How to Identify Agaricus Mushrooms

Editor’s Note:

This is yet another post in a series I am writing about identifying wild mushrooms to genus. In previous posts, I addressed Amanita mushrooms and Tylopilus mushrooms, and in the future I plan to add more.

Agaricus arvensis
Agaricus arvensis, the so-called “Horse mushroom” is an edible mushroom, but there are at least 2 distinct species that go by the name “Agaricus arvensis.” Photo by Salix. Licensed under Creative Commons Attribution-ShareAlike 3.0 Unported.

Today’s fare is a tour of the Agaricus genus, which contains more than 300 species that grow all around the world. With a little practice, you can get very good at identifying Agaricus mushrooms because they’re easy to pin down to genus, and their species traits are well studied and rather easy to discern when you have a captive specimen to inspect.

So here’s to Agaricus mushrooms! Tally ho!

Yours In Fungal Fancy,

Mushroom Anna

Agaricus Mushrooms – Key Features

Before I delve into some of the taxonomical considerations and ecological traits of Agaricus, let’s run through the basics; there are a few key features that all Agaricus mushrooms (of which there are probably more than 300 or more) share that should help you tremendously in your efforts to ID them to genus.

  1.  Terrestrial and saprobic: Mushrooms in Agaricus grow from the ground and are not found sprouting off logs, trees, or stumps. These mushrooms are decomposers, and as such they digest compost and nutrients in soil to survive, rather than growing in association with plants or trees.
  2. Chocolate-brown gills and spores: When Agaricus mushrooms are mature, they have dark brown gills that are the color of a (nice) bar of chocolate, and their spores share this color. Spore prints collected from Agaricus mushrooms look like lined smudges of cocoa powder, and they usually drop an ample load of spores. When they are young, the gills of Agaricus mushrooms are pallid, pink, or even white, but as they grow and develop, this changes.

    Agaricus campestris
    Agaricus campestris, AKA the meadow mushroom or pink bottom, has pleasant pink gills when young, which eventually turn dark brown. Photo by Andreas Kunze. Licensed under Creative Commons Attribution-ShareAlike 3.0 Unported.
  3. Ring on the stem: Agaricus mushrooms have an annulus (ring) on the stem. The ring is formed by a partial veil of tissue that protects the mushroom’s gills when it is young, and when the cap expands to expose the gills, this partial veil bursts and leaves behind a ring. For this reason, young Agaricus mushrooms may have a fully intact partial veil covering the gills. One of my favorite things is to find an Agaricus that’s in between these two states.
  4. Cap not brightly colored: As anyone who’s hunted mushrooms before is aware, some mushrooms are positively lurid: red, blue, purple, the list of possible mushroom colors is nearly endless. Not so with Agaricus mushrooms; most of them are some version of pale or brown.
  5. Clean-breaking stem: Agaricus mushroom stems are a little frail, and they break cleanly away from the cap of the mushroom in most instances. However, it’s worth noting that the stems of these mushrooms can be a bit on the chalky side, but they do not snap like chalk, as with Russula mushrooms. In fact, if you start to noodle around with an Agaricus, you will often find that you can strip pieces of the stem off without difficulty, which would be all-around impossible with the chunky, thick, and chalky flesh of Russula species.
  6. Gills attached or barely free from the stem: Unlike mushrooms with decurrent gills running down the stem, Agaricus species have gills that attach to the top of the stem faintly. They can also be free from the stem, in which case you will see a small ring of empty space between the top of the stem and the radially aligned mushroom gills.

Agaricus Mushroom Overview

Agaricus species are common all around the United States, and there are some nice edible mushrooms in the genus, as well as some poisonous species. The main things to consider when attempting to identify these mushrooms, above and beyond the features listed above, are whether the mushroom stains when it is bruised, cut, or handled and also the aroma of the mushroom.

