Bitter Bolete

Appearance

The cap of this species grows up to 15 cm in diameter, though some North American specimens reach 30 cm across. Grey-yellow to pale- or walnut-brown, it is slightly downy at first and later becomes smooth with a matte lustre. It is initially convex before flattening out with maturity. The cap skin does not peel away from the flesh. The pores underneath are white at first and become pinkish with maturity. They are adnate to the stalk and bulge downwards as the mushroom ages. The pores bruise carmine or brownish, often developing rusty-brown spots with age, and number about one or two per millimetre. The tubes are long relative to the size of the cap, measuring 2–3 cm deep in the middle part of the cap. The stalk is initially bulbous before stretching and thinning in the upper part; the lower part of the stalk remains swollen, sometimes shrinking at the base where it attaches to the substrate. It measures 7–10 cm —rarely to 20 cm —tall, and 2–3 cm wide, and can bulge out to 6 cm across at the base. It is lighter in colour than the cap, and covered with a coarse brown network of markings, which have been likened to fishnet stockings in appearance. Described as "very appetising" in appearance, the flesh is white or creamy, and pink beneath the cap cuticle; the flesh can also develop pinkish tones where it has been cut. It has a slight smell, which has been described as pleasant, as well as faintly unpleasant. The flesh is softer than that of other boletes, and tends to become more spongy as the mushroom matures. Insects rarely infest this species.


The colour of the spore print is brownish, with pink, reddish, or rosy tints. Spores are somewhat fuse-shaped, smooth, and measure 11–17 by 3–5 µm. The basidia are club-shaped, four-spored, and measure 18–25.6 by 7.0–10.2 µm. Cystidia on the walls of the tubes are fuse-shaped with a central swelling, thin-walled, and have granular contents. They possess sharp to tapered tips, and have overall dimensions of 36–44 by 8.0–11.0 µm. On the pore edges, the cheilocystidia are similar in shape to the pleurocystidia, measuring 24.8–44.0 by 7.3–11.0 µm. The hymenium of Smith and Thiers's variety ''uliginosus'', when mounted in Melzer's reagent, shows reddish globules of pigment measuring 2–8 µm that appear in the hyphae and throughout the hymenium, and a large globule in the pleurocystidia.

Several chemical tests have been documented that can help confirm the identify of this species. On the cap flesh, application of formaldehyde turns the tissue pinkish, iron salts result in a colour change to greyish-green, aniline causes a lavender to reddish-brown colour, and phenol a purplish pink to reddish brown. On the cap cuticle, nitric acid causes an orange-salmon colour, sulphuric acid creates orange-red, ammonia usually makes brown, and a potassium hydroxide solution usually makes orange.

Naming

Italian cook and author Antonio Carluccio reports this is one of the most common fungi brought to him to identify, having been mistaken for an edible species. Young specimens can be confused with many edible boletes, though as the pores become more pink the species becomes easier to identify. Some guidebooks advocate tasting the flesh, the smallest piece of which will be very bitter. The dark-on-light reticulation in the stalk is distinctive, and is the opposite colouration of that on the stalk of the prized ''Boletus edulis''. ''T. felleus'' is found in the same habitat as ''B. badius'', though the latter's yellow tubes and blue-bruising flesh easily distinguish it. ''B. subtomentosus'' can have a similar-coloured cap, but its yellow pores and slender stalk aid identification.

''Tylopilus rubrobrunneus'', found in hardwood forests of eastern North America, is similar in appearance to ''T. felleus'', but has a purplish to purple-brown cap. It is also inedible due to its bitter taste. Another North American species, ''T. variobrunneus'', has a cap that is reddish-brown to chestnut-brown, with olive tones in youth. It has shorter spores than ''T. felleus'', typically measuring 9–13 by 3–4.5  μm. In the field, it can be distinguished from the latter species by its mild to slightly bitter taste. ''T. rhoadsiae'', found in the southeastern United States, has a lighter-coloured cap that is smaller, up to 9 cm in diameter. The edible ''T. indecisus'' and ''T. ferrugineus'' can be confused with ''T. felleus'', but have less reticulated stalks. The dimensions of the spores of the Australian species ''T. brevisporus'' range from 9.2 to 10.5 by 3.5 to 3.9 μm. ''T. neofelleus'', limited in distribution to deciduous forests of China, New Guinea, Japan, and Taiwan, can be distinguished from ''T. felleus'' macroscopically by its vinaceous-brown cap and pinkish-brown to vinaceous stalk, and microscopically by its smaller spores and longer pleurocystidia .

