Mycetinis opacus (Berk. & M.A. Curtis.) A.W. Wilson & Desjardin., 2005. Mycologia 97: 677-678.

Petersen, Ronald H. & Hughes, Karen W., 2017, An investigation on Mycetinis (Euagarics, Basidiomycota), MycoKeys 24, pp. 1-138 : 37-40

8. Mycetinis opacus (Berk. & M.A. Curtis.) A.W. Wilson & Desjardin., 2005. Mycologia 97: 677-678.

Marasmius opacus Basionym. Berk. & M.A. Curtis 1849. Hooker’s J. Bot. 1: 99. ≡ Marasmiellus opacus (Berk. & M.A. Curtis) Singer. "1949"(1951). Lliloa 22: 300.

Holotype.

United States, South Carolina, Darlington Co., Society Hill, N34°30'47", W79°51'03", VI, Curtis 1241 (FH, K).

Diagnosis.

1) Basidiomata of moderate size, gracile (pileus 3-20 mm broad; stipe 10-45 × 1.5-2.5 mm); 2) rhizomorphs usually plentiful, slender, pallid, usually erect; 3) pleurocystidia fusiform, usually submammilate; 4) cheilocystidia arbuscular with coarse branches, not setulose; 5) pileipellis microstructures differing significantly from pileus margin to disc; 6) odor and taste negligible (not alliaceous); 7) distribution in eastern North America (with some extralimital reports); 8) fruiting chiefly on dead Rhododendron twigs and leaves.

Description.

Basidiomata (Fig. 50) of moderate size, marcescent, reviving. Pileus 3-15(-20) mm diam, convex with decurved margin when young, expanding with age to plano-convex or sometimes plane with decurved or uplifted margin; disc rarely subumbonate, sometimes slightly depressed; margin entire when young, remaining so throughout maturation or becoming striate to rugulose-striate in age; surface smooth, sueded-like, dry, opaque, finely powdery or granulose overall when young, disc remaining so in age and margin sometimes becoming glabrous; when young, disc colored "pale vinaceous fawn"6C2-3, “drab” 6D3, "hair brown" 6D-E3, near "Rood's brown" 7D5, "chestnut brown" 7D-E4-5, or "clove brown" 7F3-4, with a slightly paler margin, with maturation disc region retaining greyish brown tones or fading to "vinaceous buff " 6C3-4, pale greyish yellow 5D3, pale brownish orange 5C3-4, "pale vinaceous fawn" 5-6B3, "pale cinnamon pink" 5A2, or "pale yellow-orange" 4A3, margin in age fading to "tilleul buff" 5-6B2, "pale pinkish cinnamon" 6A2, "pale cinnamon pink" 5 A2, "pale yellow-orange" 4A3, pale greyish buff or white; in age pileus typically with a pale greyish brown disc region and white margin, in wet weather entire pileus sometimes becoming white. Context thin (<1 mm), buff. Lamellae adnate or shallowly adnexed, subdistant, total lamellae 12-18, non-intervenose to somewhat so, especially near pileus margin, seldom forked, sometimes wavy in outline, non-marginate, white, buff, "pale ochraceous buff" 4A2, not discoloring. Stipe 10-40 (-45) × 1.5-2.5 mm, central, terete or seldom compressed, equal, solid at first becoming hollow in age, pruinose to pubescent or fibrillose overall or with furfuraceous base, insititious, tough; apex buff, pinkish buff, "pale cinnamon pink" 5B2-3 or near "tawny olive" 5C4, base brownish grey 5C2-3, “drab” 6D3, "hair brown" 6-7D-E3, "buffy brown" 6-7D4-5 “fuscous” 6E4, "chestnut brown" 6-7E4. Rhizomorphs (Fig. 50) abundant, well-developed, cord-like, ranging from buff to orange white to pale brownish orange, greyish brown or light brown, often branched and forming tangled masses. Odor not distinct or faintly sweet; taste not distinctive.

Habitat and phenology.

Scattered or gregarious on fallen twigs and leaves of Rhododendron maximum and Tsuga canadensis , rarely on debris of Quercus spp., Pinus spp or undetermined deciduous hardwoods; common in temperate eastern North America, rare in southwestern United States and Japan. Known U.S. distribution: Arizona, Connecticut, Georgia, Illinois, Missouri, New Jersey, New York, North Carolina, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, West Virginia.

