Iodine Bolete

The Iodine Bolete is a handsome, pale bolete that is easy to overlook among the many buff-capped species of summer oak woodland, until the stipe base is broken and the characteristic chemical odour gives it away. It is one of the few boletes best confirmed by smell rather than by a colour change.

Formerly Boletus impolitus, it was moved to Hemileccinum, a small genus intermediate in character between the true boletes and Leccinum. Members of the genus share a dry, finely felted or scurfy cap, small yellow pores, and a stipe finely granular or pruinose but without the raised black scabers of Leccinum or the coarse net of Boletus proper.

It is a thermophilic fungus of southern and central Europe, reaching its northern edge in southern Britain, where it is a local speciality of ancient oak parkland on heavy soils.

Cap

Hemispherical then convex and finally almost flat, 6–15 cm or more across. The surface is dry and matt with a fine felty or minutely fibrillose texture, often becoming areolate, that is broken into small mosaic-like cracks, in dry weather. The colour is a soft pale clay-buff, greyish ochre or pale tan, sometimes with faint pinkish or olive flushes, and it does not blue on handling.

Pores

Small, round and bright lemon to chrome yellow, darkening to olive-yellow with age. They typically remain unchanging when bruised, or at most stain faintly brownish; there is no distinct blue reaction. The tube layer is yellow and does not blue in section.

Stipe

Stout, cylindrical to club-shaped and often swollen at the base, 6–12 cm tall. The ground colour is pale to bright yellow, sometimes flushed with reddish-brown or rusty tints in the lower half, and the surface is finely granular, pruinose or minutely scurfy. Crucially there is no raised reticulation and no dark scabers.

Flesh and spore print

Thick and firm, pale lemon yellow, essentially unchanging when cut or at most acquiring a faint pinkish tinge in the stipe base. The flesh of the stipe base emits a strong iodoform-like chemical odour, most obvious when the base is split and left for a minute. The spore print is olive-brown.

Hemileccinum impolitum is ectomycorrhizal, associating almost exclusively with oaks and occasionally with other broadleaves such as beech and sweet chestnut. It favours heavy clay and calcareous soils in warm situations: old oak parkland, wood pasture, sunny woodland margins and grassy rides under veteran trees.

Fruiting runs from June to September, tied closely to warm weather with intermittent rain. The species is widespread in central and southern Europe and around the Mediterranean, thinning northwards; in Britain it is largely confined to southern England on clay and chalk.

As a mycorrhizal partner of oak, this fungus links veteran trees to a wide soil volume, extracting mineral nutrients and buffering summer drought stress on the heavy, cracking clays where it thrives. Its habit of appearing in open wood pasture and parkland reflects the preference of both fungus and host for warm, well-lit conditions with an undisturbed soil profile.

Because it depends on mature and often ancient oaks and on unploughed ground, it is treated as an indicator of long continuity of tree cover, and it is listed as scarce or declining in parts of its northern range.

Hemileccinum depilatum is the closest relative, differing in its distinctly wrinkled and pitted cap surface and its association with hornbeam rather than oak. Butyriboletus appendiculatus has a browner cap, flesh that blues at least faintly, and a fine yellow network over the upper stipe.

The Penny Bun (Boletus edulis) is separated by its brown, slightly greasy cap, its white pores that only later turn yellow-olive, and the prominent white network on its stipe. Members of Xerocomellus are smaller and more slender with cracked caps showing pink flesh in the fissures and pores that blue readily on bruising.