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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), 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!

Spore print

Dried specimens in section Arvenses

KOH

Spores

Cheilocystidia