Macrocystidia cucumis (Pers.) Joss. - Cucumber Cap

Macrocystidia cucumis - Cucumber Cap

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

This caps of this attractive red-brown fungus are difficult to spot among the dead beech leaves that litter their most usual habitat. You are quite likely to find these mushrooms in disturbed sites beside country lanes, and they are increasingly common now that woodchip mulch is used so extensively in gardens and parks.

The specific epithet comes from the odour of cucumber (although one of our colleagues says that to him they smell rather more like salmon sandwiches).

Macrocystidia cucumis - Cucumber Cap, underside view of a group of fruitbodies

Distribution

In Britain and Ireland the Cucumber Cap is widespread but rather uncommon. This swarthy, gregarious mushroom occurs throughout mainland Europe and many other parts of the world including North America.

Taxonomic history

This saprobic fungus was described in scientific literature in 1796 by Christiaan Hendrik Persoon, who gave it the binomial name Agaricus cucumis. (Most gilled fungi were placed in the Agaricus genus in the early days of fungal taxonomy, but the majority have since been relocated to new genera.)

In 1934 French mycologist Marcel Josserand (1900-1992) transferred this species to its current genus, thus estabishing its currently-accepted scientific name Macrocystidia cucumis.

Macrocystidia cucumis - Cucumber Cap, group of a group of fruitbodies, southern England

Synonyms of Macrocystidia cucumis include Agaricus cucumis Pers., Agaricus nigripes Trog, Agaricus pisciodorus Ces., Naucoria cucumis (Pers) P. Kumm., Agaricus piceus Kalchbr., Nolanea nigripes (Trog) Gillet, Nolanea picea (Kalchbr.) Gillet, Nolanea pisciodora (Ces.) Gillet, and Naucoria cucumis var. leucospora J. E. Lange

Macrocystidia cucumis is the type species of the genus Macrocystidia.

Etymology

Macrocystidia, the genus name, means 'posessing very large cystidia'. Cystidia (singular cystidium) are large, (usually inflated) sterile cells that occur in between the spore-bearing basidia, and in mushrooms of this genus they are very big indeed..

The specific epithet cucumis comes from Latin and simply means 'of cucumber'.

Identification guide

Cap of Macrocystidia cucumis

Cap

The cap is pruinose or velvety and conical at first, becoming more shiny and bell-shaped with age; rarely expanding to become completely flat but more often retaining a shallow umbo; diameter 1 to 6 cm; red-brown or date brown with the rim area a paler yellow-brown; hygrophanous, old caps eventually turning dingy buff in dry weather.

Gills and stems of Macrocystidia cucumis - Cucumber Cap

Gills

Cream at first, becoming pale pinkish-beige with age, the gills are broad, crowded and free or nearly so.

Stem

3 to 8mm diameter, often slightly flattened; cream or beige at the apex, graduating to dark brown at the base; velvety surface.

Cheilocystidia of Macrocystidia cucumis

Cystidia

The mushrooms in this genus are characterised by the presence huge spear-shaped cystidia all over the caps, gills and stems that are so big that they are visible with a low-powered microscope or even a good hand lens. It is the cystidia which give the caps and stems their velvety texture.

Above: Cheilocystidia of Cucumber Cap.

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Spores of Macrocystidia cucumis

Spores

Ellipsoidal, thin-walled, smooth, 7.5-10 x 3.5-5µm; inamyloid.

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Spore print

Pale pinkish-brown.

Odour/taste

Odour of cucumber; taste rather unpleasant but not distinctive.

Habitat & Ecological role

Saprobic, usually in small groups on disturbed soil that is rich in woody debris such a wood chip mulch. Elsewhere it is most commonly seen under broadleaf trees, particularly beech, but also in hazel coppices and very occasionally under conifers.

Season

Fruiting from July through to December in Britain and Ireland, but most abundant in October and November.

Similar species

Pluteus cervinus is more viscid and grows on rotten wood; its gills are much paler than those of Cucumber Cap, and the cap usually flattens, whereas caps of Macrocystidia cuccumis tend to remain bell shaped much longer.

Flammulina velutipes, Velvet Shank, is a winter-fruiting species that appears on trees; it also has a dark and velvety stem but the cap is orange and does not smell of cucumber; its spore print is white.

Macrocystidia cucumis - Cucumber Cap, young fruitbodies

Culinary notes

Despite its common name, Cucumber Cap is inedible, and some authorities state that it may possibly be slightly poisonous. This mushroom should not be collected for eating.

Reference Sources

Fascinated by Fungi, Pat O'Reilly 2016.

Josserand, M. (1934) [1933] Notes critiques sur quelques champignons de la région lyonnaise. Bulletin Trimestriel de la Société Mycologique de France 49(3-4): pp 373, 376.

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

Simon Harding has kindly contributed a picture to this page.

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