Identifying a field mushroom
Disclaimer : These notes are provided as a guide only. While every attempt has been made to try to assist in the identification, the risk of eating any wild mushroom rests with the individual and I do not accept any responsibility for consequences that may arise from the action of anyone eating wild mushrooms. See also inedibles and lookalikes
The fungus that we know in Australia as a field mushroom is a member of the genus Agaricus. This is a large genus with a single ancestor (monophyletic) and within the genus is a number of sections each containing a range of species. While it is tempting to try to assign a species name to any mushroom that you might find, this can be difficult in Australia because many of the species are undescribed or if they are described they aren’t reported in popular guide books. In addition, it might require the use of a microscope and other detailed analysis. For our purposes though, it isn’t important to have a name; edibility can be determined by features that can be simply observed. Our main objectives are:
make sure we have an Agaricus
Make sure it is not in the section Xanthodermatei
Examples of some well known members of the Agaricus genus are:
Agaricus bisporus – the classic small supermarket mushroom
Agaricus bitorquis – marketed as a larger form of supermarket mushroom
Agaricus arvensis – the almond mushroom or horse mushroom.
Agaricus campestris – the classic if oft mis-identified field mushroom.
This list of features has been put together to assist the average person to identify an edible field mushroom.
1. Cap colour and texture
The cap of the edible Agaricus species varies from white though dun and on to a slightly pinkish colour in species like A. sylvaticus. The cap may be slightly scaley, and may be cracked. It is always dry and is never slimy to the touch. Both the colour and texture of the cap are influenced by the environmental conditions as well as the genetics.
Any mushroom with any hint of green in the cap colour should be rejected as this is the colour of the deadly Amanita phalloides.
top of small field mushroom, typical of those found in lawns (Dave Freer)
top of large field mushroom, similar to supermarket field mushroom (Dave Freer)
top of Agaricus arvenis, showing scales
A forest mushroom, showing red tones on the top
Top of Agaricus bitorquis. A clean off-white, with undulations
A horse mushroom from the Riverina district of NSW. Note scales.
A mushroom from a backyard in Penhurst, Victoria, showing some radiating spots.
A mushroom from Digby in Victoria, showing red/brown central region and radiating scales.
2. Gill colour
The gill colour may vary from brown to pink or off-white in the young mushroom, but it will always darken to a dark brown in a mature specimen. ‘Not black, but dark brown. Never eat a ‘field’ mushroom with white gills. This eliminates the poisonous Amanita and Chlorophyllum molybdites.
Pink colour of immature specimen of a small field mushroom. Picture courtesy of Dave Freer.
3. Spore print
The spore print is always dark brown. Not pink, not rusty, not black or purple or white. Dark brown only.
How do we take a spore print? Easy. Place the mushroom, or a piece of it, on a piece of waxed paper and place a glass over the top, with the edge of the jar just propped up by a matchstick or something similar to allow water vapour to escape. Place in a position away from draughts, overnight.
A simple setup for taking spore print
A spore print of an Agaricus species
4. The stem snaps away from the cap
The stem of an Agaricus has a texture that comprises a bundle of stringy cells running axially. The cap has a different texture. At the point of the junction of these two textures, there is region where the two will break apart cleanly. Try this for yourself with a supermarket mushroom. Note in this mushroom the dark gill colour.
The point of separation should be between the top of the stem and the flesh of the cap. In some species the stem appears to break away cleanly, but close inspection will show that there is a piece of the flesh from the cap attached to the stem and the position of the separation is actually between the surface of the cap and the flesh of the cap.
This test serves to separate Agaricus from members of the family Cortinaraceae, such as Hebeloma, Inocybe, Cortinarius and Galerina, some of which are seriously poisonous. It does not separate it from Amanita though.
Field mushrooms have a distinctive smell that is either ‘mushroomy’ due to a chemical called octenal, or almond/aniseed due to the presence of benzyl alcohol and benzaldehyde.
If the mushroom has a smell of phenol, which is the smell of India ink, or phenyl disinfectant, coal tar soap, creosote or sometimes described as ‘chemical’, then it should be rejected. Another common product that has the phenol smell is wheelie bin cleaner. It contains cresols, which are related and smell the same. Interestingly, and I don’t know why, at high dilutions wheelie bin cleaner smells like Clag glue.
