MycoBank number: MB 811563, Faces of Fungi FoF 08036.
Basidiomata medium-sized. Pileus about 50 mm diam, broadly convex; surface dry, densely subtomentose, greyish brown (8F3) to black; margin straight; context 6–10 mm thick, dull yellowish white (4A2) or paler, slowly staining pale yellowish-grey (paler than 4B2) when cut. Hymenophore poroid, adnexed; tubes 2.5–6 mm long, dull yellowish white (4A2), inconsistently staining pale orange-grey (paler than 5B2) when bruised; pores 0.3–0.8 mm diam, angular, irregular surface, dull yellowish white (4A2). Stipe 80 mm long, 10–15 mm wide, central, cylindrical and tapering downward, solid, almost entirely reticulate; surface dry, background color cream (4A3) in the upper part, darker to the base; reticulum concolorous with the pileus; context fibrous, upper part yellowish white (4A2) unchanging or slightly yellowing when bruised, lower part yellow (3A6). Basal mycelium off-white to pale beige. Spore print not obtained. Odor rubbery. Taste acidulous.
Macrochemical reactions: Hymenophore orange in KOH solution; stipitipellis, pileus context and stipe context pink in KOH solution; pileipellis reaction subnull in KOH solution.
Micromorphology: Basidiospores [55/1/1] 9.5–12 × 4.0–5 µm, 10.69 × 4.48 µm on average, Q = (1.9–)2.1–2.7(–3), 2.39 on average, subfusiform to ellipsoid, greenish grey in water (1C2) and olive grey (1D2) in KOH, smooth. Basidia (25–)26–36(–37) × (7.5–)8–10 μm, 30.9 × 8.9 μm on average, clavate, four-spored, hyaline in KOH; sterigmata 2–4 μm long. Hymenophoral trama bilateral, of the boletus type; hyphae 5–7 µm diam. Cheilocystidia 31–43 × (5–)6–8 µm, 36.7 × 7 µm on average, subfusiform or narrowly fusiform, hyaline with some brown to yellowish-brown contents (5E5–8) when observed in water, no encrustations. Pleurocystidia 28–42 × 6–10 µm, abundant, 32.5 × 7.4 µm on average, subfusiform or narrowly fusiform, hyaline with some brown to yellowish-brown contents (5E5–8) when observed in water, no encrustations. Pileipellis a trichodermium about 70–120 μm thick, composed of more or less vertically arranged, interwoven, dark brown (7F8) to black hyphae in water, 4–10 μm wide; terminal cells 30–60 × 6–10 μm, narrowly clavate or subcylindrical, with some obtuse apex. Pileal trama composed of smooth hyphae 5–7 μm diam. Stipitipellis hymeniform, composed of smooth, hyaline hyphae, or dark blonde to honey yellow (5D4–6) hyphae in the reticulum, with clavate, subcylindrical, or subfusiform terminal cells; 25–30 × 10–13 μm and occasionally with clavate, four-spored basidia. Stipe trama composed of cylindrical, smooth, parallel hyphae 4–6 μm diam. Clamp connections not seen in any tissue.
Habitat: Solitary on the forest ground, near Pinus kesiya Royle ex Gordon.
Known distribution: Japan, southwestern China, and northern Thailand.
Material examined: THAILAND Chiang Rai Province, Mae Fah Luang District, 17 Jul 2014, Benjarong Thongbai, OR0738 (MFLU14-0460).
Notes: Retiboletus fuscus can be recognized by its greyish brown or dark grey pileus, dull yellowish white context sometimes slightly yellowing when bruised, concolorous stipe with darker reticulum on surface, and off-white mycelium at base. Zeng et al. (2016), elevated Boletus griseus var. fuscus Hongo to species rank based on a multi-gene (LSU, ITS, tef1) phylogeny. The macro- and micro-morphological characters of the specimen from Thailand were similar to R. fuscus from China and Japan. The combined phylogenetic analyses indicated the sequences from the Thai specimen clustered with those of R. fuscus from China with BS = 100%, PP = 1.0. Zeng et al. (2016) reported the species from Fagaceae forests. In Thailand, the specimen was collected in the vicinity of P. kesiya trees.
Fig. 1 Maximum likelihood phylogenetic tree inferred from the three-gene dataset (atp6, rpb2, tef1), including Retiboletus brevibasidiatus, R. fuscus, R. nigrogriseus and selected Retiboletus species. Four species were used as outgroup taxa: Borofutus dhakanus, Ionosporus longipes, Rhodactina rostratispora, Spongiforma thailandica. Bootstrap frequencies ≥ 70% and posterior probabilities ≥ 0.95 are shown above supported branches. The prefix “GB” before voucher numbers indicates that sequences were retrieved from GenBank.
Fig. 2 Basidiomata of Retiboletus fuscus (OR0738). A: Whole basidiome. B: Cut basidiome showing yellow tinge of context, especially in the lower half of the stipe. Bars: 2 cm.