Distribution

Like all ''Tylopilus'' species, ''T. felleus'' is mycorrhizal. It is found in deciduous and coniferous woodland, often under beech and oak in well-drained acid soils, which can be sandy, gravelly or peaty. If encountered on calcareous soil, it will be in moist areas that have become waterlogged and have ample leaf litter. Fruit bodies grow singly or in small groups, and occasionally in small clusters with two or three joined at the base of the stem. Fruit bodies have also been growing in the cavities of old trees, on old conifer stumps, or on buried rotten wood. The fungus obtains most of its nitrogen requirements from amino acids derived from the breakdown of proteins, although a lesser amount is obtained from the amino sugar glucosamine . The mycorrhizal plant partner benefits from the fungus's ability to use these forms of nitrogen, which are often abundant in the forest floor. Fruit bodies appear over summer and autumn, anytime from June to October or even November, in many of the northern temperate zones. Large numbers may appear in some years, and none in others, generally proportional to the amount of rainfall. Variety ''uliginosus'', known from Michigan, grows among lichens and mosses under pines.

In North America, it is known from eastern Canada, south to Florida and west to Minnesota in the United States, and into Mexico and Central America. Its European distribution is widespread; it is relatively common in many regions, but rare or almost absent in others. In Asia, it has been recorded from the vicinity of Dashkin in the Astore District of northern Pakistan, and as far east as China, where it has been recorded from Hebei, Jiangsu, Fujian, Guangdong and Sichuan provinces, and Korea.

The strong taste of the fruit body may have some role in insects avoiding it. The small fly species ''Megaselia pygmaeoides'' feeds on and infests the fruit bodies of ''T. felleus'' in North America, though it seems to prefer other boletes in Europe. Fruit bodies can be parasitized by the mould ''Sepedonium ampullosporum''. Infection results in necrosis of the mushroom tissue, and a yellow colour caused by the formation of large amounts of pigmented aleurioconidia .

The bacterium ''Paenibacillus tylopili'' has been isolated from the mycorrhizosphere of ''T. felleus''; this is the region around its subterranean hyphae where nutrients released from the fungus affect the activity of the microbial population in the soil. The bacterium excretes enzymes that allow it to break down the biomolecule chitin.

Fruit bodies of ''T. felleus'' have a high capacity to accumulate radioactive caesium from contaminated soil, a characteristic attributed to the deep soil penetration achieved by the mycelium. In contrast, the species has a limited capacity to accumulate the radioactive isotope 210Po.

Habitat

Like all ''Tylopilus'' species, ''T. felleus'' is mycorrhizal. It is found in deciduous and coniferous woodland, often under beech and oak in well-drained acid soils, which can be sandy, gravelly or peaty. If encountered on calcareous soil, it will be in moist areas that have become waterlogged and have ample leaf litter. Fruit bodies grow singly or in small groups, and occasionally in small clusters with two or three joined at the base of the stem. Fruit bodies have also been growing in the cavities of old trees, on old conifer stumps, or on buried rotten wood. The fungus obtains most of its nitrogen requirements from amino acids derived from the breakdown of proteins, although a lesser amount is obtained from the amino sugar glucosamine . The mycorrhizal plant partner benefits from the fungus's ability to use these forms of nitrogen, which are often abundant in the forest floor. Fruit bodies appear over summer and autumn, anytime from June to October or even November, in many of the northern temperate zones. Large numbers may appear in some years, and none in others, generally proportional to the amount of rainfall. Variety ''uliginosus'', known from Michigan, grows among lichens and mosses under pines.

In North America, it is known from eastern Canada, south to Florida and west to Minnesota in the United States, and into Mexico and Central America. Its European distribution is widespread; it is relatively common in many regions, but rare or almost absent in others. In Asia, it has been recorded from the vicinity of Dashkin in the Astore District of northern Pakistan, and as far east as China, where it has been recorded from Hebei, Jiangsu, Fujian, Guangdong and Sichuan provinces, and Korea.

The strong taste of the fruit body may have some role in insects avoiding it. The small fly species ''Megaselia pygmaeoides'' feeds on and infests the fruit bodies of ''T. felleus'' in North America, though it seems to prefer other boletes in Europe. Fruit bodies can be parasitized by the mould ''Sepedonium ampullosporum''. Infection results in necrosis of the mushroom tissue, and a yellow colour caused by the formation of large amounts of pigmented aleurioconidia .

The bacterium ''Paenibacillus tylopili'' has been isolated from the mycorrhizosphere of ''T. felleus''; this is the region around its subterranean hyphae where nutrients released from the fungus affect the activity of the microbial population in the soil. The bacterium excretes enzymes that allow it to break down the biomolecule chitin.

Fruit bodies of ''T. felleus'' have a high capacity to accumulate radioactive caesium from contaminated soil, a characteristic attributed to the deep soil penetration achieved by the mycelium. In contrast, the species has a limited capacity to accumulate the radioactive isotope 210Po.