Tuber bituminatum

Morphological Features:

Fruit body that in younger and immature specimens adopts a practically spherical shape, but once developed may become more oblong or irregular, although it is not a species characterized by having numerous protrusions. All collected specimens exhibit an evident and broad basal cavity, which at times may extend to occupy the entire radius of the ascoma. It is dark brown to blackish in color, and its size is rather small, generally 2 to 4 cm in diameter, occasionally reaching up to 6 cm. Its overall appearance is quite compact, fleshy, and warty. 

Peridium very dark brown to practically black in color, composed entirely of highly marked and prominent pyramidal warts, with 4 to 6 faces, each measuring approximately 4 to 7 mm at the base. They are occasionally somewhat flattened, but predominantly pointed at the apex, notably featuring longitudinal striations that impart a somewhat rugose appearance. It is a fairly compact peridium that is very difficult to separate from the gleba; it typically does not detach when cleaning off soil with a brush or similar tool.

Gleba compact and solid, fully filled, without internal cavities. It is light-colored in immature specimens but rapidly acquires a more or less dark brown hue in its fertile portion, while retaining sterile veins of a much lighter color, which typically do not change color upon maturation.

Flesh, which is essentially the gleba itself, quite compact and firm, with an odor that in immature specimens is not as pronounced as in other related species, but which becomes very strong upon maturation, reminiscent of pitch or tar, and is, in our opinion, quite unpleasant. However, it does not possess an analogous flavor as might be expected; nevertheless, it does not taste sufficiently well to be considered a choice edible.

Habitat:

As is the case with all species of the genus Tuber, this is a hypogeous species; although it typically does not grow at great depth and may even break the soil surface causing cracks, locating it is difficult without the aid of specially trained dogs. It typically fruits from spring onwards, maturing in summer and autumn. It is documented as forming mycorrhizae with broadleaf trees such as beech (Fagus sylvatica), hazel (Corylus avellana), or various Quercus species. The two collections used to compile this profile were made in the province of Álava, in the Basque Country, both forming mycorrhizae with lime trees (Tilia platyphyllos), collected in June.

Observations:

This is a species that must be classified as uncommon, given the limited information available on it, likely because it is difficult to distinguish from other species within the same genus. It is commonly confused with the excellent Tuber aestivum, with which it may share habitat and fruiting period; we must note that the latter lacks any basal cavity, and its aroma is highly pleasant, reminiscent of nuts. It is much more difficult to separate it from Tuber mesentericum, a species that is practically identical both macroscopically and in its organoleptic characteristics. The latter emits an intense tar-like odor even when young, has slightly narrower spores, and there is a slight difference in the arrangement of the wart cells.