Mottled Bolete

The Mottled Bolete is a close relative of the brown birch bolete, distinguished by two features that are easy to check in the field: the uneven, marbled colouring of the cap surface and a striking blue-green reaction in the base of the stem when it is cut open. Where Leccinum scabrum is a flat, uniform mouse-brown, this species looks as though its cap has been dabbed unevenly with paler grey, cream, and darker brown.

It belongs to the scabrum group of scaber stalks, sharing their tall slim stature, small round pale pores, and dark stem tufts. Like the white birch bolete it inclines toward damper ground, though it is less strictly confined to sphagnum and can be found in ordinary moist birch woodland as well as on wet heath.

The blue-green base reaction is worth understanding as a genus theme rather than a one-off oddity: several Leccinum species show it, several do not, and it results from oxidation of pigments in the flesh on contact with air. Slicing the whole fruit body from cap to stem base and watching for a couple of minutes is the standard way to see it.

Cap

The cap is 5 to 12 cm across, convex then flattening. Its ground colour is grey-brown to mouse-brown, but it is conspicuously variegated, with irregular paler grey or cream patches and darker blotches giving a mottled, camouflaged effect. The surface is finely felted to smooth, slightly greasy when wet.

Pores

Tubes are whitish to pale grey, becoming dingy grey-buff or faintly olive with age. Pores are small, round, and crowded, depressed around the stem apex, and stain slowly ochre-brown to slightly pinkish when bruised.

Stipe

The stem is long and slender, 8 to 15 cm tall, whitish to pale grey and covered with fine scabers that darken from greyish to blackish-brown. It tapers upward from a slightly thickened base and lacks both a ring and a volva.

Flesh and spore print

The flesh is white and fairly soft. When the fruit body is sliced lengthwise, the flesh of the stem commonly flushes pink to rose over a few minutes, while the flesh in the extreme base turns a clear blue-green, sometimes almost turquoise. The cap flesh changes little. The spore print is snuff-brown.

Leccinum variicolor grows with birch, favouring damp to wet ground: moist birch woodland, boggy heath, mossy woodland flushes, birch scrub on peaty soils, and the drier fringes of mires where the white birch bolete takes over the wettest ground. It also occurs in damp birch plantings in parks.

It is widespread in northern and central Europe and extends across the birch zone of Asia, with records from northern North America. Fruit bodies appear from midsummer through autumn, usually singly or in small scattered troops among moss, grass tussocks, and birch litter.

This species is an ectomycorrhizal partner of birch, forming a fungal sheath on the tree's fine roots and an exploratory network through the surrounding peaty soil. On acid, waterlogged, and nutrient-poor ground, the fungal partner mobilises nitrogen and phosphorus from organic matter that the roots could not access directly, and takes photosynthetic sugars in return.

Birch supports an unusually rich assemblage of specialist mycorrhizal fungi, and the Mottled Bolete is one of several Leccinum species that partition the same host across different moisture regimes: L. scabrum on drier ground, L. variicolor on damp ground, and L. holopus in true bog. This niche separation lets multiple related fungi coexist under a single tree species across a habitat gradient.

Leccinum scabrum is the main confusion but has an evenly coloured, unmottled grey-brown cap and shows no blue-green stain in the stem base. Leccinum holopus also stains blue-green at the base but is whitish to pale greenish overall rather than mottled brown, with softer, waterier flesh and a strong preference for sphagnum.

Leccinum duriusculum shares the blue-green base reaction and can look greyish, but it is much firmer, has a deeply rooting stem, and partners with poplar rather than birch. Leccinum roseofractum also reddens in the stem flesh, but lacks the mottled cap and the clear turquoise base tint. Cutting a whole fruit body from top to bottom and comparing cap patterning is the fastest way through this group.