Perigord Black Truffle

The Perigord Black Truffle is the best known member of Tuber, a genus of ascomycetes in the family Tuberaceae that complete their entire fruiting cycle underground. Instead of raising a stalk and cap into the air, they form a closed, potato-like body in which spores mature inside asci and are never actively shot away.

Because the fungus cannot use wind for dispersal, it invests instead in a powerful volatile aroma that develops as the interior ripens, drawing in soil-dwelling and burrowing animals. This scent, together with the geometry of the surface warts and the pattern of veining inside, is the practical basis for identifying the species.

Tuber melanosporum is a winter species of limestone country in southern Europe, and its mycelium is strongly associated with particular host trees. Ground above an established colony often shows a brûlé — a bare, scorched-looking ring where competing ground vegetation has been suppressed.

Fruiting body

Subglobose to irregularly lobed, usually 2-8 cm across but occasionally much larger, and often dented or flattened by stones in the soil. The outer skin (peridium) is black to rusty black and covered in small, closely packed warts that are distinctly pyramidal with four to six flat sides and a shallow depression at the top. Warts are firmly attached and cannot be rubbed off.

Interior (gleba)

The flesh is white and firm in immature specimens, ripening through grey-brown to a deep purple-black. Running through it is a dense network of fine, narrow white veins that branch repeatedly and give a marbled appearance; the veins commonly take on a reddish or brownish tone when the cut surface is exposed to air.

Aroma and texture

Ripe bodies release a strong, persistent, penetrating aroma quite unlike ordinary woodland mushroom smell. The texture is firm and slightly crunchy when sliced, and the cut face is even rather than chambered or hollow.

Spores

Spores are formed in asci embedded in the gleba, usually one to four per ascus, and are broadly elliptical, brown, and covered with stout curved spines. There is no spore print, since spores are only liberated when the body decays or is broken open.

This truffle is a species of well-drained, calcareous soils with a high pH — typically thin limestone soils on open, sunny slopes. Its principal hosts are downy oak, holm oak and pubescent oaks, together with hazel, hornbeam and occasionally lime and pine, and colonies develop on the fine roots of these trees.

The native range centres on southern France, northern Spain and central Italy, with plantations established more recently in other Mediterranean-climate regions including parts of Australia, New Zealand, Chile and the western United States. Fruitbodies form in summer and ripen slowly, reaching maturity in the cold months from roughly late autumn to early spring, which gives the species its alternative name of winter truffle.

Tuber melanosporum is ectomycorrhizal. Its hyphae sheathe the fine root tips of the host tree, forming a mantle and a Hartig net between root cells, and exchange soil-derived water, phosphorus and nitrogen for tree sugars. The relationship is obligate: the fungus cannot complete its cycle without a compatible host.

The bare zone often visible around a colonised tree reflects the mycelium's strong suppression of competing herbaceous plants in the soil it occupies. Because spores are enclosed, dispersal depends on animals that dig up ripe bodies and pass spores through their digestive tracts; wild boar, rodents, deer and various insects all play this role, and the intense aroma is the fungus's signal to them.

Tuber brumale is the closest match in the field: it shares a blackish warty exterior, but its warts are smaller and less regularly pyramidal, and its gleba is greyer black with coarser, whiter, more widely spaced veins that do not redden. Tuber aestivum has much larger, coarser pyramidal warts and, decisively, a pale hazel-brown interior with broad white veins rather than a purple-black one.

Tuber indicum and its Asian relatives look very similar externally but tend to have flatter warts and a coarser vein network, with spores whose spines are shorter and denser under a microscope. Earthballs (Scleroderma species) are sometimes confused with truffles when partly buried: they have a thick, tough, ochre-yellow scaly rind, a solid black-purple interior that becomes powdery, and no white marbled veining at any stage.