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PROFILES OF FUNGI: BOLETUS AND SIMILAR FUNGI

The group of fungi commonly termed the "boletes" produce mushroom-like fruitbodies (basidiocarps) but instead of having gills they have narrow pores on the underside of the cap. The pores are lined with basidia, and at maturity the basidiospores are "popped" a short distance into the pore, and then fall vertically so that they escape from the pores and are dispersed by wind. Almost all of these fungi grow in symbiotic association with trees, forming mycorrhizas that aid the uptake of mineral nutrients from soil. The three main genera of boletes are Boletus, Suillus and Leccinum. These genera often produce large, fleshy fruitbodies. One of the classic examples is Boletus edulis - the highly prized "Cep" or "Penny Button" (see Fig. 1) which is often collected and dried as flavouring for soups in Continental Europe. [But even a relatively young specimen will contain a large number of maggots of the fungus gnat!].

Fig. 1. Boletus edulis. The shape of this fungus is unmistakable. It has a large, swollen white stipe, often showing a reticulate pattern (seen on the right-hand side of the stipe) just below the cap. The cap ranges from 8 to 15 cm diamter, is typically brown, with a whitish bloom initially but then becoming brown. The specimen shown was photographed with a flash and does not show the characteristic cap colour. As in all Boletus species, the underside of the cap consists of tubes rather than gills. Boletus edulis can be found in coniferous, deciduous or mixed woodlands, but perhaps most commonly under beech (Fagus sylvatica) in Britain. [© Jim Deacon]

Another Boletus species, Boletus calopus (Figs 2, 3) is usually associated with beech or oak trees. Old specimens of this fungus can be large, with a muddy-brown cap up to 15 cm diameter. The underside of the fruitbody reveals the pale lemon pores and a stout, red, tapering stipe with a reticulate pattern. But one of the most notable features is the ease with which this fungus is bruised and rapidly turns a deep blue colour. Several other Boletus spp. also bruise in this way.

Fig. 2. The cap of Boletus calopus, about 15 cm diameter, heavily damaged by slugs. [© Jim Deacon]

Fig. 3. Underside of the cap of Boletus calopus (the same specimen shown in Fig. 2) showing the red, reticulate pattern on the stipe and the lemon-coloured pores. The deep blue-stained area on the left developed within 10-15 seconds when the hymenium (spore-bearing tissue) was scratched with a fingernail. [© Jim Deacon]

Fig. 4. Part of a Boletus fruitbody broken apart to show the narrow pores, about 2 cm deep. One of the characteristic features of the genus Boletus is that the pore layer can easily be removed from the rest of the cap tissue. [© Jim Deacon]

Leccinum species differ in several ways from Boletus spp., despite their overall similarity. They have dry caps, relatively tall stems (stipes), and the stipe is covered with small scales that usually darken with age. Leccinum scabrum, the "Brown Birch Bolete", illustrates these features (Figs 5 and 6 below).

Fig. 5. Leccinum scabrum, a common mycorrhizal fungus on birch trees (Betula spp.) [© Jim Deacon]

Fig. 6. Leccinum scabrum seen from below, showing the many pores and the scales on the stem. [© Jim Deacon]

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