Bitter Bolete

Tylopilus felleus has a broad distribution across temperate regions of the Northern Hemisphere.

• Native to Europe, North America, and parts of Asia
• In Europe, found from Scandinavia southward to the Mediterranean region
• In North America, ranges from eastern Canada through the northeastern and midwestern United States, extending southward to the Appalachian region
• Also reported in parts of Central America and East Asia, though some of these populations may represent closely related cryptic species
• The genus Tylopilus was established by Petter Adolf Karsten in 1881, with T. felleus designated as the type species
• The species was originally described as Boletus felleus by Pierre Bulliard in 1788 before being transferred to the genus Tylopilus

The Bitter Bolete is a robust, fleshy mushroom with distinctive macroscopic features that aid in identification.

Cap:
• 5–15 cm in diameter (occasionally up to 30 cm in exceptional specimens)
• Convex when young, becoming broadly convex to nearly flat with age
• Surface dry, smooth to slightly velvety, sometimes cracking in dry conditions
• Color ranges from light tan to dark brown or chestnut brown
• Margin often slightly overhanging the pore surface

Pore Surface:
• White when young, maturing to pinkish or flesh-colored — a key diagnostic feature distinguishing it from Boletus edulis, which retains white to yellowish pores
• Pores are small, angular, approximately 1–2 per millimeter
• Tubes are 1.5–3 cm deep, adnate to slightly depressed around the stipe
• Bruises pinkish-brown when injured

Stipe (Stem):
• 6–12 cm tall, 2–4 cm thick
• Solid, firm, roughly equal in width or slightly swollen at the base
• Surface covered with a prominent raised brown reticulation (net-like pattern) over a paler background — a distinguishing characteristic
• Color pale tan to brown, darker toward the base

Flesh:
• White to pale cream, unchanging or very slowly pinkish when cut
• Firm and solid when young, becoming softer with age
• Taste intensely and persistently bitter — the single most reliable identification feature
• Odor mild, not distinctive

Spore Print:
• Pinkish-brown to reddish-brown

Spores:
• Smooth, ellipsoid to subfusiform, measuring 10–15 × 3.5–5 μm
• Produced on basidia in the tube hymenium

Tylopilus felleus is an obligate ectomycorrhizal fungus, forming symbiotic relationships with the roots of various hardwood and conifer trees.

Host Trees:
• Primarily associated with oaks (Quercus spp.) and beeches (Fagus spp.)
• Also found in association with pines (Pinus spp.) and other conifers in some regions
• The mycorrhizal partnership enhances the host tree's ability to absorb water and mineral nutrients from the soil, while the fungus receives photosynthetically derived sugars

Habitat:
• Found in deciduous, mixed, and coniferous forests
• Prefers well-drained, acidic to neutral soils
• Often appears on slopes, along forest edges, and in clearings
• Fruits from summer through autumn (June to November in the Northern Hemisphere, depending on local climate)
• Typically solitary or in small scattered groups rather than dense clusters

Ecological Role:
• Contributes to forest nutrient cycling through decomposition of organic matter
• Mycorrhizal networks facilitated by T. felleus may aid in inter-tree nutrient transfer
• Serves as a food source for various invertebrates, including fungus gnats and slugs, despite its bitterness to humans

Tylopilus felleus is not cultivated commercially or in home gardens, as it is a wild mycorrhizal species that cannot be grown using conventional mushroom cultivation techniques.

• Mycorrhizal fungi like T. felleus require a living host tree root system to complete their life cycle
• Unlike saprotrophic mushrooms (e.g., oyster mushrooms, shiitake), they cannot be grown on sterilized substrates such as sawdust or straw
• Attempts at commercial cultivation have not been successful to date
• The species is best appreciated in its natural forest habitat during foraging season
• For those interested in finding it in the wild, look under oaks and beeches in well-drained woodland soils during summer and autumn