Agaricus pallidobrunneus R.L. Zhao, in Zhao, Zhou, Chen, Margaritescu, Sánchez-Ramírez, Hyde, Callac, Parra, Li & Moncalvo, Fungal Diversity 78: 271 (2016)
Index Fungorum number: IF 570225; MycoBank number: MB 570225; Facesoffungi number: FoF 01182; Figs. 1, 2
Pileus 80–97 mm in diam., 6 mm thick at disc, convex to applanate; surface dry, white (1A1) fibrillous cap along with light brown (6D8) to grayish brown (7D3) appressed squamules denser at the center and in a scattered manner throughout the pileus; margins were decurved. Lamellae free, reddish gray to rose gray (12B2–12B4) in the young stage and chocolate brown (9E8–9F8) at maturity, 8.5 to 10 mm broad with 1 to 2 series of lamellulae being present. Stipe 98–105 × 15–19 mm, somewhat cylindrical but minutely wide towards the base, hollow, surface yellowish white (1A2) basal region is surrounded by minute grayish brown (7D3) squamules, no bruising was observed, and on cutting no significant color change was observed. Annulus membranous, pendant upper layer, smooth, white (1A1) and floccose lower layer, white (1A1). Context white (1A1) to both in pileus and stipe, no color observed on air exposure. Odor pleasant.
Basidiospores (6.4–)7.3–7.7–7.9(–8.2) × (3.8–)4.2–4.6–5.0(- 5.2) µm, Q = 1.6–1.7–1.8, ellipsoid to oblong, smooth, brown when viewed with water, inamyloid, thick-walled, 1–2 guttule present. Basidia 20–27 × 6.4–8.96 µm, clavate to broadly clavate hyaline, smooth, 4 spored. Basidioles 19–21 × 5.0–5.5 µm, clavate, hyaline, thin-walled. Pleurocystidia absent. Cheilocystidia 18–24 × 11.5–15.5 µm, abundant, simple, clavate to ellipsoid or pyriform, hyaline, smooth. Annulus composed of hyaline hyphae of 23.5–24.5 × 8.0–9.0 µm. Pileipellis a cutis, composed of septate hyphae 23–39 × 7.5–13.0 µm, slightly ellipsoid to cylindrical, with brownish pigmentations. Stipitipellis composed of cutis hyphae of 6.0–9.0 µm width, cylindrical and hyaline. Clamp connection absent.
Macro-chemical reactions – Yellow on addition of KOH in all tissues; Schäffer’s reaction negative.
Ecology and Distribution – Growing in gregarious form on decomposed humus of leaf litter mixed soil. Earlier known to occur from China and Thailand.
Material examined – INDIA, Assam, Kamrup metro district, Jalukbari, 26°09’07.7″N 91°39’36.2″E, 79.0 m asl., 11 Oct 2022, A.K. Dutta & P. Chattopadhyay, PC22/GU/AG22 (GUBH 20457); Assam, Kamrup metro district, Jalukbari, 26°09’07.6″N 91°39’35.9″E, 79.0 m asl., 24 Aug 2023, A.K. Dutta & P. Chattopadhyay, PC23/GU/AG38 (GUBH 20459).
GenBank – GUBH 20457: OR875413 (ITS); GUBH 20459: OR875414 (ITS).
Notes – The taxon Agaricus pallidobrunneus was first discovered in Yunnan Province, China (Zhao et al. 2016). Later, it was also reported from Thailand (Jaichaliaw et al. 2021). The characteristic features of the taxon include a comparatively well-developed universal veil, no discoloration on bruising or exposure of context to air, larger basidiospores, and fibrillose squamules over pileus and stipe (Zhao et al. 2016). In the phylogenetic analysis (Fig. 2), the newly generated two Indian sequences cluster together with the type specimen sequence of Agaricus pallidobrunneus (GenBank accession No. KT951370) and two sequences of A. pallido brunneus, earlier deposited from Thailand (GenBank accession No. JF727860, MW255649), with strong support values (MLBS = 97%, PP = 0.96) suggesting all the sequences to be conspecific. Morphologically, the Indian collection nicely matches the type specimen description along with the morphology of the specimens reported from Thailand. In the tree, Agaricus erectosqumulosus appears to be sister to A. pallidobrunneus (MLBS = 95%, PP = 1.00), and this cluster together represents A. sect. Flocculenti. Morphologically, A. erectosqumulosus differs from A. pallidobrunneus by having erect squamules all over the pileus surface colored brown, an annulus that is often cogwheel-like colored white with a light brown margin, orange coloration with KOH, and somewhat smaller basidiospores (mean of 7.2 × 4.4 µm vs. 7.7 × 4.6 µm in A. pallidobrunneus; Zhao et al. 2016).
Figure 1 – Agaricus pallidobrunneus (GUBH 20457). a Fibrillose squamules over the pileus surface. b Lamellae and stipe view. c Basidiospores. d Basidia. e Cheilocystidia. f Pileipellis hyphae. Scale bars: a, b = 10 mm, c = 5 µm, d–f = 10 µm.
Figure 2 – Maximum Likelihood phylogram (-InL = 5828.769149) generated using GTR + I + G model of nucleotide evolution for nrITS sequences. The MLBS support values = 50% is shown on the left of “/” and Bayesian posterior probabilities (PP) support of greater than = 0.50 on the right side. Type sequences are shown in black bold and tagged “T” at the end. All the taxa are represented along with their GenBank accession numbers and respective country of origin. The sequences of Agaricus pallidobrunneus generated for this study are marked in red front.