Caloboletus calopus

Description

Caloboletus calopus is a fungus of the bolete family, found in Asia, Northern Europe, and North America. Appearing in coniferous and deciduous woodland in summer and autumn, the stout fruit bodies are attractively colored, with a beige to olive cap up to 15 cm (6 in) across, yellow pores, and a reddish stipe up to 15 cm (6 in) long and 5 cm (2 in) wide. The pale yellow flesh stains blue when broken or bruised. It is often found under oak trees as well as under Beech, but nearly always on chalky soil.

Although not poisonous, this mushroom will ruin any dish you put it in due to its extreme bitterness.

Common names: Bitter Beech Bolete, Scarlet-stemmed Bolete.

Mushroom Identification

1. Cap

   5 to 14 cm across, often irregularly lobed; various shades of smoky-gray, sometimes with an olivaceous flush, the cap of Caloboletus calopus is initially slightly downy, becoming smooth at maturity; occasionally cracking or developing small scales in the cap center.

2. Tubes and Pores

   The yellow tubes of the Bitter Beech Bolete terminate in tiny yellow pores that turn blue-green when cut or bruised. (Small areas of blueing can be seen in the close-up picture on the left.)

3. Stem

   7 to 9 cm tall and 3 to 5cm in diameter; often curved at base; lemon yellow at the apex and flushed red below, becoming a deeper red at maturity; covered in a pale yellowish net pattern (left). In all but the driest of weather, it is not uncommon to find chunks of the stem missing long before a fruitbody has fully developed and taken on its blushing appearance.

4. Flesh

   The cap and stem flesh is pale straw yellow, quickly turning white when cut and then later turning a striking turquoise blue. The yellow tubes turn blue-green when cut and exposed to air.

5. Spores

   12-16 x 4.5-6μm, subfusiform (narrowly spindle-shaped).

6. Spore Print

   Olivaceous snuff-brown.

7. Odor and Taste

   Bitter taste; strong and unpleasant fungal smell.

8. Habitat & Ecological Role

   Mainly on alkaline or neutral soil beneath Beech trees and deciduous oaks. In common with other boletes found in Britain and Ireland, Caloboletus calopus is an ectomycorrhizal fungus, which means that it forms symbiotic relationships with the root systems of trees. The Bitter Beech Bolete has been found to associate with pine and spruce trees as well as with its more usual hosts, beeches and oaks.

Look-Alikes

1. Suillellus luridus

   Has a darker cap and orange flesh in the stem base; it blues instantly when cut.

2. Rubroboletus satanas

   Has a white cap and orange or red pores when mature; its flesh turns pale blue when cut and then fades back to its original pallid color.

3. Xerocomellus chrysenteron

   Fruit bodies in poor condition could be confused, but the stipes of this species are not reticulated.

History

Caloboletus calopus was first named and described scientifically in 1801 by Christiaan Hendrik Persoon, who called it Boletus calopus. In 2014 Italian mycologist Alfredo Vizzini transferred this bolete to the new genus Caloboletus based on new DNA findings; its name then became Caloboletus calopus, the type species of this new genus.

Synonyms of Caloboletus calopus include Boletus calopus Pers.

The generic name Caloletus comes ​​from the Greekcalo- is derived from Greek Calo- meaning pretty, and -bolos meaning 'lump of clay'. In similar vein, the specific epithet calopus means 'pretty foot' - a reference to the graduated yellow-to red coloring of the reticulate (netted) stem of this mushroom.

Caloboletus calopus Biochemistry

Although it is an attractive-looking bolete, Caloboletus calopus is not considered edible on account of its very bitter taste, which does not disappear upon cooking. There are reports of it being eaten in far eastern Russia and Ukraine. The bitter taste is largely due to the compounds calopin and a δ-lactone derivative, O-acetylcyclocalopin A. These compounds contains a structural motif known as a 3-methylcatechol unit, which is rare in natural products. A total synthesis of calopin was reported in 2003. The frustosus variety is reported as causing severe sickness in Europe.

The pulvinic acid derivatives atromentic acid, variegatic acid, and xerocomic acid are present in B. calopus mushrooms. These compounds inhibit cytochrome P450—major enzymes involved in drug metabolism and bioactivation. Other compounds found in the fruit bodies include calopin B, and the sesquiterpenoid compounds cyclopinol and boletunones A and B. The latter two highly oxygenated compounds have significant free-radical scavenging activity in vitro. The compounds 3-octanone (47.0% of total volatile compounds), 3-octanol (27.0%), 1-octen-3-ol (15.0%), and limonene (3.6%) are the predominant volatile components that give the fruit body its odor.

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: Björn S... (CC BY-SA 2.0)

Photo 4 - Author: Tomasz Sobczak (CC BY-SA 4.0)

Photo 5 - Author: James Lindsey (CC BY-SA 2.5)