Rooting Boletes, Caloboletus radicans

In late summer one of my favourite finds is the Rooting Bolete, Caloboletus radicans. I usually see it when I’m out on the bike, for roadside verges under trees are one of its favourite haunts. It can also be found by paths where the soil is slightly more compacted.

When young, the Rooting Bolete is a very handsome fungus. It has a pale grey cap, yellow pores and the stipe (stem) is yellow and sturdy, with a fine reticulum (net-like pattern) over it. The base of the stipe is sometimes slightly red and tends to taper at its base. The bottom of the stipe often has root-like mycelial strands attached it (note 1).

As it ages, the Rooting Bolete acquires more character, if less beauty. Older specimens often have misshapen, dented and pitted caps.

By maturity a Rooting Bolete’s cap can reach 20cm (nearly 8 inches) across but in spite of its size Caloboletus radicans can be surprisingly inconspicuous. Most people won’t notice it as they speed by in a car and those who do will dismiss it as a large stone.

Seen on a Norfolk verge: Rooting Boletes, Caloboletus radicans. Near Themelthorpe, 29th August 2023.

The Rooting Bolete is mainly found in the southern half of England. It forms an ectomycorrhizal relationship with deciduous trees, nearly always Beech (Fagus sylvatica) and oak (Quercus), though it will also form relationships with Hornbeam (Carpinus betulus), Lime (Tilia) and even Common Rock-rose (Helianthemum nummularium).

It is a symbiotic relationship and both partners benefit from it. The plants benefit from their attachment to the fungus, taking up soil nutrients and water. In exchange the fungus uses some of the carbon compounds manufactured by the plant from sunlight by photosynthesis.

Rooting Boletes, Caloboletus radicans. The bigger specimen in front is old and has lost its good looks.

Rooting Bolete, Caloboletus radicans. This is the younger specimen – still in its prime.

Rooting Bolete, Caloboletus radicans

Many boletes change colour when cut or bruised and the Rooting Bolete bruises a lovely light blue within seconds when sliced in two or when the pores are squeezed.

Rooting Boletes, Caloboletus radicans

Admire but don’t eat

Caloboletus radicans is not an edible fungus. Its smell is described as “unpleasant, slightly astringent”, though I don’t find it at all objectionable. The taste is “intensely bitter” and, more seriously than this, a Swiss study found that Rooting Boletes can cause severe gastrointestinal symptoms when eaten, including recurrent vomiting and bloody diarrhoea (note 2).

So it’s best to look, rather than eat. For assistance with looking, there are good descriptions and pictures on the Discover The Wild, Ultimate Mushroom and Foraging Course Company websites. The First Nature website is especially good and includes pictures of the spores.

A Pretty Lump of Clay

Caloboletus radicans was formerly known as Boletus radicans, by which it is known on some websites (such as Boletales.com) and in older books. The name change dates from 2014 when DNA studies showed it belonged in a separate genus.

The genus name Caloboletus is from “Calo-” meaning pretty and “-bolos” meaning ‘lump of clay’; a very good description of the young cap. The specific name “radicans” means ‘rooting’.

If you’d like to more about boletes in general, I recommend “British Boletes With Keys to Species” by Geoffrey Kibby, currently in its eighth edition. Its taxonomy includes the split of Boletus into several different genera, including Caloboletus (note 3). 

Caloboletus kluzakii

The world of fungi is constantly changing, as mycologists discover new species and split existing species based on DNA studies. A recent article in the British Mycological Society journal, Field Mycology, investigates Caloboletus kluzakii, first described from the Czech Republic in 2006 and now found in the British Isles.

In its original description C. kluzakii differed from Caloboletus radicans as follows:

• C. kluzakii develops a pink-flushed cap due to the presence of a reddish subcuticular layer which becomes progressively exposed as the pallid cuticle collapses and/or is worn away. The reddish cap colour immediately intensifies when scratched or bruised.
• Its yellow, reticulate stem is often flushed reddish brown at the base.

No British specimens were found with these characteristics but DNA sequencing has now found six examples of C. kluzakii in the Kew Fungarium (note 4).

The conclusion is that “it may not be possible to definitively separate C. kluzakii from C. radicans based purely on morphological characters as many of these appear to overlap” (note 5).

Norfolk Fungus Study Group runs a small DNA Barcoding project for fungi. Perhaps the time has come to collect samples to check that our Caloboletus radicans are what we think they are…

Life is complicated, but I think we already knew that. Whatever its true identity, the Rooting Bolete is still worth finding and admiring.

Notes

Note 1 – Boletes have pores instead of gills. Both structures allow the release of spores.

Note 2 – Schenk-Jaeger K.M., Rauber-Lüthy C., Bodmer M., Kupferschmidt H., Kullak-Ublick G.A., Ceschi A. (2012), “Mushroom poisoning: a study on circumstances of exposure and patterns of toxicity”. European Journal of Internal Medicine. Vol. 23, pp85–91.

The full article is behind a paywall, but it is cited in Wikipedia.

Note 3 – And Rubroboletus – see last year’s blog post about Rubroboletus rhodoxanthus. The book is also available from other suppliers such as Pemberley Books.

Note 4 – A fungarium is a collection of dried fungi – the fungal equivalent of a herbarium (dried, labelled specimens of plants). See https://www.kew.org/science/collections-and-resources/collections/fungarium for more about the one at Kew.

Note 5 – Geoffrey Kibby & A. Martyn Ainsworth (2022), “Caloboletus kluzakii newly recorded from Britain (or will the real C. radicans please step forward“. Field Mycology, Vol. 23 (3), pp 95 – 98.

Thanks to the British Mycological Society for making the full article available as a PDF.