If in doubt, there are two approaches you can take; 1) put the mushroom in a plastic bag for 15 minutes and then sniff the contents or 2) heat a piece of the suspect mushroom in a microwave for a minute. The bad smell will become more apparent if there is phenol present as will the almond smell. If you can’t reliably and comfortably identify the smell, preferably with confirmation from someone else, then you should reject the mushroom.
It is often reported that some people can tolerate eating mushrooms that contain phenol. The only stories I have heard of such poisonings have involved the whole group of consumers. I suspect that the truth is that some people report eating yellow staining mushrooms without ill effect, but they have in fact consumed one of the Arvenses group rather than one that contains phenol.
Why is phenol a problem and benzaldehyde isn’t? Because phenol causes acute irritation of the gastrointestinal tract. This can cause distress and vomiting, but it will pass and will not leave any permanent damage. Benzaldehyde is a natural product that is a component of almond essence that is used in making marzipan and is without any toxic effects at the doses involved in mushroom consumption. Similarly, benzyl alcohol has low toxicity.
Some poisonous species contain hydroquinone as well as phenol. This too can also cause gastric upsets. In these ones, both the phenol smell and the yellow colour are not as intense. The smell should be determined on a fresh specimen at the base of the stem.
6. Colour of cut or bruised flesh
The colour of the cut or bruised flesh may be brown or red or yellow, or there may be no change in colour at all. Here for example is an edible mushroom, Agaricus bitorquis, which is showing red on a cut piece. Photo courtesy of Dave Freer.
Brown or red bruising is usually ok but not an infallible indicator. A yellow colour may be ok or it may indicate Agaricus xanthodermis, which will cause stomach upsets. There is much confusion about this. The yellow colour is an indicator of A. xanthodermus, which contains phenol, but it does not necessarily indicate an indedible mushroom. For a diagnosis of an indedible yellow staining mushroom, one needs to have the yellow stain in combination with a phenol smell, as mentioned above. Another term for yellow staining is ‘flavescent’.
The place to determine both the colour and the smell is the base of the stem. Both features are less prominent in other regions of the mushroom. Don’t be shy, really squash it to get the smell.
Another feature of the yellow staining inedible species is that the yellow colour changes to brown over about an hour. The yellow colour also intensifies with cooking. Another test is that the colour intensifies and stays permanent in response to a drop of a 10% solution of potassium hydroxide or the more easily obtained sodium hydroxide which does the same thing. Here is a picture of a mushroom that has been tested with sodium hydroxide.
Field mushrooms will always be found growing from some kind of soil. They never grow directly from wood and they do not grow in the middle of cow pats. There are some deadly species like Galerina that grow from wood and if it is growing from a cow pat, there is a good chance that it is the notorious hallucinogenic ‘gold top’, on the Australian east coast at least. They do not tend to grow from wood chip or bark mulch either.
Don’t pick and eat mushrooms that grow beside highways or other places where they may have accumulated things like heavy metals or other potentially toxic things. Mushrooms can be quite good at gathering these things.
8. Cap shape
Mushrooms of the Section Xanthodermatei, the yellow-staining, phenol containing species tend to have a flat top when juvenile, sometimes continuing to when they are fully grown. They are often described as having a ‘boxy’ shape.
This, however is not exclusive to this section. There are many other species that have a similar flat top, and the classic example is Agaricus augustus, known in the US in particular as The Prince, and highly prized as an edible. So the flat top has limited value as a diagnostic tool. Sometimes people declare mushrooms to be yellow strainers and therefore inedible based on shape alone without even testing for a yellow stain and smell. I suggest taking a more thorough approach as outlined above.
If you are eating an Agaricus that you have never eaten before, try out a small piece first. Sometimes we can be allergic to mushrooms for no apparent reason. If it tastes horrible, don’t eat it! This has happened to me with something I expected to taste good. If you have not experienced any ill effects by the next day, then you can move forward to eating a larger quantity.
Whatever you do, don’t gulp down a huge meal of something that you are unfamiliar with.
10. One final thing
If, after reading all of the above, you are still not sure, then there is an old mushroom gatherers maxim that applies: