Gray Bolete Identification Guide

Grey Cap, Netted Stipe, Oak Overhead

Grey is an unusual cap colour in the boletes, and it is what makes this species recognisable from a couple of metres. You are in an eastern oak wood in high summer, probably on a slope or a path edge where the litter is thin, and there is a solid, medium-large bolete standing in the soil with a dry, matt, mouse-grey cap. Turn it partly on its side without lifting it and look at the stipe: a coarse raised network, like a fishnet stocking pulled over the stem, standing proud of the surface and running most of the way down. Grey cap plus a strong net plus oak is the core of Retiboletus griseus.

Note also what is absent from the first impression: no bright yellow or red pores, no dramatic blue flush when the cap is knocked, and no dark tufted scabers on the stem.

Reading the Cap

Cap width is 4-12 cm, sometimes up to 15. The shape is convex when young, becoming broadly convex to nearly flat with age, with an even, non-overhanging margin.

The surface is dry, finely tomentose or felted when young, smoothing with age, and in dry weather it often cracks into a fine mosaic of small tiles with paler flesh visible in the fissures. Colour is the point: pale grey, mouse-grey, ash-grey or grey-brown, sometimes with darker fibrillose streaks over the disc, occasionally browning in old or sun-exposed specimens. There is no viscidity even after rain.

The Pore Surface

The tube layer is whitish to pale grey when fresh, dulling to greyish, greyish-yellow or dingy buff as it matures. The pores are small and round and the tubes are shallow, depressed around the stipe apex.

Now do the bruising test, because it is one of the two best characters. Press the pore surface with a thumbnail and wait. In Retiboletus griseus the pressed area slowly turns brown, then darker brown to blackish-brown. It does not turn blue at any stage. That brown-to-black staining, developing over a minute or two rather than instantly, is a reliable supporting feature and is echoed by the way handled fruit bodies pick up dark fingerprints in a collecting box.

The Stipe and Its Net

This is the character that names the genus (Retiboletus means netted bolete). The stipe is 4-10 cm long and 1-2.5 cm thick, roughly equal or tapering slightly downward, solid, and covered over at least the upper half - often nearly the whole length - with a coarse, raised, ridged reticulum.

Watch the net's colour, because it changes. It starts whitish to pale grey against a whitish-grey stipe, and darkens with age and handling to brown, dark brown or almost black, so an old fruit body looks as though its stem has been inked over a mesh.

Then check the base. In many collections the lower stipe is flushed chrome-yellow to yellow-orange on the surface, and the flesh inside the base is yellow. That yellow base under a grey netted stipe is a strong supporting character, and it is the family resemblance to the all-yellow Retiboletus ornatipes.

Flesh and Colour Change

Cut the fruit body in half from top to bottom and watch for a couple of minutes. The flesh is white and firm; it slowly discolours greyish, brownish or blackish where cut and handled, sometimes passing through a faint pinkish-brown first, and the flesh in the stipe base is often distinctly yellow. Crucially, there is no blue reaction anywhere - not in the cap flesh, not in the tubes, not in the stipe.

Odour and texture are unremarkable; the useful information here is colour and its timing.

Taking and Reading the Spore Print

Remove the cap, set it pore-side down on white card, cover it to keep it humid, and leave it several hours or overnight. The deposit is olive-brown to brown - the standard bolete print colour, and useful mainly for ruling out the pink-spored Tylopilus species that share the woodland.

Microscopically the spores are smooth, subfusiform and roughly 9-13 by 3-4.5 micrometres.

Substrate, Season and Ecology

Retiboletus griseus is ectomycorrhizal, and its preferred partner is oak; it also associates with beech and other broadleaves in mixed stands. It fruits from soil, never from wood, typically on well-drained slopes, roadside banks within woodland, and along path edges where the humus layer is shallow.

The season runs from early summer into autumn, with the strongest flushes a week or two after heavy rain in warm weather. Fruit bodies appear singly, in small scattered groups, or occasionally in loose arcs following the spread of the root system below. It is a characteristic species of eastern North American oak woodland.

Separating the Look-alikes on Morphology Alone

• Retiboletus ornatipes. The obvious sister species. Everything that is grey in R. griseus is yellow here: a yellow to olive-yellow cap, a bright chrome-yellow stipe with a bold yellow net, yellow flesh and yellow pores. Intermediate-looking specimens are best judged on the cap and interior flesh colour.
• Leccinum species. Similar stature and often a dry cap, but the stipe is decorated with dark tufted scabers rather than a raised net, and the cut flesh usually goes grey, vinaceous or black quickly.
• Tylopilus felleus. Carries a coarse dark net too, but the cap is pale tan to brown rather than grey and the pore surface ripens to pink, with a pink spore print.
• Tylopilus plumbeoviolaceus. Young caps and stipes are violet-tinged, pores ripen pink, and there is no strong reticulum.
• The porcini group (Boletus edulis and relatives). Also netted, but the net is fine and white on a stout, often swollen pale stipe, the cap is brown and slightly greasy in wet weather, and the pores age yellow to olive rather than staining blackish.
• Hemileccinum and Xanthoconium species. Brown caps with white to yellow pores, but the stipe lacks a coarse raised net.
• Strobilomyces species. Grey-black, but shaggily scaled all over and blackening strongly when cut.
• Blue-staining boletes generally. Any prompt blue reaction in flesh or tubes takes you out of Retiboletus griseus at once.