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Agaricus Section Xanthodermatei

Good evening, friends,

This week's mushroom is actually a mystery species in a group of mushrooms known as Agaricus Section Xanthodermatei. These mushrooms (in the genus Agaricus) are distinguished by flesh that stains yellow when bruised/handled and a pungent odor. This mushroom was found in the ramble on 6/15/2021. You may notice this mushroom is in the same genus (Agaricus) as last week's pavement mushroom (A. bitorquis), so even just using pictures we can compare/contrast two different mushrooms in the same genus.


The most conspicuous species in Section Xanthodermatei is A. xanthodermus, and that very well could be the specimen we have here, but we'll look at the reasons why we can't confidently conclude this identification.


Agaricus Section Xanthodermatei

Fun Facts (I'll be the first to admit they're not the most fun this week)

To make sure we're all current on our Greek, the prefix "Xantho" means yellow and the suffix "dermus" comes from derma meaning skin.


iNaturalist recognizes 19 different species in the Section Xanthodermatei but there are 2 with thousands of more identifications than the others: A. californicus and A. xanthodermus. However, a 2019 journal article from Mycologia recognizes 21 different species in the section (Reference 1). As fungal research expands in conjunction with the prevalence of DNA sequencing we very well may see more species added to this section.


It is noted that the species A. xanthodermus is poisonous - consumption results in gastrointestinal distress but it's not fatal - and other members of the Sect. have been found to be inpallitable as well. Cooking A. xanthodermus produces a yellow color and phenolic smell which will taint your entire meal. Agaricus mushrooms aren't a beginner's mushroom for foraging, and it's best to leave any yellow-staining mushrooms where they lie.


Agaricus Section Xanthodermatei

Ecology

Mushrooms in Sect. Xanthodermatei can be found all over the world and grow throughout all seasons. For something more tangible, the common yellow-stainer (A. xanthodermus), grows summer through fall in eastern North America - but it is also found in Europe, Asia, Africa, and has been introduced in Australia. Most of the iNat observations are from the fall, whereas our specimen was found in mid-June, so that is one reason why I'd be hesitant to say this is in fact the yellow-stainer, A. xanthodermus.


Agaricus Section Xanthodermatei

All mushrooms in Sect. Xanthodermatei, and all Agaricus mushrooms in general, are saprobic. I believe our mushroom was growing from the dead root of a living oak (photo above). There was one other mushroom growing from the dead root which also contradicts the potential A. xanthodermus identification since the yellow-stainer usually fruits with several mushrooms in a cluster.


Distinct features of mushrooms in Sect. Xanthodermatei are that they all bruise yellow, especially near the base of the stipe (seen below), and have a phenolic smell - described as an unpleasant sweet aroma - that is most noticeable when the mushroom is young and fresh (like the specimen here).


Agaricus Section Xanthodermatei

One feature that is constant across all Agaricus mushrooms is that the gills (featured below) will start pale but turn pink and then brown as they age. This is due to the dark brown spores staining the light colored gills. This mushroom was rather young when I plucked it, but you can see they're starting to take on a pinkish beige hue. If you find a mushroom in the field with a prominent ring (annulus) on the stipe and gills of this hue then you can be almost certain you have an Agaricus. If it stains yellow when handled and has a phenolic odor then you have an Agaricus in Section Xanthodermatei.


Underside of Agaricus Section Xanthodermatei

We have the Lunar New Year tomorrow and Groundhog's Day on Tuesday - quite the festive week. I was curious to see if there are any traditional Chinese mushroom dishes that are served on the lunar new year and I found that xiang gu cai xin (香菇菜心) is a Shanghainese dish comprised of braised shiitake mushrooms and bok choy. Here's a link to a recipe.


Groundhog's Day is the halfway point between the winter solstice and the spring equinox which was news to me. The traditional Celtic holiday Imbolc - or St. Brigid's Day - celebrates this celestial occasion as well.


Okay that's that. I'm impressed if you made it this far. You ran into "Xanthodermatei" in the first sentence and still pushed through all these paragraphs. Typing it was just as intimidating. We’ve earned a new year’s treat.


Hope everyone enjoyed the snow,

Aubrey


References:

1) Kerrigan RW, Callac P, Guinberteau J, Challen MP, Parra LA. Agaricus section Xanthodermatei: a phylogenetic reconstruction with commentary on taxa. Mycologia. 2005 Nov-Dec;97(6):1292-315. doi: 10.3852/mycologia.97.6.1292. PMID: 16722221. Website: https://pubmed.ncbi.nlm.nih.gov/16722221/

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