THE GENUS XEROCOMUS


Introduction

The genus (Xerocomus) is notoriously difficult.
There is a rather big confusion with regard to species concept and the names to use for them. This is partly due to the rather large variability of macroscopical characters, such as color of pileus and stipe, and the discolorations of the flesh when cut.

Gerard Oolbekkink and Wim Van Duin, students of the National Herbarium in the seventies, studied the use and value of microscopical characters of spores and covering layer of the pileus for species identification.

A recent monograph by Ladurner and Simonini (2003), published by gives the most recent state of affairs in this group.

Molecular phylogenetic studies indicate that Xerocomus is a heterogeneous group, probably of polyphyletic origin (having several ancestors). The current concept of Xerocomus probably has to be spilt off in three or four, unrelated groups, that may deserve the rank of genus, viz.:
  • Xerocomus in the strict sense, typified by X. subtomentosus and including also Phylloporus.
  • Paraxerocomus for the X. chrysenteron group.
  • Pseudoboletus to accommodate the parasitic species X. parasiticus
  • and a separate genus for X. badius

These pages will be used to share with you my own experiences with Xerocomus

Field characters of three confusing species:

X. rubellus, X. communis, and X. bubalinus

X. rubellus: Pileus usually entirely dark red; stipe red or partly yellow; flesh usually staining dark blue; red dots and lines present or absent in flesh of the stipe base. Common and wide-spread under deciduous trees. Compare also X. ripariellus below

X. communis. This species usually has a rather pale pileus with a pinkish, apricot or reddish flush. The stipe is reddish stiped on a yellow background; the flesh is hardly bluing and base of stipe shows distinct red dots and lines. This species is also rather common in deciduous woods and parks.

X. bubalinus. This species, recently described from the Netherlands, has a ochraceous pileus, frequently with a pale margin, sometimes also with a reddish flush; the stipe is (pale) yellow, freqeuntly with reddish stripes; the flesh colours pink jsut under the surface of the cap and blue just above the attachment of the tubes and in the middle of the stipe; the stipe base never shows red dots and lines. It frequently grows under Populus, more rarely Tilia on sandy soils.

DESCRIPTIONS OF THE SPECIES

Xerocomus communis (Bull.) M. Bon

macroscopic description

Pileus up to 110 mm broad, hemispherical to convex, yellow-brown, brown, grey-brown or brown with distinct reddish, pinkish or apricot tinge; when young finely tomentose, with age often breaking up in irregular patches, showing the yellow context underneath; Tubes adnate, up to 15 mm long, lemon-yellow then greenish-yellow, contrasting with pileus color pores yellow. Stipe 30-90 x 4-28 mm, cylindrical to clavate, often attenuated towards base, brown or yellow, sometimes with reddish tinges, particularly in middle part; fibrillose-striate to furfuraceous. Context yellow; staining blue in places, especially in pileus and upper part of stipe; pores staining blue-green when bruised. Stipe base often, but not always with very distinct flame-red dots.


X. communis


X. communis


X. communis, dots in stipe base


Length cut, showing discoloration and red dots in stipe base

Xerocomus bubalinus (Oolbekkink & van Duin) Redeuilh

macroscopic description:
Pileus up to 50 mm broad, hemispherical to convex, very dark red-brown when young, then red-brown (Mu. 7.5YR 3-4/2-4; 5R 4/4-5,5/6, 10YR 4/4-5/6), very irregular rugulose, entirely covered with a fine bloom only later cracking. Tubes adnate, yellow, strongly contrasting with pileus color (2.5 Y 7-6/6-8), pores remarkably yellow (2.5 Y 8-7/6). Stipe yellow as pores, with vague reddish tinges, especially when young at apex, rather strongly fibrous-grooved lengthwise. Context yellow in pileus and stipe (5 Y 8/6/8), faintly turning blue when cut, particularly in upper part of stipe and lower part of pileus just above the tubes, andalso a pink in the rest of the pileus; context in stipe base orange (5-7.5 YR 7/8)


X. bubalinus Noordel. 1582


X. bubalinus Noordel. 1584

Xerocomus ripariellus Redeuilh

macroscopic description:
Pileus up to 60 mm, usually smaller, hemispherical to convex, then expanding with involute to straight margin, deep carmine red when young, later becoming paler, finally sordid brownish red, finely velvety to rugulose at first, then breaking up in small, irregular patches, showing the pale pinkish flesh. Tubes adnate, ventricose, yellowish; pores small, rounded to irregular, yellow then olivaceous yellow, staining blue when bruised. Stipe 20-60 x 3-11 mm, straight, cylindrical, sometimes fusiform, sordid carmine-brown, fibrous. Flesh pale yellow in pileus and stipe, staining blue when bruised. Smell and taste indistinct.

In damp, marshy places under Willows, Poplars, etc.

The distribution of this species is poorly known because of its resemblance to X. rubellus. Distinctive are the remarkable cracked hat and stripes spores, and the habitat.


Xerocomus ripariellus, Leg. G. v. Zanen, aug. 2003


X. ripariellus aquarel Wakker