Agaricus and Aroma

One of the features that helps with identification of these species is the aroma of the mushroom itself; generally speaking, there are three sorts of Agaricus aromas: bland or indistinct, sweet (often like almonds), or foul (like library paste, phenol, or otherwise “icky chemically”). In general, those Agaricus species that are sweet-smelling are considered good to eat, and those that smell crappy, surprise surprise, are not on anyone’s list of great edibles.

Agaricus sylvaticus
Agaricus sylvaticus, AKA the pinewood mushroom, is an edible species that stains reddish when cut. Photo by Jersy Opiola. Licensed under Creative Commons Attribution-ShareAlike 3.0 Unported.

If you’re trying to identify the scent of Agaricus mushrooms, in my experience these aromas are fairly strong with fresh specimens. Crushing some of the stem and then taking a good whiff will usually tell you what you need to know. There are some mushroom hunters who cannot detect the phenolic, unpleasant smell of Agaricus, and I do wonder if there are some species/geographical areas where the phenolic smell is less pronounced in the local Agaricus population.

Personally, I find that phenolic-smelling Agaricus mushrooms to be very pungent and in-your-face, especially with noxious species such as Agaricus xanthodermus, which used to grow in large fairy rings all around my yard in Oregon years ago when I first started studying mushrooms.

Bruising Reactions of Agaricus Species

The bruising reactions of certain Agaricus mushrooms are pretty profound, but others are a little less pronounced. In general, bruising Agaricus species turn reddish, yellow, or dark brown when cut or crushed. In most cases, rubbing the edge of the cap will reveal whether you have a yellow-staining Agaricus, although some of them only turn yellow at the base of the stem, and so I usually cut the base of the stem up a bit and also rub the cap in order to look for this reaction. When it comes to reddish-staining Agaricus mushrooms, they usually bruise when sliced, so it’s important to hack into the cap of the mushroom in some cases to see if your specimen has this trait.

Unlike Boletus mushrooms and allied genera, which often have quickly-staining and radical coloration, the bruising of Agaricus mushrooms may seem tame and a little less than eye-poppingly exciting, but nonetheless these bruises and stains help a lot with identification, and they are rarely slow or faint, so if you have a bruising Agaricus, you will usually see the flush of color without difficulty. I make this distinction because some “staining” reactions of Boletus species can be so slow, so faint, and so frustratingly hard to notice that I typically wait an hour or more after handling an unknown bolete-type mushroom to decide whether or not it’s a stainer. Agaricus species simply do not have this problem, and whatever chemical reaction takes place that causes the bruising is relatively immediate when measured up against the slow-acting oxidization of some staining boletes.

Agaricus Lookalikes

In the grand scheme of things, I would rate Agaricus as an easy genus to identify; their brown gills, ringed stems, and sort of bland coloration sets them apart from a lot of the mushrooms in this fine and wild world of ours. However, there are a few genera that I think are worth mentioning that may be mistaken for Agaricus, especially by those who are not terribly familiar with the gestalt of different mushroom genera.

Chlorophyllum molybdites
Chlorophyllum molybdites, the green-spored parasol mushroom, is a poisonous mushroom with white gills that turn greenish in age. Photo by Byrain. Licensed under Creative Commons Attribution-ShareAlike 3.0 Unported.

Lepiota and Chlorophyllum Mushrooms

Although they are distinct genera, Lepiota and Chlorophyllum mushrooms look quite alike in the field, and some of them share the same spaces as Agaricus mushrooms; namely they have rings on the stalk, are drab in color on average, and sometimes have scaly caps that are a common feature with some Agaricus species. Also, a few of the mushrooms in Lepiota and Chlorophyllum stain mulberry, saffron, or dark reddish when cut or bruised, which could perhaps confuse you, given that some Agaricus mushrooms have similar staining reactions. For example, the shaggy parasol mushroom, Chlorophyllum rhacodesstains a cheerful orange-red when cut, and the green-spored parasol, Chlorophyllum molybdites, sometimes stains a darker crimson color when it’s sliced.

