What You Should Know
Rhizopogon luteolus is an ectomycorrhizal fungus used as a soil inoculant in agriculture and horticulture. It was deliberately introduced into Pinus radiata plantations in Western Australia after it was observed to improve tree growth. A somewhat spherical-shaped fungus, it resembles a potato, however, is not nearly as firm. May give some resistance when squeezed but not too much. The cavity inside reveals a spongy interior with olive-brown 'powder'.
It has a worldwide distribution but is almost certainly introduced with pine under which the gasterocarps are to be found buried or half-buried in the litter of fallen pine needles. It is frequently confused with Rhizopogon rubescens but may be distinguished by the smaller spores, the absence of reddish tints or stains, and the hymenophoral trama which is more strongly gelatinized resulting in a very hard gleba on drying.
Unlike Tuber species (the true truffles), which are edible and highly prized, the Rhizopogon luteolus is of disputed edibility. While many authorities describe it as edible (albeit not highly regarded), others including Roger Phillips and Leif Goodwin list it as inedible.
Other names: Yellow False Truffle.
Rhizopogon luteolus Mushroom Identification
Looking very much like a potato and just as variable in size and shape, typically 1.5 to 4.5cm across its largest dimension and can be ovoid, ellipsoidal, an oblate spheroid, or a lobed blob. There is no stem, but cord-like mycelial threads spread into the soil (and to tree rootlets from a central point under the fruitbody. Its outer skin is tougher than the interior tissue, and it is off-white initially but soon turns ochre and eventually olive-brown. The outer surface, which usually cracks irregularly as the fruitbody expands, is often randomly decorated with tawny mycelial strands that give it a slightly woolly appearance.
Internally the spore-bearing gleba of Rhizopogon luteolus is almost white at first, turning ochre and eventually olive-brown as the spores approach maturity. The interior of the fruitbody comprises many tiny chambers lined with basidia, upon which the spores develop; initially, the interior is soft and spongy, becoming dry and powdery when old.
Oblong-ellipsoidal, 7-10 x 2.5-3.5µm; covered with an irregular coarse reticulum.
Creamy white or yellowish.
Habitat & Ecological Role
Generally occur either singly or, more commonly, in small groups in pine woodland on sandy soil, often beside forest tracks. Rhizopogon luteolus is ectomycorrhizal with pine trees.
Pisolithus arhizus is darker and much larger; it, too, is mycorrhizal with pines.
Rhizopogon luteolus Taxonomy and Etymology
When Swedish mycologists Elias Magnus Fries and Johan Nordholm first described this fungus in 1817 they gave it the binomial scientific name Rhizopogon luteolus, and that is the name by which it is known today. It was not always so, however: not long after Fries and Nordholm had named this species, Carlo Vittadini (1800-1875) and other mycological authorities were treating the genus Rhizopogon as ascomycetous rather than, as it certainly is, basidiomycetous. Its taxonomy has been a challenge ever since and only recently has its affinity with the family Suillaceae been uncovered.
Synonyms of Rhizopogon luteolus include Rhizopogon induratus Cooke.
Rhizopogon, the generic name, comes from Rhiz- meaning root, and -pogon, meaning beard. You might therefore expect the false truffles in this genus have root-like appendages that look rather like beards hanging down from a chin. They are indeed attached to the ground (and ultimately to the roots of pine trees) by pale mycelial cords.
The specific epithet luteolus refers to the yellow coloring of these potato-like false truffles.
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