Lilac-brown Bolete

The Lilac-brown Bolete is a distinctive dark bolete of northern and montane mixed forest, coloured throughout in shades of lilac, purple-brown and chocolate. Young caps often carry a fine pale bloom that rubs away to reveal a deeper purple-brown beneath, giving fresh specimens a plum-like appearance.

Long known as Tylopilus eximius, it was transferred to the genus Sutorius when molecular studies showed it was not part of the Tylopilus lineage despite its pinkish spore deposit. Sutorius is a small genus of dark, finely punctate-stalked boletes with representatives in North America, Asia and Australasia.

What separates it most reliably from the true Tylopilus species it grows beside is the pore surface, which is a smoky purple-grey from the very first button rather than passing through a white stage.

Cap

The cap is 5-15 cm across, convex then broadly convex to flat. The surface is dry and matt, minutely velvety, and often covered when young by a fine whitish or greyish pruina. Colour is a deep purple-brown, chocolate-brown or lilac-grey-brown, sometimes fading to a duller brown in age or after rain.

Pores

The pore surface is smoky purple, purple-grey or lilac-grey from the button stage onward, darkening to purplish-brown with maturity. Pores are small and round, and the tubes are depressed at the stipe. Bruising produces slow darker brown stains.

Stipe

The stipe is 6-12 cm long and 1.5-3 cm thick, equal or slightly swollen below, and coloured lilac-grey to purplish-brown, usually paler than the cap. Its surface is densely covered with fine dark purple to blackish dots, scurf and granules, which under a hand lens resolve into minute punctate scales rather than a raised net.

Flesh & spore print

The flesh is pale lilac-grey to whitish, sometimes with a purplish tinge, and changes little when cut or slowly darkens. The spore print is reddish-brown to pinkish-brown.

This bolete grows on soil in coniferous and mixed woodland, especially under spruce, fir, hemlock and pine, and also in mixed stands with birch, beech and oak. It favours mossy, acid, humus-rich ground and often fruits in numbers along shaded trails and on cool north-facing slopes.

The season runs from midsummer to early autumn, chiefly July through September. Its range covers northern and montane eastern North America, from Canada and New England south along the higher Appalachians, and closely related species in the genus occur in eastern Asia and Australasia.

Sutorius eximius is ectomycorrhizal, forming root sheaths with conifers and, in mixed stands, with hardwoods such as birch and beech. As with other ectomycorrhizal boletes the fungus builds a mantle over the fine roots and a Hartig net between the cortical cells, delivering nitrogen and phosphorus scavenged from cool, acid, slowly decomposing humus in exchange for host sugars.

Because it draws on humus-bound nutrients that trees cannot access directly, it is an important contributor to nutrient cycling in northern forest soils where litter breakdown is slow. Fruitbodies persist for several days before softening and are commonly grazed by slugs and mined by fly larvae.

The Violet-gray Bolete (Tylopilus plumbeoviolaceus) is the nearest match in colour, but its pore surface is white when young and only later turns pink, and its stipe is smooth to faintly netted rather than densely dotted. Tylopilus rubrobrunneus has a purple-brown cap too but pale buff, essentially smooth stipe and white pores ageing pink.

Porphyrellus species share dark caps and a pinkish-brown spore print, but their flesh typically reacts blue-green then reddish when cut, and their caps are browner and more velvety. Dark Leccinum species have prominent coarse scabers rather than fine purplish punctation and drop brown, not reddish-pink, spore prints.