Agaricus section xanthodermatei
(Agaricus xanthodermus, Agaricus malangelus)
Urban lawns, fields and parks along the front range.
Agaricus in section xanthodermatei are still being studied in Colorado. We always called these bad smelling, yellow stainer's Agaricus xanthodermus. A. xanthodermus is a similar mushroom in this section known from California and other states. 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. 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 this section and Agaricus section arvenses (also yellow staining but edible) but it takes some time to become familiar with them.
An ITS sequence that we ran in 2022 suggests that we found something undescribed that is more closely related to Agaricus malangelus (from New Mexico). See iNaturalist link for more info and also first picture. More collections should be analysed to see if we have several Agaricus in this section or just a single one floating around the front range.
Feb 11 2023 11:58 AM
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