Slippery Jack

Slippery Jack (Suillus luteus) is the type species of Suillus, the genus of slimy-capped boletes tied almost exclusively to conifers of the pine family. Its two most obvious characters are stated in its common name and its structure: a cap coated in a thick layer of gluten that makes it genuinely slippery to hold, and a conspicuous membranous ring around the stipe that no other widespread European Suillus of similar colouring possesses.

Suillus differs from the true boletes in several respects beyond the slime. The tube layer tends to be pale yellow with somewhat larger, more angular or radially arranged pores, the stipe frequently carries glandular dots or resinous granules, and many species have a partial veil that leaves a ring or a marginal fringe. Molecular studies place the genus in the family Suillaceae, apart from Boletus.

The species has one of the widest distributions of any ectomycorrhizal fungus, having travelled the world in the root balls and nursery soil of planted pines. Wherever Pinus has been introduced for forestry, from the southern Andes to New Zealand, Australia and southern Africa, Slippery Jack has usually followed and now fruits abundantly in the plantations.

Cap

5-14 cm across, hemispherical then convex and finally almost flat, sometimes with a low broad umbo. The surface is chestnut-brown, purple-brown, dark yellow-brown or occasionally almost olive, and is coated in a thick glutinous layer that makes it shiny and slimy in wet weather. When it dries the gluten sets into a glossy, varnished-looking skin. The cuticle peels away readily from the flesh.

Pores

The tube layer is pale lemon-yellow when young, deepening to a duller ochre-yellow and finally olive-tinged with age. Pore mouths are small, round and neatly arranged, and the layer is adnate to slightly decurrent at the stipe. Bruising causes little change beyond a slow brownish discolouration. In very young specimens the pores are hidden by the partial veil.

Stipe

5-10 cm tall and 1-3 cm thick, roughly cylindrical, pale yellow at the apex and whitish below. It carries a large, soft, membranous ring, white on the upper surface and characteristically stained dark purple-brown to violet-brown on the underside; below the ring the stipe often takes on the same purplish-brown tone. Above the ring the surface bears fine yellowish to brownish glandular dots.

Flesh & spore print

The flesh is soft, pale yellow to whitish, and does not change colour appreciably when cut. The spore print is clay-brown to dull ochre.

Suillus luteus is ectomycorrhizal and strongly specialised on two-needle pines, above all Scots pine (Pinus sylvestris), and also with other members of the genus including introduced Monterey and radiata pines. It grows on the ground in pine woodland and plantations, on sandy, acidic and often disturbed soils, along tracks and firebreaks, on dune slacks with planted pine, and in shelterbelts and urban plantings.

Fruiting begins in late summer and continues right through autumn into early winter, with the species often persisting after the first frosts. Native to Europe and northern Asia, it has been introduced with plantation forestry to North America, South America, southern Africa, Australia and New Zealand, where it is now one of the most frequently encountered fungi in exotic pine stands.

The fungus wraps the fine roots of pines in an ectomycorrhizal mantle and sends hyphae into the surrounding sandy soil, supplying the tree with phosphorus, nitrogen and water while receiving photosynthetic carbon in return. It is a pioneer species, quick to colonise young pine plantings and newly disturbed ground, which is precisely what has made it such a successful global traveller: nursery seedlings carry its mycelium with them.

The glutinous cap coating is thought to help the fruiting body retain moisture and to shed water and spores efficiently in the open, exposed conditions typical of sandy pine ground. Fruiting bodies are soft, decay rapidly and are used heavily by fly larvae and by small mammals; the ring and veil remnants often persist as a collar even after the rest of the mushroom has begun to collapse.

Weeping Bolete (Suillus granulatus) is the nearest match in cap colour and slime but has no ring at all, and its young pore surface exudes milky droplets that dry as brown granular dots. Larch Bolete (Suillus grevillei) also has a ring but a bright golden-yellow to orange, strongly glutinous cap, and it grows only with larch. Bovine Bolete (Suillus bovinus) has a duller pinkish-tan cap, no ring, and much wider, angular, compound pores arranged in a coarse honeycomb.

Bay Bolete (Imleria badia) can be a similar brown but has a dry to merely tacky velvety cap, no ring, and pores that flush blue-green within seconds of being pressed. In every case the combination of a slimy chestnut cap, small neat yellow pores and a large membranous ring stained purple-brown beneath is unique to Suillus luteus among common species.