Larch Bolete

The Larch Bolete is one of the most reliably identifiable boletes in the northern temperate zone, because it grows exclusively beneath larch and looks quite unlike anything else that does. Its cap has a lacquered, almost varnished appearance in wet weather, ranging from bright chrome-yellow through golden-brown to rich chestnut, and it can be spotted from a distance shining on a plantation floor.

It belongs to Suillus, the slippery jacks, a genus within the Suillaceae that is set apart from most boletes by glutinous cap cuticles, glandular dots or a ring on the stem, and an unusually tight fidelity to conifers, especially those in the pine family. Most Suillus species have a single host genus, and S. grevillei has larch.

Because larch is widely planted well beyond its native range, this bolete has followed it across Europe, into North America, and to other continents. Wherever a larch plantation matures, this species tends to appear within a decade or two, making it a good illustration of how mycorrhizal fungi travel with their partner trees.

Cap

The cap is 4 to 12 cm across, convex then flattening, sometimes with a low central bump. Colour is variable within a warm range: chrome-yellow, golden-yellow, orange-brown, or dark chestnut, often darker at the centre and paler at the rim. The cuticle is smooth and distinctly glutinous or sticky in damp weather, drying to a shiny, almost polished finish, and it peels easily.

Pores

The tube layer is bright lemon to golden-yellow when young, dulling to olive-yellow or dirty ochre with age. Pores are small and angular and become larger and more irregular near the stem. They discolour to cinnamon or rusty-brown where bruised or handled.

Stipe

The stem is 5 to 10 cm tall, yellow above and progressively browner or reddish-brown below, and it bears a distinct membranous ring formed by the remains of a partial veil that covered the tubes when young. Above the ring the surface is finely dotted and yellow; below it the stem is often streaked or netted with brown. There is no volva.

Flesh and spore print

The flesh is pale to bright yellow, sometimes flushed reddish-brown in the stem base, and it may stain pinkish-brown or slightly bluish-green in places when cut. The spore print is ochre-brown to olive-brown.

Larch is the sole requirement for this species. It fruits in larch plantations, mixed forest containing larch, amenity larch plantings in parks and estates, avenues, and shelterbelts, and native larch stands. It is entirely absent from woods where larch is missing, however suitable they may otherwise look.

It is common across Europe and temperate Asia and is well established in North America wherever European larch, Japanese larch, or native tamarack grows. It appears from late summer through to the first hard frosts, often abundantly and in troops, and can be one of the most numerous large fungi on a plantation floor in a good autumn.

Suillus grevillei is ectomycorrhizal with larch, forming fungal sheaths on the tree's short roots and extending mycelium and, in the Suillus fashion, rope-like strands of aggregated hyphae into the surrounding litter and soil. These strands transport water and nutrients efficiently over relatively long distances, which suits the fast-growing, deciduous-needled larches.

The genus Suillus is unusually host-specific across the board, and this species is a textbook case: its distribution maps almost exactly onto planted and natural larch. Because larch sheds all its needles each autumn, its litter layer turns over quickly, and the fungus plays a part in recovering nitrogen and phosphorus from that annual pulse of decaying material and returning it to the tree. The glutinous cap is thought to help retain moisture and may deter some grazing invertebrates.

Other Suillus species are the nearest relatives but differ in host and detail. Suillus luteus, the slippery jack, has a dull brown to purplish-brown cap, whitish then yellow pores, and a conspicuous purplish membranous ring, and grows with two-needle pines. Suillus granulatus and S. collinitus lack a ring altogether and have glandular dots on the stem, again with pines.

Suillus tridentinus and S. bresadolae also grow with larch but have orange to rusty-orange tones and much larger, more radially arranged pores. Suillus cavipes, another larch associate, is separated at once by its dry, densely woolly brown cap and a hollow stem. The combination of a shiny golden cap, small yellow angular pores, a ring, and larch overhead is diagnostic for S. grevillei.