The genus Actidium Fries
Fries, Syst. Myc. II: 595-596 (1823)

by Eric W.A. Boehm

webassets/A_hysterioides.jpgActidium hysterioides Fries. From Ellis & Ellis (1985).

Bulliardella (Saccardo) Paoli, 1905

Mytilidion Duby, subgen. Bulliardella Saccardo, 1883

Ostreionella Seaver, 1926

The genus Actidium was established (Fries 1823) to accommodate A. hysterioides Fr., a stellate mytilinidiaceous fungus found on Pinus and Picea in Europe, with two-celled, symmetric, ascospores, light olive to reddish-brown in color, faintly longitudinally striate in age (Barr 1990). Fries (1823) was the first to note its similarity to the genus Glonium in the Hysteriaceae, with hyaline to yellow, slightly asymmetric didymospores (Zogg 1962). Although only one species, A. nitidum (Ellis) Zogg, is known from temperate North America (Barr 1990) from the Cupressaceae, Zogg (1962) recognized an additional three species from Europe, namely A. hysterioides, A.baccarinii (Paoli) Zogg and A. pulchra (Teng) Zogg, all on the Pinaceae. Due to similarities in ascospore morphology, the genus Actidium may have affinities with other didymospored hysteriaceous genera (e.g., PsilogloniumActidiographium and Glonium), although molecular data are presently lacking.

Barr (1990): “Ascomata superficial, gregarious, conchate or triangular or two to four grouped in radiating configuration, small to medium sized; apex cristate, opening by longitudinal slit; base rounded applanate; surface smooth, shining black, longitudinally striate; peridium firm, brittle, narrow, composed of several layers of elongate compressed cells, heavily pigmented externally. Asci cylindric.. Trabeculae sparingly branched & anastomosed, sparse. Ascospores light brown, oblong fusoid or straight or inequilateral, one septate median, slightly constricted; walls thin, smooth or delicately longitudinally striate; homogeneous; overlapping uniseriate in the ascus. Anamorphs not known, not formed in culture (Lohman, 1938; Zogg, 1962). On wood & twigs, occasionally leaves of gymnosperms, infrequent. Type species: Actidium hysterioides Fries. Notes: Zogg (1962) included Ostreionella Seaver (in Seaver & Chardon, Sci. Survey Porto Rico & Virgin Islands 8: 77. 1926) as a synonym of Actidium, but the type specimen of OstreionellaO. fusispora Seaver has globose ascomata with an abrupt compressed short apical papilla & is instead more closely related to Ostropella in the Melanommataceae. Only one species, Actidium nitidum, is known from temperate North America. This species seems to be closely related to Actidium hysterioides but differs in not forming radiating groups of ascomata, in habit on members of the Cupressaceae rather than of the Pinaceae & in slightly narrower ascospores. The fine longitudinal striations on walls of ascospores have not been reported previously. Actidium baccarinii (Paoli) Zogg is known from Europe & Actidium pulchra(Teng) Zogg from China. Both of these species have larger ascospores that the other two”.


Key to species of Actidium Fries

  1. Fruitbody star-shaped or astral; spores 11-14 x (1.5-)2-3 μm; Europe on PinusPicea (Pinaceae→ Actidium hysterioides Fries

1′. Fruitbody bivalve shell-shaped (conchate), not star-shaped, nor forming radiating groups of ascomata → 2


  1. Ascospores (9-)11-14(-16) x (1.5-)2-3 μm; Europe, N. America (USA) on Cupressus, Thuja (Cuppresaceae→ Actidium nitidum (Ellis) Zogg

2′. Ascospores larger → 3


  1. Ascospores (16-)18-22(-24) x (3-)4-5(-6) μm; Europe on PinusPicea Thuja → Actidium baccarinii (Paoli) Zogg

3′. Ascospores 23 – 28 x 6 – 7.5 μm; China → Actidium pulchra (Teng) Zogg

1) Actidium hysterioides Fries; (2) Actidium nitidum (Ellis) Zogg;
(3) Actidium baccarinii (Paoli) Zogg; Illustration from Zogg (1962).

Actidium nitidum (Ellis) Zogg

Zogg, Ber. Schweiz. Bot. Ges., 1960, 70, 195-205.

Glonium nitidum Ellis, 1879
Solenarium nitidum (Ellis) Kuntze, 1898
Bulliardeilla nitida (Ellis) Lohman, 1938
Psiloglonium nitidum (Ellis) Petrak, 1953
Bulliardiella sabinae Kirschstein, 1924

Rare. North America & Europe. Collected from the bark & wood of weathered PinusPicea Juniperus. Important synonyms: Glonium nitidum Ellis ex Cooke et Ellis, 1879; Bulliardella nitida (Ellis) Lohman 1938; Psiloglonium nitidum (Ellis) Petrack 1953.

Ellis & Everhart (1892) for Glonium nitidum Ellis: “Fruitbodies densely gregarious, superficial, minute (0.25 – 0.5 μm long), lying parallel in the direction of the fibers of the bark, sub-conchiform, faintly striate; lips closed so as to form a narrow ridge or crest along the apex of the fruitbody. Asci cylindrical, sessile, 35-45 x 4 μm. Paraphyses if present not distinct (?). Ascospores uniseriate, clavate-oblong, uniseptate, hyaline, 6 – 7 x 2 – 2.5 μm. Collected from the inner surface of the loosened bark of cedar Cupressus thyoides on stumps, not too much decayed; fairly common around Newfield, NJ.”

Barr (1990): “Ascomata gregarious, 200 – 400 μm long, 130 – 150 μm in diameter & in height. Asci 45 – 60 x 3.5 – 5 μm. Ascospores (6-) 7.5 – 10 (-12) x 2 – 3 μm, apex obtuse, base more acute; wall finely longitudinally striate. Distribution on twigs & leaves of conifers, north temperate zone. Collected from New Jersey on Thuja occidentalis. Note: Petrak’s (1953) combination in Psiloglonium was made on the basis of a collection from Florida. The small sizes of asci & ascospores suggest that Petrack had a different species according to Zogg (1962).”

Actidium nitidum (Ellis) Zogg. Photo credit: © Alain Gardiennet, France, #AG09080, 18/3/2009, on Juniperus communis. Illustration: Zogg (1962).


Actidium hysterioides Fr.
Fries, Syst. Myc. II, 1823, 596.

Rare. Europe. Old wood of Pinus & Picea

Actidium hysterioides Fr. Illustration from Zogg (1962)

Actidium baccarinii (Paoli) Zogg
Zogg, Ber. Schweiz. Bot. Ges., 1960, 70, 195-205.

Rare. Europe. Collected from old wood of PinusPicea &Thuja.

Actidium baccarinii  (Paoli) Zogg. Illustration from Zogg (1962).

Actidium pulchra (Teng) Zogg
Zogg, Ber. Schweiz. Bot. Ges., 1960, 70: 195-205

Rare. China.