Hypochnus rhacodium Berk. & M.A.Curtis ex Burt, Ann. Missouri Bot. Gard. 13(3): 322 (1926).

Svantesson, Sten, Larsson, Karl-Henrik, Koljalg, Urmas, W. May, Tom, Patrik Cangren,, Henrik Nilsson, R. & Larsson, Ellen, 2019, Solving the taxonomic identity of Pseudotomentellatristis s. l. (Thelephorales, Basidiomycota) - a multi-gene phylogeny and taxonomic review, integrating ecological and geographical data, MycoKeys 50, pp. 1-77 : 53-55

Hypochnus rhacodium Berk. & M.A.Curtis ex Burt, Ann. Missouri Bot. Gard. 13(3): 322 (1926). Fig. 21

Type.

UNITED STATES OF AMERICA. Pennsylvania: on underside of decaying logs of apparently a frondose species, E. Michener 1435 (syntypes: Mo. Bot. Gard. Herb. 5095 [BPI US0291002]!; FH Curtis Herb. 4061; K Curtis Herb. 4061, designated by E.A. Burt in Ann. Missouri Bot. Gard. 13: 322 (1926)).

Description.

Basidiome annual, resupinate, membranaceous, effused. Mature parts continuous, with a hard and rather brittle texture. Hymenium smooth; pale brown to brown with a pink hue. Subiculum loose, fibrous and dark brown with an orange hue. All characters recorded in dried state.

Hyphal cords lacking, but loose bundles of subicular hyphae sometimes present.

Hyphal system monomitic, clamp connections and reaction in Melzer’s reagent absent from all hyphae.

Subicular hyphae noticeably long and straight, thick-walled; forming a loose tissue. Individual hyphae (5.6-) 5.7-7.3 (-8.0) μm wide, with a mean width of 6.5 μm; pale to dark orange brown in both KOH and water, sometimes with granular contents which turn blue green in the presence of air.

Subhymenial hyphae often somewhat sinuous, thin to thick-walled; forming a rather dense tissue. Individual hyphae (3.7-) 3.8-5.3 (-6.1) μm wide, with a mean width of 4.7 μm; brown to pale orange brown or pale green in KOH (but not with the blue green reaction present in other species); orange brown to brown in water, with strongly granular contents.

Encrustation granular, probably amyloid (hard to observe due to the colour); brownish-black in KOH, dark blue green in the presence of air; brownish-black in water; scattered in occurrence on the subicular hyphae.

Basidia with four slightly curved sterigmata, occasionally two-sterigmate; narrowly clavate or sometimes narrowly clavopedunculate, thin-walled, with one-three slight constrictions. Dimensions: 73-105 (-109) × (8.8-) 8.9-10.1 (-11.2) μm; mean dimensions: 94 × 9.6 μm. Sterigmata (8.5-) 9.5-12.1 (-12.5) μm long, with a mean length of 10.9 μm. Colours and reactions the same as for the subhymenial hyphae.

Cystidial organs lacking.

Basidiospores in frontal face generally with a subcircular basic shape and an angular to nodulose or sometimes cross-shaped outline, covered in bi- or trifurcate, sometimes singularly attached, echinuli. Nearly all spores with three-five distinct, rounded to square lobes; abnormally large spores originating from two-sterigmate basidia infrequently occurring. Frontal dimensions: (7.8-) 8.0-9.1 (-9.3) × (7.7-) 7.8-8.9 (-9.0) μm; mean dimensions: 8.3 × 8.3 μm; Q-value: 0.9-1.1; mean Q-value: 1.0. Echinuli (0.9-) 1.0-1.6 (-1.7) μm long, with a mean length of 1.3 μm. Lateral face ellipsoid to ovoid or sometimes subcylindrical, usually with angular edges, sometimes with one-three lobes. Lateral dimensions: (7.9-) 8.2-8.9 × (5.4-) 5.9-6.8 (-7.0) μm; mean dimensions: 8.5 × 6.3 μm; Q-value: 1.2-1.4 (-1.6); mean Q-value: 1.3. Colour in KOH brown to orange brown, in the presence of air with a blue green reaction; in water greenish-orange to orange brown; inamyloid.

Chlamydospores lacking.

Habitat.

The only specimen of H. rhacodium recorded to date is the type collection, which was collected in Pennsylvania, United States. No further information on habitat or any further locality description is available.

Distribution.

Basidiome encountered in: the United States.

Remarks.

The hymenium of H. rhacodium is very thick and dense in comparison to all other Pseudotomentella species. It consists of tightly packed basidia, which are overlapping in length and have a total thickness equalling four-six basidial lengths. All other morphological characters fit well within the P. tristis group, thus suggesting an abnormal basidiome.

Within the P. tristis group, the basidiome of H. rhacodium can be recognised by its lack of hyphal cords and skeletal hyphae and its hard and brittle texture after drying. This feature makes it unique within the group and the risk for confusion with any other described species should hence be small.