Scarlet-stemmed Bolete

Caloboletus calopus is one of the handsomest of the blueing boletes, and its scientific name, meaning 'beautiful foot', refers to the striking stalk: yellow at the apex and washed a deep carmine or crimson over the lower two-thirds, the whole overlaid with a fine raised network.

It was long placed in Boletus but now sits in Caloboletus, a genus assembled for firm-fleshed, pale-capped, blue-staining boletes with yellow pores. Members of this group typically have thick, hard flesh that keeps its shape well and reacts strongly and instantly to bruising.

A fungus of acid, often upland soils, it associates with both broadleaved and coniferous trees and is most frequent in beech woods on sandstone or granite and in mountain spruce forest. Its combination of pale matt cap, brilliant lemon pores and red netted stalk makes it one of the more easily learned large boletes of European woodland.

Cap

5-15 cm across, thickly convex then broadly flattened, dry and finely felted or matt, often becoming areolate and cracking in dry weather. Colours are muted: pale grey-buff, olive-buff, clay or dull brownish-grey, sometimes with an ochre flush. The margin is thick and long incurved.

Pores

Small, round and bright lemon-yellow when young, ageing olive-yellow. They stain a strong, immediate blue at the slightest pressure, one of the most reliable characters.

Stipe

7-15 cm tall and 2-5 cm thick, stout, club-shaped or barrel-shaped, yellow at the extreme apex and carmine to blood-red over most of its length, sometimes browning at the very base. The entire surface bears a fine raised reticulum, red over the lower part and yellowish above.

Flesh & spore print

The flesh is firm, pale yellow to whitish, and turns blue promptly when cut, most intensely in the cap and upper stalk. The spore print is olive-brown.

Ectomycorrhizal with a range of trees, most commonly beech and oak among broadleaves and spruce, fir and pine among conifers. It prefers acidic, sandy or stony soils and is particularly associated with upland and montane woodland, often appearing on banks and in mossy ground beneath mature trees.

The season runs from midsummer to mid-autumn. It is widespread across Europe from Scandinavia to the Mediterranean uplands, extends into western Asia, and closely related members of the Caloboletus complex occur in western North America.

This bolete forms ectomycorrhizas, sheathing the fine root tips of its hosts and creating an interface through which soil nutrients pass to the tree in exchange for photosynthetic sugars. Its ability to partner both broadleaves and conifers gives it a wide potential habitat, though it is largely restricted by soil acidity.

The thick, hard flesh of the fruiting body resists insect attack and desiccation better than that of softer boletes, so specimens often persist for a week or more in the field. The rapid blueing reaction results from oxidation of pigments in the flesh when cell walls are damaged and air reaches the tissue.

Caloboletus radicans has a similar pale, matt cap and blueing lemon pores but its stalk is yellow with at most a faint pale reticulum, never carmine, and its base tends to be rooting. Neoboletus luridiformis has a dark velvety brown cap, orange-red pores and a stalk covered in fine red dots rather than a raised network.

Rubroboletus satanas is far paler and chalkier in the cap, with red to orange pores and a fat bulbous stalk carrying a red net on yellow. Butyriboletus appendiculatus has a warm brown cap, yellow pores that blue only weakly and a yellow stalk with a fine yellow reticulum, lacking any red zone.