Mottled Bolete Identification Guide

The Setting

Think wet birch. This bolete belongs to damp, acidic, often peaty ground: birch carr at the edge of a bog, sphagnum-cushioned hollows in a birch wood, mossy heath with scattered birches, the wet corner of a heathland where the ground squelches underfoot. It is ectomycorrhizal with birch and will not be found away from it. Season is high summer through to the first frosts, with the best flushes a few days after prolonged rain.

The first impression is a tall, upright bolete with a rather sombre cap and a slender, boldly speckled stem standing clear of the moss. Because the fruit bodies often push up through sphagnum, the lower half of the stipe may be pale and stretched where it has grown through the moss layer.

Reading the Cap

A mature cap is 5 to 12 cm across, hemispherical when young and becoming broadly convex, rarely flattening completely. The surface is dry, matt and finely felted, never slimy, and in dry weather it often develops a fine network of cracks over the crown.

Colour is the character the name refers to. Rather than a single even brown, the cap is irregularly marbled, blotched or streaked: a base of dark grey-brown, mouse-brown or umber broken up by paler patches of grey, buff, ivory or almost white. Some individuals are strongly two-tone, others only faintly mottled, but the unevenness is usually visible on at least part of the cap. The margin often carries a very narrow sterile overhang of skin.

Reading the Pores

Turn the cap over. The tubes are adnexed to nearly free and clearly depressed around the top of the stipe. Pore mouths are small and round, whitish or pale grey at first, then dull greyish-buff and finally pale ochre-brown. Press them firmly with a thumb and the bruise develops as an ochre to dull brown mark, not blue. The tube layer is thick and separates cleanly from the cap flesh if you push it with a thumbnail, which is a general bolete character worth confirming.

Reading the Stipe and Its Scabers

The stipe is long relative to the cap, 8 to 15 cm by 1 to 2.5 cm, cylindrical or tapering slightly upwards, and sometimes slightly swollen at the very base. The ground colour is white to greyish, and over it runs a dense covering of small tufted scales called scabers, arranged in more or less vertical rows. They start whitish to pale grey and darken with age to blackish-brown, giving the stem a rough, speckled, birch-bark look. There is no ring, no net-like reticulation and no volva.

The Cut: the Decisive Test

Split a whole fruit body lengthways from the top of the cap to the very bottom of the base and then watch it for two or three minutes. Three zones behave differently.

Cap flesh is white and slowly flushes a soft pink or rose, sometimes only faintly, sometimes clearly.

Upper and middle stipe flesh flushes pinkish to reddish, often more strongly than the cap, and can pass through to a dull grey.

The base of the stipe turns a distinct blue-green or verdigris, usually within a minute, and the tint frequently extends into the basal mycelium and the soil-bound strands. This is the anchor character. A mottled cap plus pink in the upper flesh plus blue-green in the base is the signature of this species.

Make sure you actually dig the base out rather than snapping the stem off at ground level, because the reaction is confined to the lowest few centimetres.

Taking and Reading a Spore Print

Cut a mature cap free of the stipe, lay it pore-side down on white paper, cover it with a bowl to hold humidity, and leave it three to six hours. The deposit is olive-brown to snuff-brown, distinctly warm and dark. Under a microscope the spores are smooth, subfusiform, with a shallow suprahilar depression, and measure roughly 12 to 17 by 4.5 to 6 µm. A pale ochre or cinnamon-toned print would point away from Leccinum.

Substrate and Season Reinforce the Identification

Host specificity does a lot of the work here. If your bolete has scabers, an olive-brown print and grows under birch in wet mossy ground, you are already inside a small group; the flesh reactions then finish the job. Habitats that are dry, sandy, or dominated by aspen, poplar, oak or conifers push you towards other species in the genus.

Separating It From Its Closest Relatives

Leccinum scabrum is the commonest confusion. Its cap is evenly coloured dull brown to greyish brown without mottling, its flesh changes little or turns only faintly pinkish, and critically the base does not go blue-green. It also tolerates much drier birch ground.

Leccinum holopus grows in the same wet sphagnum birch habitat but has a pale, whitish to faintly greenish cap that is never dark and blotchy; its flesh may show a weak blue-green at the base, so cap colour is the separator.

Leccinum cyaneobasileucum also blues at the base but has a more evenly coloured greyish-brown cap and weaker pink flushing in the upper flesh; separating extreme forms may need microscopy.

Leccinum versipelle has a bright orange to tawny cap, coarse blackish scabers, and flesh that darkens through wine-grey to nearly black rather than turning pink then blue-green.

Leccinum aurantiacum and relatives have orange-red caps and associate with aspen and poplar.

Suillus species under birch or conifers have glutinous caps and often a ring or glandular dots instead of scabers.

Tylopilus felleus can look superficially similar in stature but has a smooth, netted stipe rather than scabers and pale pink pore mouths.