Butter Bolete

Butyriboletus appendiculatus is the type of the butter boletes, a genus split from Boletus to hold a group of firm, thick-fleshed species with buttery yellow pores and flesh, fine reticulation on a yellow stipe and a slow, weak blue reaction. In North America the same common name is applied to several regional Butyriboletus species, so the name covers a group of similar-looking fungi worldwide.

The European species is recognised by its combination of a warm tan to reddish-brown dry cap, a bright chrome-yellow pore surface and a stipe that tapers downward into a distinctly rooting, appendage-like base - the feature that gives the epithet appendiculatus.

It is a warmth-loving fungus of southern and central European broadleaf woodland, and one of the more substantial boletes to be found under oak on rich calcareous soils in high summer.

Cap

The cap is 7-20 cm across, hemispherical then broadly convex. Colours are warm tan, ochre-brown, hazel or reddish-brown, sometimes with a slightly paler margin. The surface is dry and finely velvety when young, becoming smoother, and often cracks shallowly in dry weather to expose pale yellow flesh in the fissures.

Pores

The pore surface is bright chrome to lemon yellow when young, deepening to golden and finally olive-yellow. The pores are small and round. Bruising produces a blue-green stain that develops slowly over several seconds and is often modest in intensity, sometimes fading again.

Stipe

The stipe is 6-15 cm tall and 2-6 cm thick, firm and club-shaped at first, then tapering downward to a narrowed, rooting base. It is yellow at the apex, usually flushed brownish or reddish-brown over the lower portion, and carries a fine, delicate raised network in the yellow upper part. The net is much finer than in the porcini group.

Flesh & spore print

The flesh is thick, firm and pale yellow, deeper yellow beneath the tubes, and turns pale blue slowly and weakly when cut, most noticeably in the cap and stipe apex, sometimes flushing reddish in the stipe base. The spore print is olive-brown.

This bolete grows with broadleaf trees, chiefly oak and beech and also sweet chestnut and hornbeam, on warm, well-drained calcareous or base-rich soils. It favours sunlit woodland edges, open coppice, parkland and wooded slopes, and is scarce in dense shade and on cold acid ground.

The season runs from midsummer into early autumn, generally from July to September. It is widespread in central and southern Europe, common in France, Italy, Spain and the Balkans, reaching its northern limit in southern Britain and southern Scandinavia, and extends east into western Asia. Comparable Butyriboletus species replace it in North America and eastern Asia.

Butyriboletus appendiculatus is ectomycorrhizal, forming a hyphal sheath around the fine roots of oak, beech and related trees and an internal network between the root cortex cells. The tree supplies carbohydrates and the fungus returns water, phosphorus and other mineral nutrients gathered from a large soil volume. The tapering rooting stipe base is a growth form rather than a functional root, produced as the fruit body pushes up through compacted litter and soil.

Its association with warm, base-rich soils places it among the thermophilic elements of European woodland mycota, and its distribution has been used as one indicator of climatically favourable broadleaf woodland. Fruiting follows summer rain, and the firm flesh allows fruit bodies to persist for a week or more in dry conditions.

Butyriboletus regius has a distinctly rose-pink to purple-red cap rather than a tan-brown one, with the same yellow netted stipe. Butyriboletus fechtneri has a pale grey to whitish cap and a more strongly reddening stipe base. Boletus edulis is separated by white pores that only slowly yellow, white flesh with no blueing at all, and a coarse white net on a pale stipe.

Rubroboletus species that share the habitat have red to orange pore mouths and blue rapidly and strongly. Neoboletus luridiformis has a dark brown cap, orange-red pores and a stipe densely dotted in red without a raised net.