Lilac-brown Bolete Identification Guide

The First Look

Most boletes are brown, tan, red or yellow. This one is purple-grey, and that unusual colour is what stops you. Sutorius eximius, long known as Boletus eximius or Tylopilus eximius, is a medium to large bolete in which cap, pores and stipe are all variations on the same smoky chocolate-lilac, so the whole fruitbody reads as one dusky colour rather than as contrasting parts.

You will typically be in mixed eastern North American woodland in high summer, under oak with hemlock or pine nearby, on well-drained slopes with a thin litter layer; the species also occurs in eastern Asia in comparable forest. Fruitbodies come singly or in small groups, and they often push up through the litter with a fine grey bloom still intact on the cap.

Reading the Cap

Caps run 5 to 15 cm, occasionally larger. The shape is hemispherical at first, then broadly convex, and finally almost flat with a slightly incurved rim; the margin is even and not appendiculate.

Colour is the point. Fresh caps are dark purple-brown, chocolate-lilac or greyish violet, and are covered when young in a fine pale grey pruina, a bloom that softens the colour and rubs away under a thumb to leave a darker, glossier patch. Older caps lose the bloom entirely, dull towards greyish brown, and can look almost sepia in bright sun.

The surface is dry to slightly tacky after rain, smooth, and neither scaly nor cracked. Check under the extreme margin for a narrow sterile band of cap flesh overhanging the tubes.

The Pore Surface

Turn the cap over and expect the underside to be dark from the beginning, which is unusual and immediately useful. Young pores are pale greyish-lilac or vinaceous grey; they deepen with age to a distinctly purple-brown, chocolate-vinaceous or dull reddish brown. There is no white or bright yellow stage at all.

The pores are small and round, and the tube layer is shallow, depressed around the top of the stipe. Press the surface firmly with a thumb and wait a minute or two. The reaction is muted: a slow dulling to a darker brownish or greyish tone, sometimes with a faint vinaceous flush, but no rapid blue and no bright colour change.

The Stipe and Its Dots

The stipe is 6 to 12 cm long and 1.5 to 3 cm thick, usually more or less equal but often slightly swollen in the lower half or narrowed abruptly at the very base. It is solid and firm.

Its ground colour is the same lilac-brown or greyish violet as the cap, paler towards the apex and sometimes almost whitish at the very base. Over this ground lies the key character: dense, fine, dark purplish-brown to blackish scurfy dots or punctae, covering most of the surface. Look at them with a lens. They are flat to very slightly raised granules, not the coarse projecting scabers of a Leccinum and not a raised network. There is no reticulation anywhere on the stipe, and no ring.

At the base, whitish mycelial threads may bind soil and litter.

Flesh and Colour Change

Slice the fruitbody in half lengthways. The flesh is whitish to pale greyish or faintly lilac, firm, and often slightly darker directly under the cap skin where the cap colour bleeds inwards.

Watch the cut face for two or three minutes. Most collections change slowly and undramatically: a smoky grey, pale pinkish or dull vinaceous tint spreads through the cut, occasionally with a hint of blue-grey near the tubes, and then stalls. It never floods a strong ink-blue and it never blackens the way a few dark boletes do. Recording that slow, weak, greyish reaction is important, because a strong or immediate colour shift moves your specimen to another genus.

Taking a Spore Print

Cut the stipe off flush with the tubes and place the cap pore-side down on white paper. A dark card under one half is useful too, because the print colour is easier to judge against contrasting backgrounds. Cover with a bowl and leave for six to twelve hours in still air at room temperature; thick boletes need the full period.

The deposit is reddish brown to purple-brown or vinaceous brown, not the olive-brown of Boletus and not the cinnamon of Suillus. Under a microscope the spores are smooth, ellipsoid to sub-fusiform, roughly 11 to 16 by 4 to 5.5 micrometres. Print colour alone rules out the great majority of large boletes.

Substrate and Season

Ectomycorrhizal, growing from soil in mixed forest. Reported partners include oak, beech, hemlock, spruce and pine, and it is most often found where broadleaves and conifers grow together. It is a summer species in most of its range, appearing from late June into September after warm rain, and it does not grow on wood or in open grassland.

Separating It From Look-alikes on Morphology Alone

Tylopilus plumbeoviolaceus is the nearest confusion. Its cap is a brighter violet or purple when young, fading to dull brown, its pores start white and become clearly pink, and its stipe is smooth to obscurely reticulate at the apex without the dense dark scurfy dots.

Tylopilus rubrobrunneus has a larger, dull reddish-brown to purple-brown cap, but again the pores pass through a whitish stage into pink, and its stipe is pale and unspotted.

Tylopilus felleus has a tan to olive-brown cap, pores that turn distinctly pink, and a coarse, strongly raised, dark network over the upper stipe, which is quite different from fine flat dots.

Tylopilus alboater shares dark tones but the cap is charcoal to black with a grey bloom, the pores start white and stain blackish, and the flesh reddens and then blackens after cutting, a far more emphatic reaction.

Harrya chromapes has a pink stipe covered in pink scabers with a bright chrome-yellow base and a pinkish cap, quite unlike this species.

Leccinum species carry coarse, projecting, roughened scabers rather than fine flat punctae, and their flesh generally darkens grey or black on cutting.

Strobilomyces species are covered in coarse shaggy grey-black scales on both cap and stipe, with a black spore print.

Porphyrellus species are dark brown to blackish with brown pores and a reddish-brown print but lack the lilac tones and the dense dark stipe dots.

Field Summary

A large summer bolete in mixed oak and conifer woodland, uniformly smoky lilac-brown, with a grey bloom on the young cap, purple-brown pores that never pass through white or yellow, a stipe densely peppered with fine dark scurfy dots but no net, pale flesh with a slow weak greyish or vinaceous change, and a reddish to purple-brown spore print.