What You Should Know
Rubroboletus rhodoxanthus is a species of bolete in the family Boletaceae, native to Europe. It produces large, colorful fruit bodies with pink patches on the cap, red pores in the hymenial surface, and has a robust stem decorated in a dense, red-colored network pattern.
When longitudinally sliced, its flesh is distinctly bright yellow in the stem and discolors blue only in the cap, an excellent diagnostic feature distinguishing it from similar species. It is inedible and may cause adverse gastrointestinal symptoms if consumed.
The fungus is more widespread in warm broad-leaved forests of southern Europe, where it grows in mycorrhizal symbiosis with trees of the family Fagaceae, particularly oak (Quercus) and beech (Fagus). However, it is rare in northern regions and regarded as critically endangered or extinct in some countries.
Previously known as Boletus rhodoxanthus, it was transferred in 2014 to the newly erected genus Rubroboletus, based on DNA data.
Other names: Ruddy Bolete, Rosensopp (Sweden), Papegøyerørsopp (Norway), Blasshütiger Purpurröhrling (Austria), Roodnetboleet (Netherlands), Hřib Nachový (Czech Republic).
Rubroboletus rhodoxanthus Mushroom Identification
The cap is at first hemispherical, gradually becoming convex to almost flat as the fungus expands, with a diameter of 10 to 20 cm (4 to 8 in), but can sometimes grow up to 30 cm (12 in). It is at first slightly velvety and colored mostly whitish-gray, but soon becomes smooth, pinkish-gray, pinkish-beige, or pinkish-red, especially towards the margin or when handled.
Tubes and Pores
The tubes are adnate to emarginate, 0.5 to 1.5 cm (0 to 0.5 in) long and initially yellow, becoming somewhat olivaceous-yellow in very mature fruit bodies and staining blue when cut. The pores (tube mouths) are orange to deep red and instantly bluing when handled.
The stem is 8 to 12 cm (3 to 4.5 in) long by 3 to 6 cm (1 to 2.5 in) wide, bulbous or clavate when young, becoming more elongated and cylindrical at maturity. It is orange or orange-yellow at the top (apex), gradually becoming orange-red to carmine-red in the lower part and bears a dense, orange-red to carmine-red reticulation (network pattern).
The flesh is distinctly bright yellow and unchanging in the stem, but paler and turning blue when cut only in the cap. It has a mild taste.
The spores are olive-brown in mass. When viewed under the microscope they are ellipsoid to fusiform (spindle-shaped), measuring 10–15.5 by 4–5.5 μm. The cap cuticle is a trichodermium of septate cylindrical hyphae, sometimes finely incrusted.
Rubroboletus rhodoxanthus Look-Alikes
Very similar, but has a distinctive smell of chicory or hay and whitish flesh that stains blue in the cap, as well as the stem when cut.
Has a whitish cap without flushes of pink and whitish flesh that usually stains pale blue also in the stem when cut.
Is mycorrhizal with spruce (Picea) or fir (Abies) and has pale yellow flesh that stains blue throughout.
Has brighter colors on the cap ranging from pale grey to blood-red or purple, has yellow flesh that stains weakly to moderately blue throughout, and microscopically has smaller spores, measuring 12.5–14 × 4.5–5 μm.
Differs by its pinkish-red to crimson-red cap that has a roughened or "hammered" appearance and stains blue when handled, but also by its flesh that stains intensely dark blue throughout.
Rubroboletus rhodoxanthus Taxonomy and Etymology
The fungus was first described in 1836 by Czech mycologist Julius Vincenz von Krombholz, who considered it to be a variety of Boletus sanguineus. In 1925, it was recombined as a distinct species by German mycologist Franz Joseph Kallenbach, and the fungus remained in genus Boletus until 2014. The species epithet is derived from the Ancient Greek words ρόδο (rhódo, "rose" or "pink") and ξανθός (xanthós, "blonde" or "fair").
The first extensive phylogenetic studies on Boletaceae in 2006 and 2013, indicated that Boletus was not monophyletic and hence an artificial arrangement. A 2014 study by Wu and colleagues recognized 22 generic clades within Boletaceae, concluding that Boletus dupainii and some closely related red-pored species belong to a distinct clade, distant from the core clade of Boletus (comprising Boletus edulis and allied taxa). The new genus Rubroboletus was therefore described to accommodate species in this clade and B. rhodoxanthus was transferred to this genus. The placement of the species in genus Suillellus, following online recombination by Blanco-Dios, was not supported by molecular data and has been subsequently rejected by later authors.
Photo 1 - Author: Irene Andersson (irenea) (Creative Commons Attribution-Share Alike 3.0 Unported)
Photo 2 - Author: GLJIVARSKO DRUSTVO NIS from Serbia (Creative Commons Attribution 2.0 Generic)
Photo 3 - Author: amadej trnkoczy (amadej) (Creative Commons Attribution-Share Alike 3.0 Unported)
Photo 4 - Author: MichelBeeckman (MichelBeeckman) (Creative Commons Attribution-Share Alike 3.0 Unported)
Photo 5 - Author: Gerhard Koller (Gerhard) (Creative Commons Attribution-Share Alike 3.0 Unported)
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