Probably Agaricus subrutilescens
or Agaricus augustus

Joe posted on Mushroom Observer

Notes: Agarricus but what species? I was going to guess A. subrutilescens,
but then noticed that cap flesh stains yellow near cuticle when cut.
Both Arora and Trudell & Ammirati say that A. subrutilescens,is non-staining. In duff (and possibly bark dust). Nearest tree was a spruce.
I do not have a mature specimen, so cannot tell real maximum sizes and other characters.

Cap
• appears brown from a distance, but actually white, with abundant brown fibrils
• max diameter 6.5 cm
• mild almond odor and taste
• flesh white, 1 cm thick
• cap flesh stains yellow near cuticle when cut. See 3rd photo.

Gills
• free
• close
• young gills off-white
• 6mm

Stipe
• shaggy white starting at partial veil downward for about 1/2 length of stipe, then brown fibrils
• almost all the stipe was below the duff; only the cap was showing. See 2nd photo
• 17.5 cm long
• 2.5 cm thick
• slightly bulbous base
• almond taste
• hollow
• membranous partial veil still present in all specimens, so I cannot be certain what annulus will look like. But by how it’s attached to the stipe, I’m guessing it will be skirt-like
• stipe does not stain on cutting or bruising

Dick said: Have you ruled out A. augustus or A. subrufescens?--

Joe said

• Thank you for the response and the suggestions.
• I had not ruled out either A. augustus or A. subrufescens, but did think they were less likely because:
• A. augustus: My caps lack the scales which I see in all the guidebook photos. (I am relying on the guidebooks because I have not previously identified anything in this genus, except A. xanthodermus.)
• A. subrufescens: Arora says this has a strong odor and taste. Mind have a mild odor and taste. Arora also says (photo caption, top of p.337) this "is a rare ... species." (Obviously this doesn't rule it out, but does make it less probable.)
• But now on reexamination, (including looking at Matchmaker), A. augustus seems more likely:
• Most everything seems like A. agustus. As to lack of scales on the cap as shown in photos, Arora says "fibrils or fibrillose scales". Stem could best be described as rooting; my largest specimen had about 3 cm of cap above ground, with about 17 cm of stem below the duff surface. My smaller specimens had similar proportions. Matchmaker says the flesh "sometimes becoming yellow near cuticle after exposure".
• In addition to what's in Arora. Matchmaker says that A. subrufescens has a "strong nutty" taste. Mine is mild and almond. Matchmaker says "A. subrufescens is usually rather stocky ...." Mine are not stocky. However my stipe could be described as a gradual bulb, which fits subrefescens, and Matchmaker says flesh "white, unchanging or very faintly yellowish near top of stem ... (Kerrigan)."
• Matchmaker says A. subrutilescens odor is "mild or slightly fruity (Arora), fruity / spicy after exposure (Kerrigan)". Mine is almond.
• I will rummage around to see if I can find some KOH, as it seems that cap staining color with KOH would differentiate (a) subrutilescens from (b) augustus and subrufescens.
• Do you have any other suggestions about what to give attention to in order to pin this down?

Dick said: Sometimes we just have to accept that we can't tell for sure between closely related species. The microscope can often help us out but not always. I quite often say it's probably "this" because the variability factor is always in play. Especially with Agaricus.
I would agree that A. subrufescens seems unlikely but the cap color does match pretty well. I would also agree that A. augustus seems the most likely. Your photo of the lone cap does seem to indicate a little purple color but the group shot does not. I would definitely expect to see some purplish tones if it were A. subrutilescens. Sometimes you do not see the scales until the cap has expanded.
Contact Judy Roger for some KOH. I'm going to get some from her in the next couple of days and could pick some up for you also

John said: One idea to think about is that Arora describes subrufuscens as common in S. California, rare elsewhere