Pileus margin pileipellis a modified rameales-structure, composed of two conspicuously clamped elements: 1) hyphae repent, diverticulate (Fig. 52 A–C), 3-5.5 µm diam, firm-walled, producing side branches; side branches 2-7 × 3-4 µm diam, broadly digitate or cylindrical, often dichotomous; and 2) broom cell-like pileocystidia (Fig. 52 D–G) 31-42 × 9-14 µm overall, arbuscular, stalked (stalk 4-35 × 3.5-5.5 µm, firm-walled, clamped at base), unexpended distally, producing apical branches irregularly in 360°(branches 2-7 × 3-4.5 µm, rounded at apex, rarely dichotomous); branches resembling those of diverticulate hyphae, often evacuating but not collapsing (i.e. remaining as “exoskeletons” in pileipellis). Pileus disc pileipellis constructed of the following: 1) occasional pileal hairs (Fig. 51) 10-80 × 3.5-5.4 µm, cylindrical, arising as side branches of repent hyphae, often minutely roughened, usually subcapitulate; 2) an irregularly hymeniform layer of inflated hyphal termini (Fig. 53) 14-34 × (6-)8-15 µm, ranging from digitate, gnarled-digitate, ellipsoid to broadly clavate, almost always thick-walled over inflated portion [wall -1.5 µm thick, refringent (PhC), sometimes vaguely pigmented yellowish], obscurely clamped, apparently immersed in a thin slime matrix; 3) widely scattered pileocystidia as in pileus margin, but generally crumpled and difficult to recognize; and 4) underlying a thicker tightly interwoven thatch of hyphae 3-7 µm diam, smooth, thick-walled (wall -1 µm thick, not gelatinized), conspicuously clamped. Pileus trama hyphae interwoven. Hymenophoral trama regular; hyphae 3.0-8.0 µm diam, filamentous, smooth or encrusted nearest the pileipellis, non-gelatinous, hyaline or subhyaline, inamyloid, thin- to thick-walled (up to 1.5 µm thick). Pleurocystidia (Fig. 54) common, 22-28 × 6-7 µm, fusiform to narrowly fusiform, usually submammilate, conspicuously clamped; contents homogeneous with 1-2 vaguely vacuolated areas in mid-section. Basidioles clavate to ampulliform; basidia (Fig. 55) (16-)25-31 X (5-)8-10 µm, clavate, often subcapitulate, 4-sterigmate, clamped; contents multi granular to multiguttulate. Basidiospores (Fig. 58B) (6.5-)7.5-10(-11) × (2.8-)3.5-4.5(-5) µm (Q = 1.50-2.43; Qm = 1.98; Lm = 8.23 µm), ellipsoid to subamygdaliform, flattened somewhat adaxially, collybioid (hardly tapered proximally), thin-walled, inamyloid; contents often multigranular. Cheilocystidia (Fig. 57) 17-36 × 4.0-8.5 µm, resembling broom cell-like pileocystidia, sporadic (often absent, apparently produced on selected basidiomata), very variable, ranging from (occasionally) irregularly fusiform to bifurcate to (usually) stalked (stalk 7-17 × 3-5.5 µm, obscurely clamped, firm-walled), not expanded distally, producing a cluster of gnarled-digitate diverticula; diverticula 2-8 × 2-2.5(-3) µm, rounded at apex, occasionally inflated somewhat and occasionally dichotomous. Stipe medullary hyphae monomitic, 2.0-4.5(-6.0) µm diam, strictly parallel, free (not involved in slime matrix or adherent in sheets), firm- to thick-walled (wall -1.5 µm thick, hyaline), conspicuously clamped. Stipe cortical hyphae cylindric, ranging from hyaline (stipe apex) to pale yellow (stipe base), non-encrusted, inamyloid, thick-wlled (wall -1.5 µm thick). Stipe vesture a poorly developed Rameales -structure with numerous, erect caulocystidia; caulocystidia (Fig. 58A) digitate to vermiform, often cudgel-shaped, similar to pileus hairs, mostly hyaline and thin-walled on stipe apex, pale yellowish brown to brownish orange and thick-walled (wall -1.5 µm thick) near stipe base. Rhizomorph tissue similar to that of stipe; rhizomorph medullary and cortical hyphae undifferentiated, parallel or subparallel, firm- or thick-walled. Rhizomorph vesture absent, in some areas composed of a thin layer of loosely interwoven, irregular-shaped hyphae with scattered, short and broad diverticula.

Commentary.

Three characters are inconspicuous but diagnostic: 1) pleurocystidia are usually submammilate, unlike the acute forms found in sect. Androsacei and sect. Perforantia ; 2) cheilocystidia are generally of the siccus -type, but branches are coarse and not setulose; and 3) pileipellis microstructures differ significantly from pileus margin to disc.

Basidiomata are usually accompanied by numerous rhizomorphs, often as tall as stipes (or taller), unlike other members of the clade in which rhizomorphs, if present, are short and extremely thin (but pallid, unlike the black or dark brown of the rhizomorphs of sects. Androsacei and Perforantia ).

