Agaricus is a distinctive genus, with free gills, dark chocolate spores and often a robust, stocky stature. They always have a partial veil that leaves at least some trace of a ring around the stem after the cap expands. They have dry caps, often fibrillose or scaly. The spores will sometimes turn to pink and then dark chocolate, so you may find specimens of intermediate age with pink gills. But the spores will never be discharged from the gills until they are mature and dark chocolate, so if you take a spore print, you will never be fooled. Most of them are >5cm across (with some up to 20-30cm). A few small species are <5cm, but they are still stocky with a thicker stem than most other mushrooms of that size (5mm or more thick). They are all saprophytic.
Agaricus flesh may stain yellow or reddish when cut, in
varying amounts. Some also have a pleasant sweet almond odor (like almond
latte syrup) that some think smells more like black licorice. It is very
interesting to eat a mushroom with a sweet component to it (I like them on
pineapple pizza where there is already a sweet/
Agaricus' closest relatives are Lepiota (which have lost their spore pigment and have white spores), the Coprinus inky cap and a whole surprising array of non-gilled mushrooms that do not seem related at all. Bird's nests and puffballs are more closely related to Agaricus than Agaricus is to any other gilled mushroom. There are also gastroid (semi-truffle) forms of Agaricus, but that is true of many groups of mushrooms.
Key to Agaricus sections: crush the stem base to detect all odors. Cut in half if you still don't see a staining reaction.
Arvenses - these species may stain slightly yellow when you cut them in half. They have a pleasant sweet almond odor, best detected by crushing and smelling the base of the stem. KOH yellow.
A. fissuratus ('arvensis') - "horse" mushroom - found in grass. White. Thick veil. Slight yellow staining. Compare A. 'campestris'. ~10cm.
A. arvensis (nivescens/
A. crocodilinus - huge (<40cm), cap may crack like crocodile skin. Usually in grass, was common 30 years ago.
A. macrocarpus - with bulbous stem base, northern species. Usually in forests?
A. albolutescens - in coastal forests. Whitish. Squat, strong yellowing and almond scent. Up to 15cm.
A. mesocarpus - eventually yellows.
A. sandianus - cap gets many brown scales.
Minores - related to the Arvenses, these are our smallest Agaricus. Often <5cm, but still somewhat stocky. Also yellowing (or turning orange) with an almond scent. They are difficult to tell apart. KOH yellow.
A. kerriganii ('semotus') - in forests, turns yellow-orange. Up to 7cm (largest small species) may have a bulb and rhizomorphs.
A. diminutivus - slender in forests, also staining, sometimes purple tinged.
A. comptuloides (friesianus?) - with a more pronounced umbo.
A. 'purpurellus' - purple capped specimens may not represent distinct species from others found here.
Agaricus and Subrutilescentes - no colour staining, little or no odor. KOH- (Agaricus) or green (Subrutilescentes).
A. 'campestris' - in grass, white, like Arvenses grp but weak ring and tapered stem. Margin of cap overhanging. ~10cm. /Agaricus
A. 'altipes' - longer stem, in forests.
A. 'chionodermus' - slight yellowing, with a persistent ring in forests.
A. porphyrocephalus - related, similar grass species, staining reddish. Either smooth white (var. pallidus) or brown fibrillose cap (shown).
Xanthodermati - possibly poisonous species that stain yellow (possibly slowly and usually then browning somewhat) and have the smell of creosote, phenol, glue, formaldehyde or bad science lab when you crush the stem base. KOH yellow.
A. hondensis - red-brown scales of varying amounts, not staining yellow. Stocky, often >10cm, felty ring.
A. subrufescentoides - always red-brown and even scalier, not as stocky (<10cm) and ring not felty.
Bivelares/Chitoniodes - these species may stain red in places when cut or rubbed. They have no odor in common. The veil is often sheathing, meaning that instead of hanging down like a skirt from above, it looks upside down with the veil flaring up and out from below. KOH-. Agaricus bisporus, the store bought button mushroom, is such a species. The portobello is also the same species, it is just bred to be larger. Check them out at the grocery store next time, and you may actually see the shape of the veil and a slight red staining before you get kicked out of the produce section, but these features are subtle. KOH-.
A. bitorquis - white cap, very squat, tapering stem, thick sheathing veil, in hard packed, disturbed soil. Little red staining. 5-20cm or so. /Bivelares
A. bisporus - sometimes found in the wild in compost or gardens. White or brownish cap, not as thick, tapered nor with as developed a veil. Little red staining. 5-15cm or so. /Bivelares
A. bernardii - like A. bitorquis but found in sand with a salty odor and taste. Flesh does turn red. 5-15cm or so. /Chitonioides
A. porphyrocephalus - related to A. 'campestris' (in grass, little ring, etc.) but staining reddish. Either smooth white or brown fibrillose cap.
Sanguinolenti - these red stainers stain more strongly then the Bivelares/Chitoniodes, even stronger (and more quickly) than A. bernardii. They are usually more slender than stocky. None are very common in the Pacific Northwest, so unfortunately not a lot is known about the species to be found here. KOH-.
A. thujae - brown scaly capped group. Slender with lower stem scales. With red cedar.
A. haemorrhoidarius var fumosus - sturdier with very dark cap and smooth lower stem.
A. benesii - sturdier with scaly lower stem. With spruce. Not white capped like in Europe. The California white capped species needs a new name.
|Bill Isaacs' 1963 UW Master's Thesis noted a bunch of
new species that have never been validly published, nor in many cases
confirmed, but specimens matching their description should be looked out
for so they can be studied better. More information can be found in
Rick Kerrigan's new book, Agaricus of North America is a great comprehensive treaty on all that is understood so far about this genus in North America.