Russula quercophila Buyck & Halling
Cap (60-) 75-100 mm diam., convex, firm, irregularly waving-sinuous near the margin, the latter smooth to very shortly subsulcate; pellis not separable, entirely cottony or finely woolly when young, later only so near the extreme margin, dry to somewhat greasy, not viscid when humid, smooth or with minute veins or ridges, continuous, very pale cream when young because of the cottony hairy covering, becoming later pale brown to coffee with milk or greyish brown, finally dark reddish brown to almost black when old.
Gills 8-12 mm high, decurrent, unequal with numerous lamellulae, brittle, widely spaced, somewhat attenuate and strongly anastomosing towards the cap margin, not forked or only close to the stipe; edge looking paler when young, entire.
Stipe (30-) 50-70 x (14-) 28-38 mm, slightly eccentric, subcylindrical or narrowing to the base or sometimes to the top, very hard and firm, compact, whitish to cream or slightly brownish, paler than cap, more or less irregularly veined-wrinkled and showing some white mycelium at the base. Veils absent.
Context 6-8 mm thick above gill attachment, dirty cream to pale brown, turning browner with time, locally with distinct orange tinge. Mild. Odour not strong at first, but strongly disagreeable to nauseous when old. FeSO4 very slow and ambiguous reaction, developing greenish as well as a reddish orange tint.
Spore print not obtained.
Spores (8.2-) 8.3-8.64-8.9 (-9.1) x (7.6-) 7.7-7.89-8.0 (-8.2) µm (Q = (1.03-) 1.10 (-1.13)), subglobose, subreticulate with irregular, often large meshes; ornamentation of interconnected, low and obtuse warts, locally fused in short ramifying crests, up to 0.5 µm high, distinctly amyloid; suprahilar spot large but not amyloid.
Basidia 61-70 (-77) x 10-11 µm, clavate-pedicellate, 4-spored, exceptionally 2-spored; sterigmata stout, 6-7 x 1-1.5 µm.
Cystidia appearing as very abundant on gill sides but, when counting SV+, turn out to be only rather abundant (800/mm2), hardly emergent or if so, then only up to 20 µm, 76-95 (-130) x 6-9 µm, mostly subcylindrical to narrowly fusiform and often with one or more constrictions near the apex, very thin-walled, distinctly greying in sulfovanillin although contents not abundant and often poorly visible, refringent-granular to finely crystalline. Marginal cells occupying the whole edge, which is sterile, terminal cell measuring 30-70 x 5-9 µm, resembling the extremities of stipiti- and pileipellis, also subcylindrical to more irregularly inflated, typically originating from a shortly cylindrical cell which is often inserted on a more inflated, sometimes ramified basal cell, not pigmented and optically empty, thin-walled.
Subhymenium small-celled. Lamellar trama with conspicuously abundant, narrow, oleiferous hyphae, mainly composed of chains of inflated cells, rather than with loosely arranged sphaerocytes.
Pileipellis orthochromatic in cresyl blue, a one-layered, narrow cutis lying immediately on the sphaerocytes rosettes, composed of narrow to moderately inflated, thin-walled hyphal extremities, the latter also more or less aggregated in tufts towards the cap margin, mostly 4-5 µm diam., many hyphae with conspicuous zebroid incrustations, some filled with brown diffuse pigment, especially in the cap center; end-cells often irregularly inflated or sinous, towards the cap margin mostly subulate or narrowing upwards. Pileocystidia not observed.
Stipitipellis with a remarkably abundant network of oleiferous hyphae or fragments of hyphae; covered with sinuous, more or less aculeate to fusiform, optically empty cells, reminiscent of empty hymenial cystidia. No caulocystidia with differentiated contents. Clamps absent.
In Quercus seemannii woodland mixed with Quercus rapurahuensis and Q. copeyensis, in small groups of a few dispersed individuals, on soil.
Russula quercophila is in Costa Rica associated with endemic Quercus. Using the existing American literature for identification, it would key out to the northeastern North American Russula imitatrix Homola & Shaffer, placed in section Compactae s.l. This is yet another extremely rare fungus, which is - to our knowledge - only known from the type locality near Milford in Maine, USA, and from one other collection near Kentville.
Under the microscope, R. quercophila is strongly similar to R. imitatrix, because of more or less tortuous elements in the pellis and on the gill edge; both species also have a small-celled, almost filamentous subhymenium, both have a remarkably abundant network of slender (often fragments of) oleiferous hyphae under the surface of the stipe, and in both species, pellis of cap and stipe is covered with optically empty cells, reminiscent of empty hymenial cystidia.
When looking more closely, however, R. imitatrix has pileocystidia, which - although small and having very poorly differentiated, granular-refringent contents - are easily found because of the presence of several minute apical appendices. Its pileipellis is also composed of even more strongly irregular, contorted terminal elements. We were unable to demonstrate the presence of pileocystidia in the Costa Rican quercophila-collections.
Russula quercophila can also very easily be mistaken for the North American R. eccentrica and especially for the Costa Rican R. cartaginis because of the brown coloured cap, pale stipe, spaced, pinkish ochraceous gills and mild taste. R. cartaginis, however, has a more sulcate cap margin and the cap is not woolly-pubescent (only the stipe), gills are not anastomosing towards the margin and are white when young (pinkish ochraceous from the start in R. quercophila), and the stipe is much more irregularly veined and often wider near the base, instead of narrowing downwards as in R. quercophila. The former species turns also very dark in the herbarium, whereas R. quercophila remains pale upon drying, especially in the gills. Under the microscope, differences in spore characters and pileipellis composition exclude any confusion.
Because of its features, both macro- and microscopical, R. quercophila fits perfectly in the subsection Albospissinae, until now still monospecific and only known from tropical Africa. R. albospissa differs essentially by features of the spores and the presence of long, cylindrical dermatocystidia in the pileipellis having an incrusted wall exactly as observed in the zebroid-encrusted - but empty - hyphae in the cap of R. quercophila.