Cyanoboletus pulverulentus

Description

Cyanoboletus pulverulentus is an edible bolete mushroom. The cap is convex, flat when old, dark reddish-brown becoming lighter with age, and grows up to 8 cm (3.1 in) in diameter. The flesh is yellow, with a mild taste, and immediately turns blackish-blue when handled.

It is found in deciduous and mixed forests, particularly on moist soil on slopes and under beech and oak trees. A common species, it is found in northern Asia, Europe, North Africa, Central and northern South America, and eastern North America. All parts of the mushroom will stain dark bluish-black after handling.

The mushrooms used to be considered edible, but a recent study has revealed this mushroom hyperaccumulates arsenic (with concentrations exceeding 1000 mg/kg in dry weight) and therefore its consumption should be restricted.

DNA testing moved this mushroom from Boletus to a newly erected genus called “cyanodoboletus” (“cyano” referring to the characteristically brilliant blue stains). The parenthesis are there on the name because pulverulentus is a European name, DNA has proven the American lookalike is a different species, but the replacement North American name has yet to be adopted.

Common names: Ink Stain.

Mushroom Identification

1. Cap

   Immature specimens are finely velvety and convex, expanding to between 4 and 10cm across and becoming smooth and broadly convex but not flattening completely. The dry cap surface is a dirty dark brown, sometimes cracking and turning reddish in and around the cracks; the cap surface turning black when bruised.

   The yellow cap flesh of Boletus pulverulentus turns blue-black if it is cut and exposed to air.

2. Tubes and Pores

   Beneath the cap, yellow spore tubes terminate in large angular pores that are at first yellow (occasionally pale orange) but turn blue-black if touched. When cut or broken open, the tubes also turn blue-black.

3. Stem

   1 to 2.5cm in diameter and 4 to 8cm tall, the stem of a Blackening Bolete is cylindrical or slightly tapering towards the base. A brilliant yellow at the apex, the stem surface is reddish-brown towards the base, and like the rest of this scruffy-looking mushroom, it turns blue-black wherever it is bruised in handling.

   The yellow stem flesh turns rapidly blue-black when cut and exposed to air.

4. Spores

   Subfusiform to broadly ellipsoid, smooth, 11-14 x 4.5-6µm.

5. Spore Print

   Olive-brown.

6. Odor and Taste

   Not distinctive.

7. Habitat & Ecological Role

   Boletus pulverulentus is most commonly found under beech or oak trees, but this ectomycorrhizal species is also sometimes found under other kinds of hardwoods. Less commonly the Blackening Bolete is recorded as occurring under conifers.

Look-Alikes

1. Neoboletus luridiformis

   Is equally sensitive to handling and its cap, pores and red-dotted stem turn dark blue when bruised; however, its pores are red rather than yellow.

2. Boletus oliveisporus

   The eastern North American lookalike can be distinguished from C. pulverulentus by the pink to reddish color in the center section of its stipe.

3. Neoboletus luridiformis

   Also go a dark blue, but the pores are orange/red and the cap a darker color, and the stem more orange/red too.

4. Caloboletus calopus

   Has a much redder stem base and the cap is paler, it does not turn as dark blue.

5. Imleria badia

   Is a much browner species with pale pores, not chrome yellow, and turns a lighter blue.

6. Rubroboletus satanas

   Has a much paler, almost white cap and a very red stem and pores and slowly turns blue when damaged then back to the pale flesh color.

History

Boletus pulverulentus was first described in 1836 by the German mycologist Wilhelm Opatowski (1810 - 1838). Its synonyms include Xerocomus pulverulentus (Opat.) E.-J. Gilbert.

The fungus was transferred to the newly created genus Cyanoboletus in 2014, where it is the type species. Based on the 28S rDNA, North American collection of this fungus reported in the Genbank database (accession number KF030313) does not match that from Europe.

The generic name Boletus comes from the Greek bolos, meaning 'lump of clay', while the specific epithet pulverulentus means 'covered in powder' - a reference to the dry, finely velvety or slightly powdery surface of the caps and stems of young specimens.

Photo sources:

Photo 1 - Author: Björn S... (CC BY-SA 2.0)

Photo 2 - Author: Björn S... (CC BY-SA 2.0)

Photo 3 - Author: Lukas from London, England (CC BY-SA 2.0)

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

Photo 5 - Author: walt sturgeon (Mycowalt) (CC BY-SA 3.0)