Hemileccinum impolitum

Description

Hemileccinum impolitum is a basidiomycete fungus of the family Boletaceae, native to Europe. It is commonly referred to as the iodine bolete, because its fruit bodies tend to emit an iodine odor when cut, more detectable in the stem base or overripe specimens. The color ranges from light tan, pale brown, chestnut-brown, grey, ochraceous-brown, grayish-brown or olivaceous-brown and the cap of young fruit bodies is initially covered in a velvety, finely filamentous silvery-gray coating that disappears in age.

Like other members of the family, H. impolitum has tubes and pores instead of gills in the hymenial surface of its fruit bodies. It is widely distributed in temperate and southern Europe, where it grows in mycorrhizal symbiosis with broad-leaved trees, particularly oak (Quercus).

Hemileccinum impolitum is generally considered edible although hardly delectable, but because of its rarity, this mushroom should not be picked for the pot.

Common names: Iodine Bolete.

Mushroom Identification

1. Cap

   Ranging from 5 to 12cm in diameter when fully expanded, the cap of Hemileccinum impolitum is clay brown to reddish beige and finely velvety at first, becoming smooth and dry except during wet weather. Young fruitbodies have rounded and domed caps, but with age, they often develop slightly unevenly as though hit with a ball-pein hammer. When cut, the pale lemon-yellow flesh of Boletus impolitus may after a long delay turn faintly pink or in some instances faintly blue.

2. Tubes and Pores

   The tubes (5 to 15mm in length) and the rounded pores ofHemileccinum impolitum are initially lemon yellow, becoming deeper yellow before age. When cut and exposed to the air the tubes do not significantly change color.

3. Stem

   The stipe of the Iodine Bolete is pale yellow, often with a red flush on its lower part, and the stem surface is granular to slightly floccose (covered in tiny fleecy or woolly scales) but never reticulate. There is a distinct iodoform odor in the lower part of the stem when it is cut or torn.

   Ranging from 5 to 15 cm tall and typically 2 to 4cm in diameter, stems are usually more or less cylindrical but slightly fatter at the base.

4. Spores

   Subfusiform, 10-16 x 4-6.5µm.

5. Spore Print

   Olivaceous brown.

6. Odor and Taste

   Young specimens have a mild taste and no distinctive odor except when the lower part of the stem is cut and releases a distinctly iodoform odor.

7. Habitat & Ecological Role

   This attractive, large bolete is found most often on heavy clay soil beneath oaks and occasionally other broadleaf trees. Very occasionally on mainland Europe, it appears also under conifers, and then nearly always pines.

8. Season

   Midsummer to the end of autumn in Britain and Ireland but sometimes continuing into the New Year in southern Europe.

Look-Alikes

1. Boletus delipatus

   Very similar and can only be distinguished from Hemileccinum impolitum by microscopic study of the cap cuticle; Boletus delipatus is even rarer than the Iodine Bolete and is currently recorded from only one site in southern England.

2. Rubroboletus satanas

   Has a chalky white cap, red pores and a bulbous red stem.

3. Caloboletus calopus

   Has a pale cap and yellow pores; its reticulate stem is yellow near to the apex and red towards the base.

4. Hemileccinum depilatum

   The sister-species of H. impolitum and morphologically very similar, differing by its wrinkled or "hammered" cap surface, and its association hornbeam (Carpinus) or hop-hornbeam (Ostrya). Microscopically it is distinguished by the structure of its cap cuticle, which is a palisadoderm composed of spherical and shortly cylindrical cells.

5. Leccinellum lepidum

   Can also look very similar, but typically has a viscid cap with a wrinkled or "hammered" surface not turning violet in NH3, while its flesh slowly turns violaceous-gray and finally grayish-black when exposed to the air. Microscopically it has longer spores, often reaching 20 μm in length.

6. Xerocomus subtomentosus

   Lacks scabrosities on the stem surface, while its pores are larger, angular and stain bluish when bruised. When longitudinally cut, its flesh is pinkish-brown in the lower part of the stem and sometimes discolors faintly bluish in the cap.

History

The iodine bolete was first described by Elias Magnus Fries, an eminent mycologist of the 19th century, who placed the fungus in genus Boletus. The Latin epithet impolitum (meaning "rough"), likely refers to the cap of the species, which is initially felty and covered in a finely filamentous coating when viewed under a magnifying glass. The species' taxonomic position had long remained uncertain and various authors had placed it in different genera in the past, including the now-abandoned genera Tubiporus and Versipellis.

Based on preliminary analysis of the 28S ribosomal RNA locus, mycologists Manfred Binder and Halmut Besl placed the species in Xerocomus in 2000. However, in 2008 Josef Šutara transferred the fungus to the new genus Hemileccinum, based on its distinctive morphology. More elaborate phylogenetic studies by Wu and colleagues in 2014, confirmed that the iodine bolete does not belong in Boletus, Xerocomus or Leccinum, since collections identified as this species occupied a distinct phylogenetic lineage within the subfamily of Xerocomoideae, closely related to Corneroboletus. Subsequent contributions by R. Halling and colleagues, and M. Loizides and colleagues, have since confirmed the monophyly of the genus, which presently includes just two European species: H. impolitum and H. depilatum.

Photo sources:

Photo 1 - Author: Lukas from London, England (CC BY-SA 2.0)

Photo 2 - Author: Lukas from London, England (CC BY-SA 2.0)

Photo 3 - Author: George Chernilevsky (Public Domain)

Photo 4 - Author: Lukas from London, England (CC BY-SA 2.0)