Aureoboletus minimus Ming Zhang, C.Q. Wang & T.H. Li, sp. nov.

Index Fungorum number: IF 900314; Mycobank number: MB 900314; Facesoffungi number: FoF 14038; Fig. 1

Etymology – “minimus” refers to the small basidiomes.

Holotype – GDGM44400.

Macroscopic characters – Basidiomes small-sized. Pileus 0.8–2 cm wide, obtuse to convex at first, becoming broadly convex to plane in age, fleshy, surface dry or slightly viscid when wet, covered with fibrillose to tomentose squamules, light orange (6A6), pastel red (7A6–9A6) to grayish red (7B5–9B5, 8C5–9C5), usually cracking when mature; margin somewhat involute at young, nearly flat when mature, without veil remnants at margin. Context 1–2.5 mm thick at stipe, firm and tough in youth, soft when matured, white on the whole, greyish red (10B5–11B5) beneath pileipellis, slightly becoming greyish-pinkish or greyish red (10B5–11B5) when exposed. Tubes 2–3 mm deep, distinctly depressed around stipe, yellowish white, yellowish gray to pale yellow (2A2–4A3, 2A3–4A3, 2B2–3B2), often with an olive tint, unchanging when bruised. Pores 2–3 per mm, irregular angular, slightly elongated around stipe at maturity, concolorous with tubes, unchanging when bruised. Stipe 15–40×3–6 mm, central, cylindrical or narrowly clavate, solid, slightly enlarged toward the base, concolourous with pileus, covered with white to yellowish brown fibril or tomentum, with white basal mycelium. Odour not distinct. Taste mild.

Microscopic characters – Basidiospores 13–16×5–6 µm, (x=14.5 × 5.4 μm, n=20), subfusiform, inequilateral in side view, smooth, yellowish to yellowish brown in 5% KOH, thin-walled. Basidia 25–30 × 7–10 µm, clavate, 4-spored, sterigmata 2–4.5 µm long, yellowish white to hyaline in 5% KOH, without basal clamps. Pleurocystidia 32–50×10–13 μm, fusiform, thin-walled. Cheilocystidia frequent, similar to pleurocystidia in shape and size. Hymenophoral trama subparallel, yellowish white to hyaline in 5% KOH, with 4–10 μm broad. Pileipellis a trichodermium of erect hyphae 12–22 μm in diam, yellowish white to hyaline in 5% KOH; terminal cells 37–70×12–22 µm, cylindrical, clavate or nearly fusoid. Stipitipellis a layer of repent to suberect branching hyphae 4–8 μm in diam, hyaline in 5% KOH. Clamp connections absent in all tissues.

Habit, habitat and distribution – Solitary to scattered on soil or decayed wood in broadleaf forest dominated by Fagaceae trees. Currently only known from central Vietnam.

Material examined – Vietnam, Lam Dong Province, Da Lat, Bi Doup Nui Ba National Park, alt. 1500 m, 15 October 2017, Ming Zhan, GDGM44400 (holotype), Tai-Hui Li, GDGM44401.

GenBank numbers – GDGM44400: LSU = OP901640, TEF1-α=OP918149, RPB1=OP918155, RPB2=OP918152; GDGM44401: LSU = OP901641, TEF1-α = OP918150, RPB1=OP918156, RPB2=OP918153.

Notes – Aureoboletus minimus is characterized by its small basidiomes with a dry and usually cracked pileus densely covered with fibrillose to tomentose squamules, white context slightly changing greyish-pinkish or greyish red when expose, yellowish gray to pale yellow hymenophore unchanging when bruised, subfusiform and relatively larger basidiospores. The above combination of features makes it easily distinguished from other species in Aureoboletus.

Morphologically, A. minimus is similar to A. glutinosus and A. tenuis. However, A. glutinosus differs by its strongly glutinous or mucilaginous basidiomes, reddishbrown to greyish-ruby pileus with irregular reticulation and gelatinous veil remnants at margin, and smaller basidiospores 10–13.5×4.5–5 µm (Zhang et al. 2019). Aureoboletus tenuis differs in having relatively larger baisidomata, glutinous and wrinkled pileus, and smaller basidiospores 11–12 × 4–5 μm (Zhang et al. 2014). Phylogenetic analyses based on the combined sequence of LSU, TEF1-α, RPB1, RPB2 shown that two specimens of A. minimus formed a distinct lineage in Aureoboletus, and showed close relationships with A. auriflammeus, A. garciae and A. miniatoaurantiacus but with low support value and larger genetic distance (Fig. 2). In morphology, A. auriflammeus and A. miniatoaurantiacus can be easily distinguished by their pileus minutely covered with orange yellow, orange to reddish-orange tomentum or powder (Zhang et al. 2019). Aureoboletus garciae, recently reported from Mexico, differs in having vivid blue to light blue pileus with some reddish tones, bright yellow hymenophore, and smaller basidiospores 9–14 × 4–5(–6) µm (Haelewaters et al. 2020).

Figure 1 – Aureoboletus minimus (GDGM44400, holotype). a–c Basidiomes. d Basidiospores. e Cheilocystidia and pleurocystidia. f Basidia and basidioles. g Pileipellis. Scale bars: a–c=2 cm, d–f=10 µm, g=20 µm

Figure 2 – Phylogram generated from maximum likelihood analysis based on combined LSU, tef1, rpb1 and rpb2 sequence data of Aureoboletus. Bootstrap support values for maximum likelihood greater than 50% are indicated above or below the nodes, and branches with Bayesian posterior probabilities greater than 0.95 are given in bold. The new species are in blue. The tree is rooted with Phylloporus imbricatus HKAS68642 and Xerocomus subtomentosus HKAS58865