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Paxillus obscurisporus C. Hahn

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

Paxillus obscurisporus

From its general shape a mature example of this poisonous gilled fungus might be mistaken for a 'funnel' (Clitocybe genus), except that its gills are brown rather than white, and its spores are vinaceous redish brown whereas Clitocybe species have white spores.

This gilled member of the order Boletales occurs most often under lime trees (Tillia species), birches (Betula species) and willows (Salix species); it produces ground-hugging caps that are usually a lot bigger than other members of the Paxillus genus.

Paxillus obscurisporus under Betula pendula

Distribution

Uncommon but perhaps not as rare in Britain and Ireland as records currently suggest, because of easy confusion with other Paxillus species the distribution of Paxillus obscurisporus is not yet well understood. This species has been recorded in other central and northern European countries including France, Germany, Poland and parts of Scandinavia.

Taxonomic history

This species was first distinguished as separate from Paxillus involutus and other members of that genus by the German mycologist Christoph Johannes Hahn in 1999, who gave it the scientific name Paxillus obscurosporus, later corrected to 'obscurisporus'.

Etymology

The generic name Paxillus means a 'peg' or a 'small stake', while the specific epithet obscurisporus is a reference to the dark spore print relative to other Paxillus species.

Toxicity

Paxillus invollutus is now known to be toxic, and as this is a very close relative it seems most likely that Paxillus obscurisporus is also a toxic toadstool.

Identification guide

Young cap of Paxillus obscurisporus

Cap

Initially convex, the pale ochre cap soon becomes centrally depressed; finely downy at first, becoming smoother with age; sticky when wet.

Caps expand to between 7 and 35cm in diameter - larger than other members of the Paxillus genus.

Grooved margin of mature cap of Paxillus obscurisporus

Cap margin

Initially markedly inrolled when young but eventually almost flattening and developing increasingly distinctive broad radial grooves.

Gills of Paxillus obscurisporus

Gills

Pale ochre at first, the crowded, branching gills gradually turn reddish brown (rapidly so when bruised), darkening with age. Gills are decurrently attached to the stem.

Stem

Relatively short and stout: typically 1.5 to 4cm in diameter and 3 to 5cm long, the stems are slightly downy and stain red-brown when bruised. Creamy pink mycelium is evident at the stem base.

Spores of Paxillus obscurisporus

Spores

Broadly ellipsoidal, smooth, 5-6.5 x 7-10.5µm.

Show larger image

Spore print

Quite an intense vinaceous reddish brown when taken from fresh specimens.

Odour/taste

Odour not distinctive; taste acidic.

Habitat & Ecological role

Mycorrhizal, usually under broad-leaved trees on roadside verges and in parkland.

Season

July to October in Britain and Ireland.

Similar species

Paxillus rubicundulus P. D. Orton (syn. Paxillus filamentosus) is very similar in macroscopic characters but has pale rusty brown spores 5.5-8.5 x 4-5μm; it is mycorrhizal with Alders (Alnus species).

Paxillus involutus, the Brown Roll-rim, is a smaller and much more common mushroom of acid heathland and woodland; its spores are smaller and its spore print is sienna rather than the intense reddish-brown associated with Paxillus obscurisporus.

Reference Sources

Fascinated by Fungi, Pat O'Reilly 2016.

Roy Watling & Hills, A.E. 2005. Boletes and their allies (revised and enlarged edition), - in: Henderson, D.M., Orton, P.D. & Watling, R. [eds]. British Fungus Flora. Agarics and boleti. Vol. 1. Royal Botanic Garden, Edinburgh.

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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