly so towards the edge of the cap); ring flimsy; most spores under 7.5 µm long.
Agaricus perobscurus

79.Scales medium to dark brown; ring thick and sturdy; most spores more than 8 µm long.


80.Stem more or less bald; spores 5.5–8 x 4–5 µm; known in western North America from the West Coast in disturbed ground (compost piles, lawns).
Agaricus subrufescens

80.Stem scurfy to shaggy below the ring; spores 7–8.5 x 4.5–5 µm; in western North America known from Arizona under oaks.


81.Mature cap 3–9 cm across, developing a brown center; gills pale, then pink before turning brown; most spores shorter than 6.5 µm.; known from lawns in the Bay Area, and under oaks and cypress in southern California and Mexico.
Agaricus californicus

81.Mature cap 7.5–15 (+) cm across, usually white to yellowish; gills not passing through a pink stage; most spores longer than 7 µm; possibly widely distributed throughout western North America.
82


82.Cap usually remaining whitish throughout development; most spores longer than 9 µm.
Agaricus crocodilinus

82.Cap whitish, often becoming yellowish; most spores shorter than 8.5 µm.


83.Mature cap small (3–5 cm across).
84

83.Mature cap medium-sized to large (usually more than 5 cm across).
88


84.Growing in grass (lawns, meadows, and so on).
85

84.Growing in woods.
86


85.Cap brownish pink, darkening to pinkish brown with age.
Agaricus micromegethus

85.Cap whitish, becoming yellowish or slightly brownish.
Agaricus comtulus


86.Cap bright yellow to yellow orange over the center, with yellow scales elsewhere.

86.Cap brownish pink to brown.
87


87.Stem very slender (1–4 mm thick); gills pink before turning brown; velar elements partly orange in KOH.
Agaricus diminutivus

87.Stem not notably slender (4–6 mm thick); gills not passing through a pink stage; velar elements not orange in KOH.
Agaricus kerriganii


88.Growing in grass (lawns, meadows, and so on), or under planted trees in urban areas.
89

88.Growing in woods.
96


89.Cap whitish to yellow or yellowish—sometimes becoming slightly brownish with old age.
90

89.Cap brown, or whitish with a brown center and/or brown scales.
93


90.Growing in clusters; cap yellow when young, remaining yellow over the center.
Agaricus floridanus

90.Not typically growing in clusters; cap whitish to yellowish.
91


91.Cap often very large (up to 30 cm across); most spores more than 8.5 µm long.
Agaricus crocodilinus

91.Cap medium-sized to large (up to 15 cm across); most spores under 8.5 µm long.
92


92.Cap fairly bald, white or whitish; most spores under 7 µm long.
Agaricus arvensis

92.Cap usually becoming finely scaly or fibrillose, yellowish; most spores more than 7.5 µm long.


93.Brown shades dark brown; odor phenolic.
94

93.Brown shades tan, yellow-brown, orangish brown, or reddish brown; odor reminiscent of almonds.
95


94.Cap by maturity usually whitish with a brown center, very finely scaly or nearly bald; most spores 5–6 µm long; cheilocystidia clavate to subglobose, under 20 µm long.

94.Cap by maturity whitish to brown under prominent dark brown scales; most spores 5–7 µm long; some cheilocystidia sphaeropedunculate, 20–40 µm long.


95.Stem more or less bald; spores 5.5–8 x 4–5 µm; growing in disturbed ground (compost piles, lawns).
Agaricus subrufescens

95.Stem scurfy to shaggy below the ring; spores 7–8.5 x 4.5–5 µm; growing in urban areas under planted spruces.


96.Cap brown, or white to whitish with a brown center and/or brown scales.
97

96.Cap white to whitish overall—sometimes becoming slightly brownish with old age.
101


97.Cap yellowish-brown to orangish brown overall, densely covered with fine fibrillose scales; odor strong, reminiscent of almonds.

