Bolbitius titubans (Bull.) Fr. - Yellow Fieldcap

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

Bolbitius titubans - Yellow Fieldcap

This colourful little mushroom of rich grassland and roadside verges (and found occasionally also on damp woodchip mulch) is one of the shortest-lived of all mushrooms. It goes from a yellow 'egg on a stick' via a pinkish parasol stage to a mid-to-dark brown or, in hot dry weather, light ochre mushroom in less than a day. This mushroom, better known until recently as Bolbitius vitellinus (previously thought to be a separate species but now merged with Bolbitius titubans) is still quite commonly referred to as the Egg Yolk Fungus.

Bolbitius titubans - Yellow Fieldcap, France

Distribution

Common and widespread in most parts of Britain and Ireland, this little mushroom of manured grassland is found also throughout mainland Europe as well as in many other parts of the world including North America.

Taxonomic history

Described in 1789 by French mycologist Jean Baptiste Francois Pierre Bulliard, who gave it the scientific name Agaricus titubans, this little mushroom was renamed Bolbitius titubans in 1838 - a time when most gilled fungi were initially placed in a giant Agaricus genus, now redistributed to many other genera - by Swedish mycologist Elias Magnus Fries. A very rare variety Bolbitius titubans var. olivaceus was defined in 2000, and so the nominate variety of the Yellow Fieldcap is now formally referred to as Bolbitius titubans var. titubans.

Bolbitius titubans - Yellow Fieldcap, on manured grassland,southern England

Synonyms of Bolbitius titubans include Agaricus fragilis L., Agaricus equestris Bolton, Agaricus titubans Bull., Agaricus flavidus Bolton, Agaricus boltonii Pers., Agaricus vitellinus Pers., Prunulus boltonii (Pers.) Gray, Prunulus titubans (Bull.) Gray, Bolbitius boltonii (Pers.) Fr., Bolbitius fragilis (L.) Fr., Bolbitius titubans (Bull.) Fr., Bolbitius vitellinus (Pers.) Fr., Bolbitius flavidus (Bolton) Massee, Bolbitius vitellinus var. fragilis (L.) M.M. Moser ex Bon & Courtec., and Bolbitius vitellinus var. titubans (Bull.) Bon & Courtec.

Etymology

Bolbitius, the genus name, means 'of cow dung', although note that the Yellow Fieldcap prefers well rotted dung and tends to shun fresh cow pats.

The specific epithet titubans means staggering or wavering, and these little mushrooms do indeed tend to lean and eventually fall in what could be construed as a a slow-motion stagger!

Bolbitius titubans, young caps emerging

Rotting hay and old cow pats are the main habitats of this delicate little fungus. It fruits through most of the year, often in full sunlight.

In the picture on the left, taken at midday, the caps of four young fruit bodies are just beginning to expand...

Bolbitius titubans - decaying caps

...and in this picture, taken five hours later, the caps have expanded fully, faded from yellow to white, and are now beginning to shrivel. By the following day there was almost no evidence of the fruit bodies left. 

Bolbitius titubans - cap with striate margin

Only at the in-between stage are the marginal striations clearly visible. On warm summer days the sticky surface of young caps dries to a silky finish, and then the caps begin greying from the margin. Pale brown spores fall from adnexed and almost free gills that age from straw-yellow through cinnamon to rusty-brown.

Identification guide

Yellow Fieldcap - young specimen

Cap

1 to 4cm in diameter at maturity, the caps are at first egg-shaped and chrome-yellow, soon expanding like a parasol as they mature and rapidly greying from the margin. The surface is smooth and viscid.

The cap flesh is very thin and easily splits from the edge.

Yellow Fieldcap - gills

Gills

Adnate and pale yellow, the moderately spaced gills become cinnamon coloured as the spores mature.

Stem

2 to 4mm in diameter, white with a yellow tint; slightly granular near the apex and downy as the base. The stem, which has no ring, is hollow and very fragile. 

Spores of Bolbitius titubans

Spores

Ellipsoidal, smooth, 11-15 x 6-9µm.

Show larger image

Spore print

Cinnamon to rust brown.

Odour/taste

Not distinctive.

Habitat & Ecological role

Saprobic, on rotting straw and in recently manured grassland; occasionally on woodchip mulch; appearing within two days of rain, but then lasting little more than 24 hours.

Season

June to October in Britain and Ireland.

Similar species

Pluteus chrysophaeus is a longer lasting mushroom; it grows on wood rather than on grass, and its spores are pink rather than cinnamon brown.

Bolbitius titubans, Yellow Fieldcap, grassland, France

Culinary Notes

The Yellow Fieldcap is generally regarded as inedible, and its small size and thin flesh mean that there is little temptation to try eating this insubstantial mushroom..

Reference Sources

Fascinated by Fungi, Pat O'Reilly 2016.

BMS List of English Names for Fungi

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.

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