Blue Tooth Identification Guide

The setting and the first impression

The Blue Tooth belongs to old, mossy, nutrient-poor conifer forest, and finding it usually means walking sandy or gravelly ground under pine or spruce where the moss and lichen layer is deep and continuous and the litter is thin. You are looking down at ground level for something that does not look much like a mushroom at all: a lumpy, irregular, top-shaped mass, often half-buried in moss, frequently growing in fused clusters where two or three fruit bodies have grown into one another and engulfed twigs, needles and grass stems as they expanded. From above, a young fruit body reads as a pale bluish-white to greyish cushion, while a mature one has gone rusty brown in the centre with a persistent pale rim. It is firm to the touch, corky rather than soft, and it takes real effort to prise one out of the moss intact.

Reading the upper surface

Look at the cap surface before you disturb it. It is 3-10 cm across, irregularly rounded to fan-shaped or wildly lobed, with a surface that is uneven, bumpy and often pitted, and finely velvety to matted-felty when young. Fresh growth at the margin is whitish to pale blue-grey and slightly downy; the older central region darkens progressively to cinnamon, rust-brown and finally a dull dark brown, so a single fruit body shows a colour gradient from a pale advancing rim to a dark centre. Unlike some relatives it is not strongly concentrically zoned on top, and the surface never becomes shiny. Debris embedded in the cap is normal and even helpful: this genus grows around obstacles rather than lifting them clear, so grass blades and pine needles sticking straight out of the flesh are a good genus-level cue.

The spore-bearing surface

Turn the fruit body over, and the group becomes obvious. Instead of gills or pores, the underside carries a dense forest of short spines or teeth, each 2-5 mm long, crowded, tapering to a point, and running down onto the top of the stem. When young the teeth are pale blue-grey to bluish-white, echoing the cap margin; with age they dull through greyish-purple to brown, and the tips often stay paler than the bases so the surface looks slightly frosted. Run a fingertip across them: they feel stiff and slightly prickly, not soft. Because the teeth are shallow and decurrent, the underside of a mature specimen looks brown and felty at arm's length; get the lens onto it to confirm that you really are looking at individual teeth rather than a tightly packed pore layer.

Stem, base and section

The stem is short, thick and often barely distinguishable from the cap, 1-4 cm long and up to 3 cm thick, tapering downwards and usually merging into a mass of moss, sand and needles bound by mycelium. Its surface is felty and rusty brown to orange-brown, and it may be swollen, twisted or almost absent where the fruit body has grown flat against the ground. Now cut the whole thing vertically from cap to base with a sharp knife, because the section is the single best character the species has. In fresh material the flesh is sharply two-coloured: the upper part, through the cap, is a clear blue to blue-grey, sometimes faintly zoned; the lower part, through the stem, is a strong orange-brown to rust-brown. That blue-over-orange contrast in one clean cut is what confirms Hydnellum caeruleum. The flesh is tough, fibrous and corky, hard to cut cleanly, and it does not exude droplets. In old, dried fruit bodies the blue drains away and the whole section goes brown, so cut the freshest specimen available.

Taking a spore print

Tooth fungi print readily if you can get the underside flat. Trim the base, sit the fruit body teeth-down on white paper, cover it and leave overnight. The deposit is brown, and against white paper it is easy to read. That brown print is important because it separates the whole Hydnellum and Sarcodon group from the white-spored tooth fungi such as Hydnum, and it is the difference between two genera that both look spiny from below. Under a microscope the spores confirm it: they are small, roughly 4.5-6 micrometres, subglobose to irregularly angular, and covered in coarse warts or tubercles, quite unlike the smooth ellipsoids of a white-spored tooth fungus.

Substrate, season and ecology

Hydnellum caeruleum is mycorrhizal with conifers, principally pine and spruce, so it fruits on soil in the root zone of living trees and never on wood. Its habitat is specific: nutrient-poor, well-drained, acidic sandy or gravelly ground with a thick moss carpet, typically in mature or old-growth forest that has not been fertilised or disturbed. It is a northern species, common through the boreal zone and scattered southwards in suitable montane and heathland forest, and it is treated as an indicator of long-established, undisturbed conifer woodland. Fruiting runs from late summer into autumn. Fruit bodies are annual but tough, and they persist for weeks in a blackened, mummified state long after they stop growing, so late-season finds are often too old to show the blue at all.

Separating it from look-alikes on morphology alone

The other tooth fungi in the same forests overlap in shape and colour, and section is usually the deciding step.

Hydnellum suaveolens is the closest visual match, with a white to pale bluish cap and a similar top-shaped habit. Cut it: in H. suaveolens the blue sits in the stem while the cap flesh is whitish, exactly the reverse of the blue-cap-over-orange-stem arrangement here. It also carries an intense sweet anise odour that H. caeruleum lacks.

Hydnellum aurantiacum shares the rusty tones and the felty cap, but its flesh is orange-brown throughout with no blue anywhere, and its cap surface tends towards clear orange rather than blue-grey at the margin.

Hydnellum peckii has a similar shape and a pale margin, but fresh fruit bodies exude conspicuous red droplets on the cap surface, and the sectioned flesh is pinkish-brown and zoned rather than blue over orange.

Hydnellum concrescens and Hydnellum scrobiculatum are thinner and more strongly concentrically zoned on the cap, often with pinkish or reddish-brown banding, and their flesh in section is brown and zoned with no blue.

Sarcodon species, including the scaly Sarcodon squamosus and S. imbricatus, are much larger, with a cap surface broken into coarse upturned scales, longer teeth, and brownish flesh with no blue.

Hydnum species, the white or buff-coloured tooth fungi, are entirely different in texture: soft and brittle rather than corky, cream to pale orange throughout, with a white spore print.

Coltricia perennis grows in the same mossy pine woodland and has a comparable zoned funnel shape and central stem, but its underside is a smooth brown pore layer, not teeth.

Field checklist

Mossy conifer forest on poor sandy soil, irregular top-shaped fruit body engulfing debris, felty cap with a pale bluish advancing margin and a rusty centre, crowded decurrent spines that are bluish when young, short thick rusty stem, tough corky flesh that sections blue in the cap and orange-brown in the stem, brown spore print, warty spores. Make the vertical cut early; almost everything else is shared with its relatives.