Suillellus luridus (formerly Boletus luridus), commonly known as the lurid bolete, is a fungus of the family Boletaceae, found in calcareous broadleaved woodlands in Europe. Fruit bodies appear in summer and autumn and may be locally abundant. It is a firm bolete with an olive-brown cap up to in diameter, with small orange or red pores on the underside (yellow when young). The stout ochre stem reaches high and wide, and is patterned with a red network. Like several other red-pored boletes, it stains blue when bruised or cut.
While edible and good when cooked, it can cause gastric upset when eaten raw and can be confused with the poisonous Boletus satanas; as a result, some guidebooks recommend avoiding consumption altogether. When consumed with alcohol, Suillellus luridus has been implicated in causing adverse reactions similar to those caused by the compound coprine, though laboratory testing has not revealed any evidence of coprine in the mushroom.
First described in 1774, the species has been transferred to various Boletaceae genera in its taxonomic history, although it retained the original name given to it by German botanist Jacob Christian Schaeffer until a transfer to genus Suillellus in 2014. Several varieties, a subspecies, and a form have been described by European mycologists. Suillellus luridus is mycorrhizal, forming a symbiotic association with broad-leaved trees trees as oak, chestnut, birch and beech, and has been found to have a growth-enhancing effect on conifers in experiments. The fruit bodies are highly attractive to, and often infested by, insects, and several species of fly have been recorded feeding on them. Chemical analyses have revealed some aspects of the mushroom's components, including its volatile flavour compounds, its fatty acid and amino acid compositions, and the identities of the carotenoid compounds responsible for its colour.
Taxonomy and phylogeny
Boletus luridus was described by German botanist Jacob Christian Schäffer in 1774, in his series on fungi of Bavaria and the Palatinate, Fungorum qui in Bavaria et Palatinatu circa Ratisbonam nascuntur icones. The specific epithet is the Latin adjective luridus, 'sallow'. French botanist Pierre Bulliard's 1791 Boletus rubeolarius is a heterotypic synonym (based on a different type). The following year, Johann Friedrich Gmelin called it Boletus subvescus, from the Latin words sub "nearly" or "under", and vescus "edible". However, this is a nomen nudum. Several taxonomical synonyms arose when the species was transferred to different genera within the family Boletaceae by different authorities, including Leccinum by Samuel Frederick Gray in 1821, Tubiporus by Petter Karsten in 1881, Dictyopus by Lucien Quélet in 1888, and Suillellus by William Murrill in 1909.
The variety Boletus luridus var. erythropus, published as "beta" by Elias Magnus Fries in his 1821 Systema Mycologicum, is synonymous with Boletus erythropus. Boletus luridus var. rubriceps was originally described from Spain (as a species of Tubiporus) by René Maire in 1937, and later formally transferred to Boletus by Aurel Dermek in 1987. Other varieties of B. luridus include Roman Schulz's var. obscurus and var. rubromaculatus published in 1924; Josef Velenovský's 1939 var. tenuipes, found in the Czech Republic; and Jean Blum's 1969 var. lupiniformis and var. queletiformis, originally described from France and Spain, respectively. Boletus erythrentheron, originally described as a distinct species by Jan Bezděk, was later recombined as the variety B. luridus var. erythrentheron by Albert Pilát and Dermek in 1979, and finally as a subspecies by Jiri Hlavácek in 1995. Carmine Lavorato and Giampaolo Simonini defined the form primulicolor from Sardinia in 1997. Rolf Singer's 1947 variety caucasicus, later recombined as an independent species, Boletus caucasicus Singer ex. Alessio, has never been validly published and is a nomen nudum. Boletus luridus f. sinensis, found in Hainan Province, China, was later elevated to species status and transferred to another genus with the name Neoboletus sinensis.
Boletus luridus was the type species of Boletus section Luridi, originally circumscribed by Fries in 1838. This section traditionally included species producing medium to large fruit bodies with thick, swollen stems, and minute pores that are coloured red, orange, or rarely yellow. However, early phylogenetic investigations indicated that Boletus is strongly paraphyletic in its traditional delimitation. As further studies have resolved phylogenetic and taxonomical relationships to a finer detail, Boletus has been fragmented and additional genera were recognised. Molecular phylogenetics inferred from ribosomal DNA sequences by Manfred Binder and David Hibbett, showed B. luridus to be related to a group containing B. torosus and B. luteocupreus (now placed in genus Imperator), with B. vermiculosus and Pulveroboletus ravenelii as more distant relatives. In a separate molecular study by Mello and colleagues, B. luridus clustered together with B. rhodoxanthus, while further genetic analyses in 2013 indicated that B. luridus and some red-pored boletes are part of a dupainii clade (named after Boletus dupainii), quite distant from the core group of Boletus edulis and its relatives. However, more refined analyses based on a larger number of sequences, have since demonstrated that B. luridus and its allied species form a distinct generic clade, and since 2014 the fungus has been placed in genus Suillellus.
Common names The English common name is lurid bolete. Both S. luridus and Boletus satanas are known as ayimantari (meaning "bear mushroom"), in Eastern Turkey.
