Lurid bolete

Appearance

''Suillellus luridus'' is a stout fungus with a thick yellow-olive to olive-brown convex cushion-shaped cap that can reach 20 cm 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 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 1–2 cm 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 , but this is not always present and subhymenial flesh can occasionally be yellow or straw-coloured. The stem is 8–14 cm tall and 1–3 cm wide, and bears a distinctive, elongated orange-red reticulum 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 , measuring 11–15 μm long by 4.5–6.5 μm wide and have a median spore quotient of 2.2. The basidia 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 are fuse-shaped with swollen middles and long necks, measuring 33–48 by 7.3–13.5 μm; cheilocystidia 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 sulphate 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.

Naming

The English common name is lurid bolete. Both ''S. luridus'' and ''Boletus satanas'' are known as ''ayimantari'' , in Eastern Turkey.''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 spores than ''S. luridus'', measuring 13.3–14.7 × 4.9– 5.5 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 , 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 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 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.

A number of extra-European boletes share a similar appearance with ''S. luridus'' and have been a source of confusion in past. ''Suillellus hypocarycinus'' and ''Boletus subvelutipes'' , can be somewhat similar, but lack reticulation on the stem. Initially collected in Michigan under oak, ''Boletus vinaceobasis'' resembles ''S. luridus'', but has shorter spores and its cystidia are dark brown in Melzer's reagent. This species' phylogenetic position also remains unresolved. Also in North America, ''Rubroboletus pulcherrimus'' can be somewhat similar, but has a more robust stem and deeper red pores. The Chinese species ''Neoboletus sinensis'', originally described as a form of ''S. luridus'' but now placed in a different genus, has considerably larger spores, reported to reach 12–17 by 5.5–7 μm. Collections closely resembling ''S. luridus'' have also been recorded in Australia, though later renamed ''Boletus barragensis'' as they differ in spore size and a preference for trees of the family Myrtaceae.

Distribution

The fungus grows in a mycorrhizal association with broad-leaved trees such as oak , birch , chestnut and beech , on chalky soils. In the Czech Republic, the variety ''rubriceps'' has been reported growing under linden . It is also suspected of being a mycorrhizal associate of subshrub rock roses in the genus ''Helianthemum''.

Field studies indicate that the fungus, when paired as a mycorrhizal partner with seedlings of the conifer ''Cunninghamia lanceolata'', increases the seedling's survival rate, augments its height and ground diameter, and increases the chlorophyll content in the leaves. A similar growth-enhancing effect had been noted earlier with ''Pinus taiwanensis'' seedlings. These beneficial effects on plant growth are a result of multiple interactions among the fungus, host plant, and indigenous soil microbes that increase the biomass of carbon, and increase the bacterial diversity in the mycorrhizosphere. In a study comparing the salinity resistance of three common ectomycorrhizal fungi , ''S. luridus'' was the most tolerant to high concentrations of salt, and is a good candidate species for the inoculation of tree seedlings to be planted on saline soil. Fruit bodies grow singly or scattered on the ground, from June to November after summer rains. ''S. luridus'' may occur in parks near a single tree, though it is very rarely found in acidic soils.

The predilection of insects for this mushroom was noted by 19th-century British mycologist Anna Maria Hussey, who wrote in 1847: there are very few of the soft-fleshed tribes, all of which are the nurseries of innumerable insects, so much in favour as the poisonous ''Boletus luridus'', on breaking an old one it is a living mass of larvae. Our present subject is so soon attacked by insects that it is very rare to find specimens devoid of wriggling life, and being a very common and abundant kind, it must be of great service in the economy of insect existence. Several fly species have been recorded feeding on the fruit bodies, including ''Phaonia boleticola'', ''P. rufipalpis'', ''Thricops diaphanus'', and, in North America, ''Drosophila falleni'', ''Pegomya mallochi'', ''P. winthemi'', ''Megaselia pygmaeoides'', and ''Muscina assimilis''. In contrast, slugs tend to avoid consuming this species.

Based on phylogenetically verified collections and belowground DNA studies of mycelial distribution, the fungus appears to be native to Europe and has been so far documented in Austria, Cyprus, Denmark, Estonia, France, Italy, Montenegro and Sweden. Its distribution may extend east to the Black Sea and eastern Anatolia regions of Turkey, and south to the Bar'am Forest in the Upper Galilee region of northern Israel, although these reports are in need of molecular verification.

A number of extra-European reports can be found in literature predating DNA studies, ranging from India and Pakistan, to Canada, the United States, Mexico, Costa Rica, China and Taiwan, but these have not been confirmed by molecular testing and are more likely to represent similar, misidentified taxa.