(Agaricus xanthodermus, Agaricus sect. Xanthodermatei)
Urban lawns, fields and parks along the front range.
We always called these bad smelling, yellow stainer's Agaricus xanthodermus. A. xanthodermus is a similar mushroom in Agaricus section Xanthodermatei known from California and other states. I have sequenced two separate collections now matching Agaricus malangelus described from New Mexico. Although there could be others I believe our Colorado "xanthodermus" is actually Agaricus malangelus. These are all considered the "loose your lunch bunch". Beginners can easily confuse these for other edible Agaricus species such as A. campestris and A. arvensis as they are very similar in appearance.
They are characterized by having a phenolic or chalkboard smell. Smell however is subjective and some people cannot smell the difference. Crush a piece of the stem before smelling to get a stronger odor. The very base of the stem when nicked, cut or injured will stain bright yellow especially when fresh. Cut a mushroom lengthwise, wait and observe the base to see if its turns bright yellow. The cap is generally deformed marshmallow like, less circular and forms a darker depressed center. The stem will generally have a thick felt like ring. I've seen both shorter, stalkier and longer skinnier versions. That could be due to weather and environmental factors. I've also seen (forever ago) and heard of a rare yellow stainer from the mountains which is likely a different species in this section. The pictures shown here are from several different collections. Note the yellow staining bases on all of them. This is the tell tail sign. If you learn these basic characteristics it becomes easy to identify these poisonous Agaricus. In general if you do not know Agaricus all that well any yellow staining ones should be avoided. There are several differences between these and those Agaricus in section arvenses (also yellow staining but are generally considered edible) but it takes some time to become familiar with them.
Stains bright yellow in KOH.
Nov 12 2023 11:29 PM
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