Porcini

Boletus edulis is the type species of Boletus and the reference point against which most other members of the family Boletaceae are described. It is a heavy, thick-fleshed fungus with a spongy pore layer instead of gills, and its combination of a brown bun-shaped cap, a bulbous white-netted stipe and flesh that refuses to change colour when cut is one of the most recognisable silhouettes in mycology.

The name covers what is now understood to be a species complex. Populations associated with different tree partners and different continents differ subtly in cap colour, stipe shape and season, and several have been split off as distinct species such as Boletus pinophilus, Boletus reticulatus and, in western North America, Boletus rex-veris and Boletus barrowsii. The core European B. edulis remains the pale-margined, chestnut-capped form of spruce and beech woods.

It is among the most intensively studied ectomycorrhizal fungi in the world, both because of its wide host range and because its fruiting is closely tied to soil moisture and temperature, making it a useful indicator of woodland conditions.

Cap

The cap is 8-25 cm across, occasionally much larger, hemispherical when young and expanding to a broad cushion. The colour ranges from pale buff through warm tan to deep chestnut, usually darkest at the centre with a distinctly paler, almost white, narrow rim at the margin. The surface is smooth, slightly greasy or tacky in wet weather and dry and matte in drought, and young caps often carry a faint whitish bloom.

Pores

The pore surface is white and stuffed in young specimens, becoming cream, then yellow, and finally dull olive-yellow with age. The pores are small and round, the tube layer thick and easily separated from the cap flesh, and the whole surface is depressed around the stipe apex. Neither the pores nor the tubes turn blue when pressed.

Stipe

The stipe is short and very stout, 5-18 cm tall and often 3-8 cm thick, markedly swollen or barrel-shaped when young and elongating to a club shape later. It is whitish to pale buff, sometimes flushing brownish in the lower half, and is decorated with a fine raised white network. The reticulation is clearest over the upper third and fades out downward.

Flesh & spore print

The flesh is thick, firm and white, remaining white when cut across cap and stipe alike, sometimes with a thin vinaceous line directly under the cap skin. Older specimens soften noticeably. The spore print is olive-brown.

Boletus edulis forms ectomycorrhizal partnerships with a wide range of trees. In Europe it is most often found with Norway spruce, Scots pine, beech and oak, and also with birch and sweet chestnut. It favours mossy woodland floors, forest rides, plantation edges and grassy banks under isolated trees, and tolerates both acidic sandy soils and richer loams.

Fruiting typically begins in mid to late summer after substantial rain and continues into autumn until frost, often in two or three distinct flushes. The species is widespread across Europe, temperate Asia and northern North America, and has been carried to the Southern Hemisphere with plantation conifers, where it is now established in New Zealand, parts of Australia and southern Africa.

This is an ectomycorrhizal fungus. Its hyphae sheathe the fine root tips of its host tree and grow between the outer root cells, forming a structure that greatly extends the root system's reach. The fungus supplies water, phosphorus and nitrogen drawn from a large soil volume, and receives sugars produced by the tree in return. It cannot be grown to fruiting without a living host, which is why cultivation attempts have had so little success.

Fruit bodies emerge when soil moisture rises and temperatures moderate, usually a fortnight or so after heavy rain. The thick fleshy caps are heavily exploited by fly larvae, small mammals and slugs, and the resulting spore dispersal through animal movement adds to ordinary wind dispersal from the tube mouths.

Tylopilus felleus is the closest general match in shape and colour, but its pore surface takes on a pink to flesh tint as it ages rather than turning yellow-olive, its stipe carries a coarse dark brown net on a paler ground, and its spore print is pinkish-brown.

Boletus reticulatus has a dry, matte, often finely cracked cap and a net that runs the full length of the stipe, and it fruits earlier in the year. Boletus aereus has a much darker bronze to sooty-brown cap and a browner stipe. Boletus pinophilus is a deeper mahogany-red with a reddish-tinged net. Any bolete with red or orange pore mouths, or with flesh that flushes blue within seconds of cutting, is not B. edulis, whose flesh stays stubbornly white.