Leccinum aurantiacum (Bull.) Gray - Orange Oak Bolete

Phylum: Basidiomycota - Class: Agaricomycetes - Order: Boletales - Family: Boletaceae

Leccinum aurantiacum, Orange Oak Bolete, New Forest, Hampshire UK

Appearing most often under poplar trees and aspens but also recorded beneath oaks, beech and birches, this summer and autumn bolete is an uncommon but not particularly rare find in Britain and Ireland. Leccinum aurantiacum is large, attractive and easy to spot, but that doesn't make its confident identification easy. As with all members of this deceptively difficult group, confident differentiation of the various Leccinum species requires study of both macroscopic and microscopic features.

Leccinum aurantiacum, Orange Oak Bolete, Hampshire, England

Distribution

Fairly common in many parts of Britain and Ireland, the Orange Oak Bolete is also found throughout most of northern and central mainland Europe. Widespread and abundant in Scandinavia and in Scotland, but it is increasingly rare further south, especially in lowland areas.

Leccinum aurantiacum recorded in North America may not be the same species as its European namesake.

Taxonomic history

The Orange Oak Bolete was described in 1781 by French naturalist Jean Baptiste Francois (Pierre) Bulliard, who gave it the binomial scientific name Boletus aurantiacus . The currently-accepted scientific name Leccinum aurantiacum dates from an 1821 publication by British mycologist Samuel Frederick Gray (1766 - 1828).

Synonyms of Leccinum aurantiacum include Boletus rufus Schaeff., Boletus aurantiacus Bull., Leccinum aurantiacum var. quercinum Pilát, Leccinum quercinum (Pilát) E. E. Green & Watling, and Leccinum populinum M. Korhonen.

Etymology

Leccinum, the generic name, comes from an old Italian word meaning fungus. The specific epithet aurantiacum means orange - a reference to the cap colour.

Identification guide

Cap of Leccinum aurantiacum

Cap

Initially globose or deeply convex and usually tomentose (finely felty), becoming shallowly convex or flattening completely and often rather misshapen with a finely scaly and less tomentose surface. The cap margin overhangs the pore surface by typically 2 to 4mm, usually in the form of roughly-triangular flaps; various shades of brick red to date brown; 5 to 15cm across when fully expanded.

Pore surface of Leccinum aurantiacum

Tubes and pores

The circular tubes are broadly adnexed or adnate to the stem; they are 1 to 3cm long, creamy-white gradually becomong pale brown, and they terminate in pores that are similarly coloured, angular, and less than 0.5mm in diameter. When bruised, the pores turn pinkish grey and eventually dark grey.

Stem flesh of Leccinum aurantiacum

Stem

White or buff and 5 to 25cm tall, the stems of Leccinum aurantiacum are 1.5 to 5cm in diameter,. Immature specimens often have barrel-shaped stems; at maturity most stems are more regular in diameter, tapering in slightly towards the cap and sometimes slightly clavate at the base. Reddish-brown woolly scales cover the whole of the stem surface but are noticeably more dense on the lower part of the stem; these stem scales become dark brown as fruitbodies age.

Stem of Leccinum aurantiacum

Stem flesh

The cap and stem flesh is white when freshly cut (far left) but darkens and often turns slightly blue towards the base when it is handled, broken or cut (near left).

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Spores of Leccinum aurantiacum

Spores

Narrowly ellipsoidal to fusiform, 12.5-18.5 x 3.5-6µm.

Spore print

Olivaceous buff.

Note: Other microscopic characters must be examined before a specimen can be conclusively recorded as Leccinum aurantiacum, especially caulocystidia and the hyphal structure of the pileipellis - see key by Geoffrey Kibby (Reference below).

Odour/taste

The faint smell and taste are pleasant but not particularly distinctive.

Habitat & Ecological role

All Leccinum species are ectomycorrhizal, and most are found only with one tree genus. Leccinum aurantiacum is mycorrhizal most commonly with poplars and aspen (Populus species) and with oak trees (Quercus species); less often it occurs with other broadleaf trees including beech and birches.

Season

July to November in Britain and Ireland.

Similar species

Leccinum versipelle has an orange cap; it bruises blue-green in the stem base.

Leccinum albostipitatum has initially white stem squamules that turn reddish as fruitbodies age; it is a rare find in Britain.

Leccinum aurantiacum, Orange Oak Bolete

Culinary Notes

Leccinum aurantiacum in Europe is generally considered to be a good edible mushroom and can be used in recipes that call for Ceps Boletus edulis (although in both flavour and texture a Cep is superior). Alternatively, use Orange Oak Boletes to make up the required quantity if you do not have sufficient Ceps.

There have been reports in North America of people suffering delayed adverse reactions to Leccinum mushrooms, although only a minority of those who eat them seem to be affected. It is far from certain, however, that the fungi involved in those North American poisoning incidents are co-specific with the macroscopically similar Leccinum mushrooms found in Britain and elsewhere in Europe.

Leccinum aurantiacum, Orange Oak Bolete, New Forest, England

Reference Sources

Pat O'Reilly, Fascinated by Fungi, 2011.

Henk C. den Bakker, Barbara Gravendeel & Thomas W. Kuyper (2004). An ITS phylogeny of Leccinum and an analysis of the evolution of minisatellite-like sequences within ITS1; Mycologia, 96(1), 2004, pp. 102-118.

British Boletes, with keys to species, Geoffrey Kibby (self published) 3rd Edition 2012.

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.

Other web pages on this species

Machiel Noordeloos (Netherlands)

Roger Phillips (UK)

Marek Snowarski (Poland)

Acknowledgements

This page includes pictures kindly contributed by David Kelly.

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