Rhizopogon truncatus is an ectomycorrhizal truffle fungus of the order Boletales. This basidiomycete was described as early as 1924 by David Hunt Linder and is perhaps most notable for the vibrant yellow color of its mycorrhizae, rhizomorphs, and sporocarps which can grow to diameters of 1-3 cm. The sporocarp is subglobose to subellipsoid in shape and has a bright lemon-yellow peridium (gray-yellow to olive-yellow when bruised) which dulls as it dries. The gleba is of a dark yellow-brown color and may exhibit sectioning with globose cavities. The 100-220 µm thick peridium may contain distended, thin-walled hyphae and, along with the gleba, reacts negatively to KOH, Fe 2(SO 4) 3, and Melzer’s reagent.
The truncated ellipsoid shape of R. truncatus spores distinguishes the species from others within the genus. These spores grow in groups of 6 or 8 per basidium and appear red-orange in KOH and Melzer’s solution. Neither columella nor clamp connections are found, but hyaline or yellow-brown sterigmal attachments are noticeable. Microscopic observation of mature trama reveals gelatinous walls of hyaline hyphae inflated to about 2-5 µm in diameter.
While the distinctive yellow color of R. truncatus would seem to make it easier to forage for than most truffles, this is not the case considering that the species is relatively uncommon (though persistent). R. truncatus is found in both emergent and hypogeous states. It forms mycorrhizal associations with conifers; lab monocultures have shown that it favors Pinus ponderosa thought it will also form associations with Pseudotsuga menziesii in dual-cuture. In keeping with its host specificity, R. truncatus follows the biogeography of P. ponderosa. It is found in the Pacific Northwest – particularly near the southern coast of Oregon – and from New Hampshire northwards into Nova Scotia.
R. truncatus is an edible truffle and has a nondescript, oily flavor. For this reason, along with its small size and sparse distribution, the North American Truffling Society notes that R. truncatus is of little culinary use.