Although the term used to describe pileipellis elements is the same as that used in Marasmiellus sect. Rameales (" ramealis -structure"), the two structures are significantly different. In its original use, the term represents a structure in which setulae are usually 2-5(-8) × 0.7-1.2 µm (i.e. Marasmiellus appalachianensis TFB 14610, MS; Ma. ramealis TFB 4727 Sweden), while the side branches and arbuscular apical branches in Mycetinis are appreciably coarser, 2-10 × 2-3 µm. In fact, Desjardin (1997: figs 6-11) included Ma. appalachianus Desjardin together with Ma. opacus (see Desjardin et al. 1993: figs 10-16) in subsect. Opacini, although pileipellis structures are quite different.

In the most detailed discussion of Ma. opacus thus far ( Desjardin et al. 1993) the following was reported: " Features diagnostic for M. opacus include: (a) convex, rugulose-striate pileus colored pale greyish brown on the disc and white on the margin; (b) subdistant, narrow, white to buff-coloured lamellae; (c) pubescent, insititious stipe with pallid apex and brownish grey base; (d) buff or pale greyish brown rhizomorphs; (e) basidiospore mean of 8.3 × 3.7 µm; (f) diverticulate cheilocystidia; (g) pileus disc with clavate to sphaeropedunculate, often thick-walled terminal cells, and pileus margin a Rameales -structure; (h) caulocystidia similar to pileipellis elements; (i) clamped, inamyloid hyphae; and (j) fructification on leaves and wood of Rhododendron , Quercus and Tsuga ." This diagnosis, however, was compiled to distinguish Ma. opacus from other Marasmiellus taxa, but not from other taxa of Mycetinis , which, after all, was not recognized as a discrete generic unit at that time.

Desjardin et al. (1993) included discussion of cultural and sexual compatibility studies and cited numerous examined specimens. Little need be added. Later, Desjardin (1997) summarized Marasmiellus taxa from the southern Appalachian Mountains, placing Ma. opacus in section Rameales subsection Opacini Singer.

Presumably, Singer’s ["1949" (1951)] placement of Marasmiellus opacus in Marasmiellus sect. Rameales presumed its conformity to the description of the section, which was attributed to Lange (1921) as " sect. Ramealis gen. Marasmii ", the type species of which was Marasmiellus ramealis (Bull. ex Fr.) Singer [≡ Agaricus ramealis Bull.: Fr] a combination transferred concurrently. Singer (1973) proposed Marasmiellus sect. Rameales subsect. Opacini to accommodate several neotropical taxa but a description of M. opacus was furnished as extra-limital. Singer (1973) summarized Marasmiellus in neotropical regions and included section Rameales Singer, which he had proposed in 1951 (Lilloa 22: 299. "1949"). Within sect. Rameales , he ( Singer 1973: 102) proposed subsection Opacini, typified by Marasmius opacus Berk. & M.A. Curtis, even though he did not explain his choice of type, nor did he take up the species. Singer’s (1973) later description of Marasmiellus opacus supplied needed taxonomic information, and among the specimens he examined were some from the southeastern United States (but with no illustrations). From all this, there is no evidence that he intimately knew M. opacus , only that he did not consider its presence in the neotropics. At the same time, subsection Opacini included eight species, so subsect. Opacini was not monotypic.

Specimens examined.

Japan, Tottori Pref., N35°30'35", E134°14'09", Kokufu Town, 4.X.1989, coll. RHP (annot. DE Desjardin), TFB 2400 (TENN-F-48740). United States, Georgia, Rabun Co., vic. Clayton, Warwoman Dell Picnic Area, N34°54'52.62", W83°21'14.21", 18.VII.1989, coll. S.A. Gordon, TFB 2788 (TENN-F- 48710); Mississippi, Harrison Co., vic. Saucier, Tuxachanie Hiking Trailhead, N30°39'43.61", W89°08'14.70", 10.VII.2014, coll. RHP, TFB 14490 (TENN-F-69190); North Carolina, Buncombe Co., Asheville, Blue Ridge Parkway, N35°34', W82°29', milepost 363, 4.VIII.1959, Coll. L.R. Hesler, TENN-F-23348); Macon Co., vic. Highlands, road to Cliffside Lake, end of road, N35°04'50.01", W83°14'03.07", 8.VII.2016, coll AS Methven, TFB 14629 (TENN69344); Highlands Biological Station, N35°04'43.99", W83°14'12.69", 19.VI.1963, coll. L.R. Hesler (TENN-F-25556); Tennessee, Blount Co., GSMNP, Cades Cove, N35°33'46", W83°50' 50, 22.V.2005, coll. J.L. Mata, JLM 1601 (TENN-F-61960); vic. Townsend, GSMNP, Turkey Pen Trailhead, N35°36'41.84", W83°44'48,29", coll. RHP, TFB 13650 (TENN-F-63084); Sevier Co., GSMNP, Indian Gap, N35°36'36.09", W83°26'19.24", 2.VIII.1936, coll. L.R. Hesler (TENN-F-10199).