It’s worth noting as well that there are some quite poisonous Lepiota mushrooms, and many and more that have the ominous “edibility unknown” label. These mushrooms tend to be a bit on the drab side on average, ranging from white to buff and brown, and many of them have little scaly bits on their caps that look like the shaggier members of the Agaricus genus.

Fortunately for mushroom hunters, Agaricus mushrooms and Lepiota and Chlorophyllum species can be told apart by gill color. Whereas Agaricus species have chocolate-brown gills in age, these two other genera have white gills, although over time those white gills my become stained by the spores. For example, the toxic (but not deadly) green-spored parasol mushroom (Chlorophyllum molybdites) starts out with lily-white gills that turn greenish-yellow as the mushroom matures.

The bottom line here is to be cautious when collecting young mushrooms. When they are newborns, Agaricus mushrooms have white gills, particularly when the partial veil is still intact and covering the gills of the baby mushroom. As the partial veil breaks and forms a ring on the stem, Agaricus mushroom gills turn pink and then brown. This issue makes it all the more important to collect several specimens of a mushroom if at all possible, because having perspective on the mushroom’s features during its life cycle can give you a lot of clues about its identity.

Lepiota brunneoincarnata
Lepiota brunneoincarnata is a deadly mushroom that, from the top, looks like some Agaricus species, but its white gills set it apart from the Agaricus genus. Photo by Strobilomyces. Licensed under Creative Commons Attribution-ShareAlike 3.0 Unported.

Cortinarius Mushrooms

Cortinarius is a large mushroom genus that comprised of several hundred species, and these mushrooms have brown gills and a webby ring on the stalk. Cortinarius mushrooms come in all sorts of colors and sizes and many of them are brownish or rather plain in appearance, and their classic cap-and-stem construction might make one tempted to confuse them with Agaricus. 

Cortinarius was once considered a safe genus overall from an edibility perspective, but it’s come to light that there are a few of them that are deadly poisonous (including Cortinarius rubellus and Cortinarius orellanus), so the prevailing wisdom is to not eat any of them. For my part, I will never eat Cortinarius mushrooms of any sort, and I encourage others to embrace this paradigm, especially because this genus has a lot of X factors and species that may not yet be clearly delineated, which means the chances that we will discover new ones that are mildly to extremely toxic is enough of a risk that I only collect them for identification purposes.

Fortunately, with a little experience, Agaricus and Cortinarius species are easy to distinguish. Firstly, the gill coloration is distinct; while Cortinarius species have rusty-brown spores that tend to make their gills a similar, light-brown and sometimes red clay color, Agaricus goes to full chocolate brown. Secondly, the ring on the stem is different; Cortinarius mushrooms have a partial veil that’s made up of fibrous, webby material that leaves a sort of cobwebbed ring on the stem. Agaricus mushrooms, on the other hand, have a fleshy partial veil that leaves a ring of tissue on the stem that’s easily separated from the fruiting body. Finally, Cortinarius mushrooms tend to be more colorful than Agaricus species. Although this is not a universal trait and there are plenty of drab brown Corts, this genus has numerous species that are flamboyantly orange, reddish, greenish, and purple, whereas Agaricus mushrooms are white, buff, tan, or brown in color.

Cortinarius caerulescens
This specimen of Cortinarius caerulescens shows the webby consistency of the partial veils of Cortinarius mushrooms. Photo by Jerzy Opioła. Licensed under Creative Commons Attribution-ShareAlike 4.0 International.

With a bit of practice, these two genera really are apples and oranges, but given their descriptions in field guides, a novice forager might be challenged to tell the differences between them.

Amanita Mushrooms

OK, if you’ve spent much time at all studying mushrooms, you probably know what an Amanita looks like, but it’s always worth mentioning, especially when we’re discussing classic “cap and stem” dealie-whoppers that grow on the ground. Amanita is a large genus of mushrooms that almost always have white gills, which right off the bat should keep you safe from confusing Agaricus with Amanita mushrooms. However, they both have rings on the stem that are often fleshy and brittle stems, and furthermore, there are loads of Amanitas (including deadly ones like Amanita phalloides and Amanita bisporigera) that are white, pale, or buff-colored.