Description
Suillellus luridus is a stout fungus with a thick yellow-olive to olive-brown convex cushion-shaped cap that can reach in diameter. The cap colour tends to darken with age, and regions of red, orange, purple, brown, or olive-green can often be present. The cap surface is finely tomentose (velvety) at first, becoming smoother with old age, and viscid in wet weather. The pore surface is initially yellowish-orange or orange, before turning orange-red to sometimes red and stains strongly blue when injured or handled. The pore surface usually has a lighter-coloured zone encircling the margin, as the pores tend to darken from their point of attachment to the stem outwards. There are 2–3 rounded pores per millimetre, and the tubes are long. The tubes are shorter around the cap margin and close to the stem, where they form a circular depression. Initially pale yellow, the tubes gradually become olive-yellow and stain bluish-green upon exposure to air. A frequent feature is the presence of a maroon layer between the tubes and the flesh (known as Bataille’s line), but this is not always present and subhymenial flesh can occasionally be yellow or straw-coloured. The stem is tall and wide, and bears a distinctive, elongated (or "stretched") orange-red reticulum (network) pattern on a paler yellowish, orange, or ochre background, often becoming darker and vinaceous towards the base. The flesh is yellowish, sometimes with red patches in the cap but almost always rhubarb to vinaceous-red towards the stem base, and stains an intense dark blue when bruised or cut. There is a faint sour smell, and the taste is described as mild. The mycelium is an unusual yellow colour.
Variety queletiformis can be distinguished from the main form by the reddish discolouration of the stem base that occurs both on the exterior surface and in the flesh. Variety rubriceps has a deep crimson red cap, while var. lupiniformis has a pale yellow or dirty ochre cap and pores, sometimes with pink tones throughout.
The spore print is olive to brownish olive. Under the microscope, the spores are elliptical to somewhat fusiform (spinde-shaped), measuring 11–15 μm long by 4.5–6.5 μm wide and have a median spore quotient of 2.2. The basidia (spore-bearing cells) are club-shaped and four-spored, and measure 29.2–36.5 by 11.0–12.4 μm. Cystidia on the sides of the tubes (pleurocystidia) are fuse-shaped with swollen middles and long necks, measuring 33–48 by 7.3–13.5 μm; cheilocystidia (on the edges of the pores) have a similar morphology. The cap cuticle is made of cylindrical hyphae 3.7–5.8 μm wide that are interwoven compactly, and the hyphal tips are erect and arranged in bundles. In contrast, the hyphae of the cap flesh is loosely interwoven with hyphae that are cylindrical and branched, measuring 3.7–8.8 μm. Hyphae do not contain clamp connections.
Some chemical tests can be used to help identify the mushroom. A drop of dilute potassium hydroxide placed on the cap cuticle will stain dark red to blackish, and orange-yellow on the flesh, while ferrous sulfate solution turns the cuticle yellow and then greenish-yellow. Melzer's reagent will turn the flesh dark blue, after the natural bluing reaction to injury has faded.
Similar species Suillellus mendax, a species described from Italy in 2014 and subsequently confirmed in Cyprus and France, is very similar to S. luridus and found under the same host-trees. It produces more robust fruit bodies with a markedly tomentose cap, has a reticulum that is less pronounced and often restricted to the upper part of the stem, and is mostly found on acidic rather than calcareous soil. Microscopically, S. mendax has more elongated, narrowly fusiform (spinde-shaped) spores than S. luridus, measuring (12.4–)13.3–14.7(–15.5) × (4.5–)4.9– 5.5(–5.7) μm, and with a higher spore quotient of 2.7. Collections from southern Europe previously classified as Boletus caucasicus on the basis of a yellow subhymenial layer (the flesh in the cap tissue immediately above the tube layer known as Bataille’s line), have been shown to phylogenetically correspond to either S. luridus or S. mendax. As shown by Vizzini and colleagues, the name Boletus caucasicus has been invalidly published (nomen nudum) and the Bataille's line is not reliable for discriminating between species in the Luridi complex, as it can be randomly present or absent in both S. luridus and S. mendax.
Another similar species is Suillellus comptus, a Mediterranean bolete sharing a lot of features with S. luridus and S. queletii. This uncommon species is also found on chalky soil under oak, but generally produces more slender and dull-coloured fruit bodies, with a rudimentary, incomplete, or at times completely absent reticulation, rarely extending below the top (apex) of the stem. Under the microscope, S. comptus has very similar spores to S. luridus, but the hyphae of its cap cuticle are more loose and prostrate, running more or less parallel to the cap. Also in the same genus, Suillellus queletii shares with S. luridus a vinaceous stem base and strongly bluing flesh, but completely lacks reticulation on the stem.
The edible Neoboletus luridiformis can be distinguished from S. luridus by its dark brown cap and absence of any reticulation on the stem; it also grows on sandy soils associated with conifers. In genus Rubroboletus, R. satanas is also found on chalky soils, but produces larger and more robust fruit bodies with a pale cap and differently patterned reticulation to S. luridus. Its flesh does not turn blue so intensely on bruising or cutting, while overripe mushrooms often carry a smell of decay. Another red-pored species in this genus, Rubroboletus rhodoxanthus, has characteristic pinkish tones in the cap and a very dense, differently patterned reticulation. When longitudinally cut, its flesh is bright yellow in the stem and stains blue only in the cap.