97.Cap with a brown to dark brown center but white to whitish elsewhere—or brown overall but if so not yellowish- to orangish-brown; odor phenolic or not distinctive.
98


98.Stature notably slender; mature stem usually about 1 cm thick.
99

98.Stature not stocky, but not notably slender; mature stem usually 1–2 cm thick.
100


99.Stem base featuring a cup-like, rimmed bulb; southeastern.

99.Stem base with or without a small, abrupt bulb, but the bulb not cuplike; southeastern and midwestern.


100.Cap often small (3–10 cm across); yellow stains from bruising sometimes becoming brownish orange.
Agaricus approximans

100.Cap medium-sized (5–15 cm across); yellow stains becoming brown.


101.Cap large (8–15 cm across); most spores longer than 8.5 µm; stem usually without a basal bulb.
Agaricus crocodilinus

101.Cap medium-sized (4–12 cm across); most spores shorter than 8.5 µm; stem with or without a basal bulb.
102


102.Stem usually club-shaped, without a prominent basal bulb.
Agaricus sylvicola

102.Stem with a prominent basal bulb.
103


103.Cap bruising strongly yellow to orangish yellow; known from red and white pine plantations in Wisconsin.
Agaricus diospyros

103.Cap not bruising, or bruising slightly to moderately yellow; variously distributed.
104


104.Spores 6–7 µm long; mature cap often becoming yellowish.

104.Spores 4.5–6 µm long; mature cap not usually becoming yellowish.
105


105.Odor reminiscent of almonds; cap surface bald or nearly so; dried specimens with a yellowish to orangish cast.

105.Odor reminiscent of phenol, or not distinctive; cap finely radially silky-fibrillose; dried specimens brown to grayish brown, without a yellowish to orangish cast.


quick links: red staining | yellow staining | not staining


106.Growing in grass (lawns, meadows, etc.) without association with trees—or growing in compost heaps or disturbed ground in urban settings. (If growing in urban grass near planted trees, try this choice first.)
107

106.Growing in association with trees in urban settings, growing in woods, or growing under Monterey cypress.
128


107.Cap a shade of pink or brown, at least over the center.
108

107.Cap white to buff, whitish, yellowish, or yellow.
115


108.Cap large, with reddish brown scales and fibrils; odor strong, reminiscent of almonds; growing in grass, compost heaps, or disturbed ground in urban areas.
Agaricus subrufescens

108.Cap varying; odor reminiscent of almonds or not; growing in grass.
109


109.Cap whitish with a brown center; surfaces yellow with KOH; known from California and Mexico.
Agaricus californicus

109.Cap pink to brownish or brown overall; surfaces yellow with KOH or not; variously distributed.
110


110.Cap small (2–4 cm across) and pinkish to pinkish brown; odor reminiscent of almonds; most spores under 5.5 µm long.
Agaricus micromegethus

110.Cap varying; odor not of almonds; most spores 5.5 µm or more long.
111


111.Found from the Great Plains eastward.
112

111.Found from the Rocky Mountains westward.
113


112.Cap with brown fibrils that become stretched out, creating fairly large scales; young cap fairly convex; stature of "meadow mushrooms"; ring if present usually fragile and torn.

112.Cap densely covered with tiny yellow-brown scales; young cap blocky; stature proportionally taller than meadow mushrooms; ring often persistent.


113.Cap essentially whitish underneath a sparse, radial covering of brown fibrils.
Agaricus argenteus
subsp. annetteae

113.Cap more brown than above (brown when young, becoming fibrillose and/or scaly with development).
114


114.Cap 4–6 cm across, grayish brown; stature of "meadow mushrooms"; ring if present usually fragile and torn.
Agaricus incultorum

114.Cap 8-10.5 cm across, reddish brown; stature taller than meadow mushrooms; ring well developed.
Agaricus depauperatus


115.Cap small (2–5 cm across); most spores less than 6 µm long.
Agaricus comtulus

115.Cap variously sized; most spores 6 µm long or longer.
116


116.Usually growing in hard-packed soil, often in gravelly places (roadsides, pathsides, etc.); stem tough, with a tightly sheathing ring that often forms bands; most spores 5.5–7 µm long; most basidia 2-spored.

