Frost's Bolete

Exsudoporus frostii, long known as Boletus frostii, is among the most photogenic fungi of eastern North America. Its cap looks polished, a deep blood or candy-apple red with a lacquered shine, and young fruit bodies bead amber droplets from the pore mouths, a habit reflected in the genus name Exsudoporus, meaning exuding pores.

It sits in the red-pored group of the Boletaceae close to Rubroboletus and Suillellus, all of which combine dark pore mouths with rapid blue bruising. What separates E. frostii is the combination of that glossy red cap with an intensely and coarsely reticulated red stipe, the network so deep and pitted that the surface resembles honeycomb.

It is a warm-season species of oak woodland, most frequent in the Appalachians and the southeastern states, and also recorded southward through Mexico into Central America.

Cap

The cap is 5-15 cm across, convex to broadly convex, and coloured deep blood-red, crimson or dark cherry, occasionally fading to brick or orange-red with age. The surface is smooth and distinctly shiny, appearing varnished or lacquered when dry and becoming tacky to slightly sticky in humid weather. The margin is often paler and may be slightly incurved.

Pores

The pore surface is dark red to blood-red, sometimes browner in age, with small round pores and a yellow-tinged tube layer beneath. In young fresh specimens the pores exude conspicuous amber to yellowish droplets that bead across the surface. Pressing anywhere on the pores produces an immediate deep blue-black stain.

Stipe

The stipe is 4-12 cm tall and 1-3 cm thick, solid, roughly equal or slightly swollen below. It is red to reddish-yellow, often yellower at the base, and is covered along its whole length with a very coarse, deep, raised network that forms elongated pits. This pronounced reticulation is the species' signature and distinguishes it from most other red-pored boletes.

Flesh & spore print

The flesh is pale yellow and turns blue rapidly and strongly when cut, particularly in the cap and upper stipe. The spore print is olive-brown.

Frost's Bolete grows in broadleaf woodland dominated by oaks, and also with beech and other hardwoods. It appears on the forest floor in leaf litter, singly, scattered or sometimes in small clusters, and is often found along trail edges, on wooded banks and in open parkland under mature oaks.

Its core range is eastern North America, with a stronghold in the Appalachian region and the southeastern states, extending north to New England and southern Ontario and west to the Great Lakes and eastern Texas. Populations continue south through the highlands of Mexico into Costa Rica. Fruiting runs from early summer to early autumn, peaking in the warm humid weeks of July and August.

This bolete is ectomycorrhizal with oaks and other hardwoods, forming a fungal mantle around the host's fine roots and a network between the cells of the root cortex. Through this interface it transfers water and soil nutrients to the tree and receives photosynthetic sugars in return, and it cannot fruit in the absence of a compatible living partner.

The droplets that gather on young pore surfaces are guttation fluid, water forced out of the actively growing tissue as the fruit body expands rapidly in humid conditions; they disappear as the fruit body matures and the tube mouths open fully. The vivid red pigments and the rapid blue reaction come from pigment compounds in the flesh that oxidise on contact with air, a feature widespread among red-pored boletes.

Rubroboletus species such as R. sinensis and their relatives share red pores and rapid blueing but have dull, dry, often pinkish or greyish caps rather than a glossy blood-red one, and their stipe reticulation is much finer. Suillellus luridus has an olive-brown to tawny cap and a fine reddish net, nothing like the lacquered red surface and deep honeycomb pits here.

Baorangia bicolor is red-capped but has clear yellow pores, a smooth unnetted stipe and a slow weak blue stain. Neoboletus luridiformis has orange-red pores, a dark brown cap and a stipe densely covered in fine red dots rather than a raised network.