Agaricus arvensis Schaeff. - Horse Mushroom

Phylum: Basidiomycota - Class: Agaricomycetes - Order: Agaricales - Family: Agaricaceae

Agaricus arvensis - Horse Mushroom

Roadside verges and permanent pasture are places where you will see this large, edible fungus - although if gathering mushrooms to eat it is not a good idea to include roadside specimens. Beware also of a toxic lookalike, the Yellow Stainer, which is also fond of roadside grassy habitats and can cause serious stomach upsets if included in a mushroom meal. Agaricus arvensis, the Horse Mushroom, first appears in summer, and these large fungi usually continue fruiting until the end of autumn.

Distribution

Fairly frequent in Britain and Ireland as well as in most countries of mainland Europe and parts of Asia and North America, the Horse Mushroom has also been reported from Australia (where it is sometimes referred to as the Almond Mushroom) as well as New Zealand.

Taxonomic history

Agaricus arvensis - Horse Mushroom in a fairy ring

First described from Bavaria in 1762 by Jacob Christian Schaeffer, who gave it the name Agaricus arvensis (although like many mushrooms it later spent some time in the genus Psalliota before reverting to its original Agaricus home), the Horse Mushroom is a cosmopolitan mushroom.

Etymology

The specific epithet arvensis means 'of the field' or 'of meadows' - a reference to the habitat in which the Horse Mushroom is most commonly found. Less obviously, the common name may not be the more obvious to horses and its apparent appetite for horse manure (and hence the common occurrence of this mushroom near stables or fields in which horses graze) but, some people have suggested, an allusion to the large size that these mushrooms can attain.

In New Zealand this species is commonly known as the Snowball Mushroom.

Toxicity

There are reports that this edible and greatly prized mushroom tends to accumulate heavy metals such as copper and cadmium, and so if eaten it is best considered an occasional treat rather than a food-for-free bonanza in those years when grassland mushrooms are plentiful.

Identification guide

Cap of Agaricus arvensis

Cap

The cap of Agaricus arvensis matures at  8 to 20cm (exceptionally to more than 30cm) diameter. White but yellowing gradually with age, smooth or finely scaly, the cap is initially spherical and expands until it is flat or nearly so. The thick flesh is white and firm. The cap turns yellowish where bruised, and old caps often take on a yellow-brown tinge.

Gills of Agaricus arvensis

Gills

At first pale pink, darkening and then becoming brown, the gills of the Horse Mushroom are free and crowded.

Stem and ring of Agaricus arvensis

Stem

Up to 10cm tall, the parallel stem usually a small bulb at its base and a robust, double ring with a cog-wheel form on the underside.

The solid stem is smooth above the ring but sometimes finely scaly below. Its diameter ranges from 2 to 3cm.

When cut at the stem base, Agaricus arvensis does not rapidly turn bright yellow - a useful visual distinction between this edible mushroom and the poisonous Yellow Stainer, Agaricus xanthodermus, whose stem base turns chrome yellow as soon as its cut flesh is exposed to air.

 

Spores

Ellipsoidal, smooth, 7-8 x 4.5-5µm.

Spore print

Dark purple-brown.

Odour/taste

Taste not distinctive. Strong odour of aniseed. (The rather similar Yellow Stainer has an unpleasant phenolic or inky odour - a useful way of distinguishing between the edible Horse Mushroom, which also bruises slightly yellow, and that toxic toadstool Agaricus xanthodermus.)

Habitat & Ecological role

Agaricus arvensis appears in manured meadows and beside bridle paths and other places where there is plenty of decaying organic matter, upon which it feeds saprophytically. The Horse Mushroom is one of the largest and most distinctive fungi in its genus, often forms fairy rings many metres in diameter in permanent pastures.

Season

August to November, typically a week or two later than the Field Mushroom, Agaricus campestris, with which the Horse Mushroom is sometimes confused.

Similar species

The poisonous Yellow Stainer, Agaricus xanthodermus, quickly turns bright chrome yellow when cut or bruised, and it smells of iodine or ink rather than of aniseed.

Agaricus urinascens var. urinascens (syn. Agaricus macrosporus) is very similar in appearance but does not turn bright chrome yellow when cut or bruised.; it grows in open grassland and in woodland clearings. The specific epithet of its older synonym refers to the large (for an Agaricus species) size of the spores this mushroom - typically 11 x 6µm - which also distinguishes it from the Yellow Stainer.

Agaricus arvensis, closed cup stage

Culinary Notes

The Horse Mushroom is a good edible species and can be used in any recipe calling for large (Portobello) cultivated mushrooms. It is great in rissotto dishes and omelettes, and it certainly has enough flavour to make tasty soups or sauces to be served with meat dishes.

The most important thing is to make absolutely sure that what you have found really is a Horse Mushroom and not a toxic toadstool such as a Yellow Stainer (Agaricus xanthodermus) or, heaven forbid, a Destroying Angel (Amanita virosa). The stem-base colour change resolves the former, and checking that the gills are pink or brown rather than white averts the latter potentially-fatal error. The cautious old mushroom gatherer's maxim is so important: 'Never munch on a hunch'. (There are not many incautious old mushroom gatherers!)

The Horse Mushroom at the closed-cup stage seen above and the specimen seen at the top of this page were photographed by Dave Kelly, with whose kind permission these picture are shown.

Reference Sources

Pat O'Reilly (2016) Fascinated by Fungi; First Nature

BMS List of English Names for Fungi

The genus Agaricus in Britain, 3rd Edition, self-published, Geoffrey Kibby 2011

Funga Nordica: 2nd edition 2012. Edited by Knudsen, H. & Vesterholt, J. ISBN 9788798396130

Dictionary of the Fungi; Paul M. Kirk, Paul F. Cannon, David W. Minter and J. A. Stalpers; CABI, 2008

Taxonomic history and synonym information on these pages is drawn from many sources but in particular from the British Mycological Society's GB Checklist of Fungi and (for basidiomycetes) on Kew's Checklist of the British & Irish Basidiomycota.

Acknowledgements

This page includes pictures kindly provided by David Kelly.

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