116.Growing in grass; stem varying; spores varying; most basidia 4-spored.
117


117.Odor faintly salty, reminiscent of brine; spores often nearly round; cheilocystidia long and sometimes mucronate; known from Colorado and Pennsylvania.
Agaricus pilosporus

117.Odor, if distinct, not reminiscent of brine; spores ellipsoid; cheilocystidia, if present, varying.
118


118.Mature cap small (1–4 cm across), often with veil tatters along the edge; spores 6-7 µm long; known from Pennsylvania.
Agaricus argyropotamicus

118.Mature cap larger than above, with or without veil taters; spores varying; distribution varying.
119


119.Mushrooms medium sized to large (7–15+ cm at maturity); odor reminiscent of almonds; dried specimens usually developing orangish yellow colors; cheilocystidia usually catenulate.
120

119.Mushrooms medium sized (3–10 cm across at maturity); odor not reminiscent of almonds; dried specimens whitish to brownish or tan; cheilocystidia, if present, not usually catenulate.
122


120.Cap often very large (up to 30 cm across); most spores more than 8.5 µm long.
Agaricus crocodilinus

120.Cap medium-sized to large (up to 15 cm across); most spores under 8.5 µm long.
121


121.Cap fairly bald, white or whitish; most spores under 7 µm long.
Agaricus arvensis

121.Cap usually becoming finely scaly or fibrillose, yellowish; most spores more than 7.5 µm long.


122.Found from the Rocky Mountains westward.
123

122.Found from the Great Plains eastward.
126


123.Spores 6–7.5 µm wide (width, not length!); northern (Alaska) and possibly montane.
Agaricus aristocratus

123.Spores narrower than above (under 6 µm wide); variously distributed.
124


124.Cap whitish underneath a radial covering of brownish to brown fibrils.
Agaricus argenteus
subsp. annetteae

124.Cap without a brown-fibrillose covering.
125


125.Most spores under 6.5 µm long.

125.Most spores 6.5 µm long or longer.
Agaricus cf. campestris


126.Most spores under 6.5 µm long.

126.Most spores 6.5 µm long or longer.
127


127.Cap bald; most spores 8–8.5 µm long.

127.Cap becoming roughened; most spores 6.5–8 µm long.


128.Found from the Great Plains eastward.
129

128.Found from the Rocky Mountains westward.
138


129.Cap brown to brownish or brownish gray, at least over the center.
130

129.Cap white to yellowish overall.
134


130.Usually found under planted trees in urban locations.
131

130.Usually found in woods.
132


131.Mushroom usually large (cap 5–15 cm across); cap sometimes becoming slightly, finely scaly; cheilocystidia cylindric to clavate; found under conifers.
Agaricus subperonatus

131.Mushroom usually medium-sized (cap 4–10 cm across); cap soon developing prominent fibrillose scales; some cheilocystidia sphaeropedunculate; found under conifers or hardwoods.


132.Stem without a bulb at the base; mature cap about as wide as the stem is long; most spores longer than 7 µm.
Agaricus griseicephalus

132.Stem with a basal bulb; mushroom tall and slender (mature stem usually longer than the cap is wide); most spores shorter than 6.5 µm.
133


133.Cap white with a brown center; stem abruptly bulbous; distributed in the southeastern United States and the southern Appalachians.

133.Cap brownish to brown overall; stem gradually bulbous; known from Florida.
Agaricus rhoadsii


134.Odor faintly salty, reminiscent of brine; spores often nearly round; cheilocystidia long and sometimes mucronate.
Agaricus pilosporus

134.Odor, if distinct, not reminiscent of brine; spores ellipsoid; cheilocystidia, if present, varying.
135


135.Cap often very large (up to 30 cm across); most spores more than 8.5 µm long.
Agaricus crocodilinus

135.Cap small to medium-sized (under 10 cm across); most spores under 7.5 µm long.
136


136.Lower stem fibrillose and shaggy; young cap the color of buttermilk; most spores 5–6 µm long.
Agaricus butyreburneus

136.Lower stem not shaggy; young cap white (but sometimes becoming yellowish with age); spores varying.
137


137.Odor reminiscent of almonds; mature cap often becoming yellowish; most spores 6–7 µm long.

137.Odor reminiscent of phenol, or not distinctive; mature cap white; most spores 5–6 µm long.


138.Cap becoming yellowish to yellow; odor reminiscent of almonds; montane.
Agaricus mesocarpus

138.Cap not becoming yellow; odor varying; ecology varying.
139


139.Cap white to whitish or buff overall, sometimes becoming a little brownish with age.
140

139.Cap a shade of brown or orange.
150


140.Appearing in low-lying areas in coast redwood forests; sometimes growing in clusters; mature stem 8–30 cm long; spores 7–8 µm long.
Agaricus sequoiae

140.Ecology varying; not growing in clusters; stem varying; spores varying.
141


141.Known from the Denver area, under planted conifers; cap round-convex; veil leaving granular remnants along the cap edge and on the stem; odor not distinctive; spores 7–8 µm long.
Agaricus bernardiformis

141.Distribution varying; cap varying; veil not leaving granular remnants on cap and stem; odor varying; spores varying.
142


142.Known from the Santa Cruz, CA area, under coast live oak; cap becoming grayish with age; odor reminiscent of almonds; most spores 6.5–7 µm long.
Agaricus cruciquercorum

142.Distribution varying; cap not becoming grayish; odor varying; spores varying.
143


143.Associated with Monterey cypress; odor strongly briny and unpleasant (according to Kerrigan [2016], "[i]t may be the world's most malodorous agaric . . . I imagine a boiling tide pool full of old gym socks"); stem with a sheathing, sometimes volva-like ring.
Agaricus vinaceovirens

143.Associated with montane conifers; odor not as above; ring sheathing or not.
144


144.Found after spring snowmelt in the Sierra Nevada; cap small (2.5–4 cm across); stature somewhat stocky; most spores 6–6.5 µm long.
Agaricus stevensii

144.Ecology varying (but not Sierra Nevada snowmelt); cap medium-sized to large (4–18 cm across); stature not stocky; spores varying.
145


145.Odor reminiscent of almonds; dried specimens usually developing orangish yellow colors; cheilocystidia catenulate.
146

145.Odor not distinctive; dried specimens whitish to brownish; cheilocystidia, if present, not catenulate.
148


146.Cap bald or nearly so; stem proportionally thick (roughly one-third the width of the cap); most spores 7–8 µm long.
Agaricus gemellatus

146.Cap usually somewhat fibrillose or scaly; stem not proportionally thick; spores varying.
147


147.Young cap pointed-convex; mature stem 1–2 cm thick; spores 6–7 µm long.
Agaricus cuniculicola

147.Young cap rounded-convex; mature stem 3–4 cm thick; spores 7–8 µm long.
Agaricus didymus


148.Flesh notably firm and tough; stem 5–8 cm long, equal, with a sheathing ring; spores 6–7 µm long.
Agaricus devoniensis
subsp. bridghamii

148.Flesh not notably firm and tough; stem and ring varying; spores 7–9 µm long.
149


149.Stem usually tall and slender, with a fragile, ephemeral ring; most spores 5 µm wide or skinnier.
Agaricus cf. altipes

149.Stem not usually tall and slender; ring often fairly substantial; most spores 5 µm wide or wider.


