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Russula cartaginis Buyck & Halling

Macroscopic features:

Sporophores occurring in small groups of a few dispersed individuals.
Cap (25-) 40-95 mm diam., broadly depressed, often irregular, near the margin becoming broadly sulcate in age; pellis not viscid when humid, not separable, not pruinose but rather felty-greasy at the touch, first breaking up concentrically, then becoming gradually completely areolate from the margin inwards, a warm dark brown.
Gills unequal, mostly alternating with 2 much shorter lamellulae, subdistant to distant, not anastomosing in between, not forking near stipe, rarely so closer to the margin, pale when young, a dirty cream, becoming much darker, dirty ochre to pinkish pale brown when old, bruising pinkish brown; edge entire, faintly brown over whole length, less so when ageing.
Stipe (15-) 20-45 (-90) x 9-20 (-40) mm, more or less cylindrical but often widening near the base, there also often irregularly and strongly furrowed, having a cottony, white to concolorous with the cap but bearing young a pure white pubescence which disappears quickly on handling, becoming glabrous and dirty cream or grayish, turning pink when injured, massive inside but quickly very brittle and irregularly hollowing under a thick cortex.
Context up to 10 mm thick, extremely brittle, breaking when touched, presenting a faint pinkish tinge when cut, especially the stipe interior, nearly insensitive to FeSO4. Taste mild but nauseous, strongly disgusting. Smell very faint, disagreeable, on drying releasing sweeter, almond-like components.
Spore print not obtained, probably white.

Microscopic features:

Spores (7.0-) 7.4-7.87-8.21-8.6 (-8.8) x (6.0-) 6.1-6.38-6.76-7.0 (-7.1) µm (Q = (1.13-) 1.16-1.21-1.23-1.27 (-1.32)), shortly ellipsoid, ornamentation of low obtuse warts of different diam. up to 0.5 µm high, distinctly amyloid and interconnected or prolonged by very fine lines in varying degrees, mostly very incompletely reticulate, some spores with almost isolated warts; suprahilar spot large but not amyloid.
Basidia 48-58 x 8 µm, clavate-pedicellate, 4-spored; sterigmata stout, 6-8 x 1-2 µm.
Cystidia probably quite numerous, 105-240 x 5-8(11) µm, not emergent, thin-walled, usually very long and slender, cylindrical or slightly tapering towards the top, locally somewhat inflated, mostly in lower part or at the base, contents not greying in sulfovanillin (SV-), although filled with abundant, minutely crystalline contents, near the gill edge dispersed and much smaller, e.g. 57 x 6 µm, sometimes with slightly thicker wall. Marginal cells occupying the whole edge, the latter completely sterile and covered by a well-developed layer of small but mostly strongly inflated cells composing highly branched tufts with virescens-like structure; cells mostly sphaerical, ampullaceous, ellipsoid to utriformous, some filled with a brown diffuse pigment.
Subhymenium except for the 1-3 narrow cylindrical cells immediately under the basidium, composed of rapidly inflating cells towards the trama, forming a well-inflated and relatively loose tissue, which seems like "perforated" by many bases of cystidia that originate in the trama. Lamellar trama mainly composed of loose sphaerocytes and relatively short, narrow and slender, often more sinuous cystidioid elements, measuring for ex. 150 x 7 µm diam.
Pileipellis orthochromatic in cresyl blue, an ill-structured layer of irregularly inflated, thin-walled elements, mostly containing a brown diffuse pigment; terminal cells very variable, cylindrical, inflated-appendiculate, ampullaceous, clavate etc... Pileocystidia dispersed but distinct, arising from deeper layers, cylindrical, 3-7 µm diam., often capitulate or irregularly constricted to somewhat inflated near the top, filled with abundant crystalline contents, not reacting in sulfovanillin.
Stipitipellis surface is a loosely interwoven tissue of thin-walled hyphae with abundant caulocystidia reminiscent of the hymenial cystidia with distinct and abundant contents, towards the base of the stipe also with more inflated elements as on the cap. Clamps absent.

Ecology:

Under Quercus oocarpa and Quercus sp.

Notes:

In the field, R. cartaginis immediately reminds one of the rare North American R. eccentrica with which it shares identical habit and overall colour, except for the gills which do not turn pink with age. Although R. eccentrica has been listed as present in Costa Rica in the past, there is - as far as we are aware - no confirmed report yet of its presence in this country, nor has this species been recorded from any neighbouring or South American country, but we have been able to examine and confirm the identity of R. eccentrica-collections associated with indigenous oak in Mexico. R. cartaginis and R. eccentrica are indeed very close, both possess very similar spores (perhaps a little less ornamented in R. cartaginis although more collections are needed to verify) and similar elements in pileipellis and hymenium. It is indeed easier to separate the two species in the field rather than under the microscope.

Nomenclature

Photographs

Microscopy

SEM photos