Notes:
Collector’s_Name: Alden C. Dirks
Habitat: Mulched ground in urban setting underneath of shrubs
Substrate: Mulched ground
Taste_and_Odor: Phenolic (unpleasant) or slightly sweet or slightly “spermatic” (maybe just where KOH was applied)
Chemical_Reactions: 5% KOH yellow on surface and context of stipe and cap, turning brown-orange over time
Spore_Print: Brown
Macroscopic_Features: Larger mature mushroom – Pileus 14 cm diameter. Stem 13.5 cm long and 1.5 cm wide with a 2.0-2.5 cm wide basal bulb and persistent ring that is thick and rubbery looking like a gasket.

Smaller mature mushroom – Pileus 7.5 cm diameter. Stem hollow, 12 cm long and 1 cm wide with an abrupt basal bulb 1.5 cm wide. White rhizomorphs extend from the base.

Stem bruises brownish yellow. When cut, the stem context and especially the basal bulb turn yellow, but darken over time to brown orange, as well as the context of the cape above the attachment point of the stipe. KOH is positive on all surfaces turning bright yellow then darkening to brown orange.
Microscopic_Features: Spores ellipsoid, length (4.8) 5.2-6.0 (6.3) x width (3.2) 3.4-3.9 (4.0), Q (1.3) 1.5-1.6 (1.7), 21 spores measured.

Species Lists

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Proposed Names

28% (1)
Recognized by sight
47% (2)
Used references: Michael Kuo, “Agaricus approximans, which is usually a bit smaller (caps 3–10 cm across) and features yellow staining that becomes brownish orange.”

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Comments

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Thank you @rwkerrigan for the feedback!
By: Alden Dirks (aldendirks)
2018-09-30 17:41:54 PDT (-0700)
Could be.
By: Rick Kerrigan (rwkerrigan)
2018-09-30 06:05:44 PDT (-0700)

It’s very difficult to separate A. approximans and A. placomyces without an ITS DNA sequence. Also, A. approximans varies in its appearance. But side-by-side, A. approximans tends to be smaller. The points noted in your analysis are all correct. It’s just tough to be sure.

I am not confident that A. approximans is the correct determination.
By: Alden Dirks (aldendirks)
2018-08-28 12:02:22 PDT (-0700)

I have to assume that the 8 or so mushrooms growing scattered over a few feet in a mulched bed were the same species, although the large size of the largest mature mushroom is confusing.

The size of the mushroom generally fits Agaricus placomyces, but Mushroom Expert says that A. placomyces typically does not have a basal bulb, that the flesh stains bright yellow at the base, and nothing about reactions of KOH on the flesh.

This specimen had a somewhat abrupt basal bulb, did not stain bright yellow at the base but slowly dull yellow, reacted strongly yellow to KOH on all parts of the mushroom.

Finally, the staining reactions turned brownish orange over time, which seems to be a feature of Agaricus approximans. However, Michael Kuo reports that A. approximans is typically significantly smaller than largest cap of this observation. Microscopic features seem to overlap considerably between these different species.