150.Found after spring snowmelt in the Sierra Nevada; cap small (2.5–4 cm across); stature somewhat stocky; most spores 6–6.5 µm long.
Agaricus stevensii

150.Ecology varying (but not Sierra Nevada snowmelt); cap varying; stature not usually stocky; spores varying.
151


151.Cap tawny orange to brownish orange, becoming yellowish orange with age; stem proportionally long, with a basal bulb; under conifers in coastal northern California and southern Oregon; odor reminiscent of almonds; most spores 7.5–8.5 µm long.
Agaricus smithii

151.Cap without orange shades; stem varying; distribution and associations varying; odor varying; spores varying.
152


152.Species usually found under planted trees in urban areas, or under Monterey cypress.
153

152.Species usually found in woods.
157


153.Cap prominently scaly; odor reminiscent of almonds; spores 6.5–9.5 µm long.
154

153.Cap bald to appressed-fibrillose or minutely scaly with age; odor not reminiscent of almonds; spores 4–7 µm long.
155


154.Scales dark brown and well-separated (increasingly so towards the edge of the cap); ring flimsy; most spores under 7.5 µm long.
Agaricus perobscurus

154.Scales medium to dark brown; ring thick and sturdy; most spores more than 8 µm long.


155.Mushroom usually robust (more than 10 cm tall); in western North America known from the Denver area, under planted conifers.
Agaricus subperonatus

155.Mushroom small to medium-sized (5–9 cm high); known from coastal California and Mexico under Monterey cypress or oaks.
156


156.Cap pale brown; ring thin and fragile; odor not distinctive; most spores 4.5–5 µm long.
Agaricus cupressophilus

156.Cap dark brown, at least over the center; ring sturdy and thick; odor reminiscent of phenol; most spores 5.5–6 µm long.
Agaricus californicus


157.Odor reminiscent of almonds; dried specimens usually developing orangish yellow colors; cheilocystidia catenulate.
158

157.Odor, if distinctive, not reminiscent of almonds; dried specimens brownish to brown; cheilocystidia, if present, not catenulate.
162


158.Found on the West Coast.
159

158.Found in the Rocky Mountains.
160


159.Cap medium sized, finely fibrillose to more or less bald; associated with coast live oak.
Agaricus cruciquercorum

159.Cap large, scaly; associated with conifers.


160.Stem more or less smooth below the ring; cap yellowish brown to orangish brown; most spores 7–8 µm long.
Agaricus didymus

160.Stem shaggy below the ring; cap variously colored; most spores either shorter (6–6.5 µm) or longer (8–9 µm) than above.
161


161.Cap light brown; most spores 8–9 µm long.
Agaricus julius

161.Cap whitish with well-spaced tan scales when young, becoming tan to tawny with age; most spores 6–6.5 µm long.
Agaricus sandianus


162.Most spores longer than 7 µm; cap gray, grayish brown, or rusty brown.
163

162.Most spores shorter than 7 µm; cap brown to dark brown or purplish brown, at least over the center.
164


163.Mature cap 5–8 cm across, gray to grayish; known in western North America from Utah and New Mexico.
Agaricus griseicephalus

163.Mature cap 8–11 cm across, rusty brown; known from California.
Agaricus depauperatus


164.Known from high elevation spruce forests in New Mexico; stem fibrillose below the ring; most spores 6.5 µm long or longer.
Agaricus sipapuensis

164.Known from the West Coast; stem fibrillose or bald; most spores shorter than 6.5 µm.
165


165.Stem fibrillose-shaggy below the ring; cap usually remaining convex with maturity; cap green with KOH.
Agaricus subrutilescens

165.Stem more or less bald below the ring; cap usually becoming broadly convex or flat with maturity; cap yellow with KOH, or not documented (possibly green).
166


166.Often growing alone, under pines (ponderosa pine, Monterey pine, Bishop pine); ring fragile, often collapsing; cheilocystidia present.
Agaricus thiersii

166.Often growing gregariously, under hemlock, Douglas-fir, or coast redwood; ring substantial, rubbery; cheilocystidia absent.



References

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Kuo, M. (2018, April). The genus Agaricus. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/agaricus.html


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