The main thing to look for, however, is the gill color; almost all Amanita species have snowy-white gills, and those that have colored gills do not have brown gills. Secondly, the majority of Amanita mushrooms have a two-part protection system in place while the mushroom grows, a partial veil that leaves a ring on the stem, and a universal veil that is like an egg that covers the entire mushroom when it’s young. As the mushroom grows, the universal veil bursts and leaves a volva or sac or cup of tissue at the base of the stem.

There are different sorts of volvas for different groups of Amanitas, but one way or the other, if you find a mushroom with tissue remnants at the base of the stem, a bulb, or some other enlargement or hanger-on, it’s quite possible that your mushroom had a universal veil when it was little. Fortunately, Agaricus mushrooms lack universal veils and so even though they have a ring on the stem like Amanitas, they do not have any remnants of tissue at the base of the stem.

Agaricus xanthodermus
Agaricus xanthodermus is a chemical-scented mushroom that stains yellow at the base of the stem. Photo by Velella. Licensed under Attribution-ShareAlike 3.0 Unported.

There are other differences between Amanita and Agaricus mushrooms, but those are the broad strokes: gill color and universal veil remnants are quite different. I would not consider Amanita to be an actual lookalike genus for Agaricus, but it’s always worth mentioning these mushrooms due to the fact that some of them are deadly poisonous. For more information on how to identify Amanita mushrooms to genus, check out my post on that topic from a few weeks ago.

Stay Tuned for Specific Agaricus Species Descriptions!

In these next couple weeks, I will assemble a few posts about Agaricus species that you should add to your edible and choice list, especially for mushroom hunters who share my hunting grounds in the North Carolina Piedmont. In the meantime, if you want to try an Agaricus mushroom and simply cannot wait for more content on this blog, go down to your local grocery store and pick up a few portobellos or white button mushrooms; these are Agaricus bisporus, the world’s most commonly cultivated mushroom, and their flavor profile, texture, and culinary performance is similar to the wild Agaricus species that I enjoy so much.

3 thoughts on “How to Identify Agaricus Mushrooms”

  1. Hi, i live in Argentina and i’ve recently been “bit” with the Mushroom hunting bug, the first two Mushrooms i’ve learned are Agaricus vs. Amanita. As luck would have it, i found some Agaricus on my first hunt today, and i need some help identifying to which of the 300 species it belongs. I read your “How To Identify Agaricus Mushrooms” and am now !00% sure these are Agaricus but i want to make sure i have an edible Species. the Cape/Stems don’t really bruise, it’s been hours now and only in a few places have they turned Yellow/Yellowish, but for the most part no color change was noted. The Smell after crushing/cutting the stem was your “typical” Mushroom smell, it didn’t smell offensive to me. Aside from these two markers are there any other distinctive Morphological Characteristics that would help me determine whether these are edible or not?
    ps: it would be nice if i could upload pictures, is there a way?

    1. Unfortunately, I don’t have the right plug-in for WordPress to allow comment photographs. The main issue for you is that there are a lot of neutral smelling Agaricus mushrooms that have not been solidly identified; my local field guide has several that are “edibility unknown,” which of course is a huge pain in the butt. What I would do, were I trying to resolve this issue, is to post photos and field notes to Mushroom Observer and some Facebook groups (the Mushroom Identification Facebook group has a lot of very active and experienced members who might be able to help). As for my own experience with Agaricus, I don’t typically eat any of them unless they smell sweet like almond extract, because those are the ones that people unilaterally agree on and have lots of experience with from an edibility standpoint. Sorry I can’t be of better help than that; I really do think your best bet is to post to some Facebook ID groups and see if you can get some consensus. Good luck!

Ask a question or share your mushroom story!