In wet habitats, on leaf sheathes of Juncus, Carex, Deschampsia etc. Very rare. Known from Sweden, Poland and France (Aronsen & Læssøe 2016). According to Emmett et al. 2018, it has been recorded in Norway, but there are no collections in the herbaria or records in the databases. Seems to be very rare in Europe.
Pileus 1.5-7 mm across, bell-shaped to convex, with or without a small umbo, shallowly sulcate, little translucent-striate, delicately pruinose, glabrescent, vinaceous buff, pale pink or greyish sepia brown, sometimes fuscous black at the centre. Odour and taste indistinctive. Lamellae 8-12 reaching the stipe, ascending, becoming subhorizontal, +/ arcuate, broadly adnate, decurrent with a short tooth, white to pinkish, the edge concave, concolorous, or somewhat darker pink. Stipe up to 30 x 0.5 mm, cartilaginous, equal for the greater part, straight to somewhat flexuous, pruinose at the apex and near the base, glabrescent, vinaceous buff above, greyish sepia to fuscous black below, becoming pinkish-hyaline, the base attached to the substratum by a patch of radiating, greyish fibrils.
Basidia 18-32 x 8-10 μm, broadly clavate, 4-spored. Spores 8-11.5 x 3.5-4.6(-5) μm, Q 2.1-3.1, elongated pip-shaped, smooth, amyloid. Cheilocystidia 13.5-27 x 6.5-11.5 μm, forming a sterile band, subcylindrical, clavate, obpyriform, somewhat irregularly shaped, covered with comparatively few, evenly to unevenly spaced, simple to branched, cylindrical, straight to curved excrescences 1-7 x 1-2 μm. Pleurocystidia absent. Lamellar trama dextrinoid, vinescent in Melzer's reagent. Hyphae of the pileipellis 1.5-8 μm wide, the narrower much branched, more or less densely covered with warts or cylindrical excrescences 1-7 x 1-1.8 μm. Hyphae of the cortical layer of the stipe 2-6μm wide, diverticulate, excrescences up to 8 x 2 μm, the terminal cells not inflated, 3.5-4.5 μm wide, diverticulate. Clamp connections abundant.
I have never seen material of M. juncicola, and the description above is based Maas Geesteranus (1986 a) and Corriol (2000) Maas Geesteranus's microscopic observations were based on one Polish collection that, according to the author, probably was not quite mature, and hence the spores may be somewhat larger in other collections. Corriol (2000) reported on one collection from France and provided some excellent photos. His description was an important addition to the poor knowledge of this very rare species.
Mycena juncicola belongs to a small group of tiny, pinkish, vinaceous coloured species growing on monocotyledoneae in wetlands. All species are rare, and there are not many collections available, so the differences are probably not yet completely understood. Because of the small size these species are not easy to describe and identify. The colours can vary, both of the pileus, the lamellae and the stipe, and the attachment of the lamellae is not a decisive feature. The attachement of the stipe to the substrate is not always easy to observe. These species are in great need of further studies, and collections of M. juncicola would be very welcome.
M. riparia is supposed to have a more pinkish cap. In addition it differs in having more irregularly shaped cheilocystidia, clavate to subglobose caulocystidia 6.5-11 μm wide, and in lacking clamps.
M. tubarioides can be separated on account of the gelatinous matter covering the hyphae of the pileipellis, the gelatinous lamellar edge, the arcuate, very broad lamellae, differently shaped cheilocystidia and the larger spores.
The North American species M. culmigena Maas Geest. can be separated from M. juncicola because of somewhat larger spores and more regularly and symmetrically shaped cheilocystidia. In addition Maas Geesteranus (1986 a) noted that the hyphae of the basal disc of M. juncicola were clamped, with colourless cell-walls, while they are clampless and with brown cell-walls in M. culmigena.
Redhead (1980) described the new species M. cariciophila, which Maas Geesteranus (1986 a) put into the synonymy of M. juncicola.