A minute (1-4 mm diam.) yellow-brown to brown discomycete growing from a lumpy black crust (Xenotypa aterrima) on Betula (birch) spp., often where the bark appears to have been stripped away.
Anamorph: not known.
Teleomorph: stromata absent. Ascomata apothecia, superficial, scattered, developing from a black crust of Xenotypa aterrima; 1-2 (-4) mm diam., cupulate, externally yellow-brown when very young and maturing to dark brown, with a paler irregular fringe-like margin; striate and finely scurfy, with a short stout stalk. Disc yellowish-brown, drying blackish brown. Outer excipulum to ca 60 µm thick, composed of globose cells 4-5 µm diam., covered with short undulating thin-walled septate hairs 3-4 µm diam. Internal tissues of loosely interwoven hyaline hyphae to ca 4 µm diam. Interascal tissue of simple cylindrical paraphyses ca 2 µm diam., slightly enlarged and pale yllowish brown at the apices. Asci 110-110 x 10-12 µm, clavate, short-stalked, with croziers, the apex rounded with a broad shallow ring staining blue in iodine, 8-spored. Ascospores (18-) 25-33 x 6-8 µm, fusiform, straight or slightly curved, hyaline, aseptate, 1-septate or infrequently 2- to 4-septate, often with several large lipid globules when living.
Description based on Ahti et al. (2000) and Dennis (1956). Previously known as Dencoeliopsis johnstonii, but reassigned to Rutstroemia on the basis of DNA evidence (Pärtel et al. 2016).
Not formally assessed. Classed as Vulnerable / D2 in the provisional Red Data List of Threatened British Fungi (Evans et al., 2006). A Northern Ireland Priority Species.
Listed as a species of principal importance to the conservation of biodiversity in the Northern Ireland Priority List, under the Wildlife and Natural Environment Act (Northern Ireland) Act 2011.
None are likely in association with Xenotypa aterrima. Other brownish members of the Rutstroemiaceae/ Sclerotiniaceae have been reported for Betula, including Rutstroemia firma (approximately 10x larger than R. johnstonii), and Ciboria spp. (present only on seeds and catkins). Encoelia furfuracea has recorded once in GB&I on Betula, but if present should be clearly distinguishable by its furfuraceous appearance ("covered in bran-like scales"). Godronia radulicola (= Cenangella radulicola) is known to also colonise Xenotypa aterrima, but the species concept is indistinguishable from R. johnstonii and is almost certainly a later heterotypic synonym (Nannfeldt, 1936). The only other known member of R. johnstonii's previous genus, Dencoeliopsis betulicola, also occurs on Betula, but has only been reported once (from New Hampshire, USA); it can be distinguished by its much smaller apothecia, asci and ascospores. (Zhuang, 1988).
Found exclusively as a parasite of the fungus Xenotypa aterrima (Diaporthales) in the UK and the vast majority of countries in which it has been found. X. aterrima is apparently specific to Betula, and has been found on the branches of both living and dead trees.
In GB&I, known from VC17 Surrey and VC H40 Londonderry. Recent surveys have discovered futher sites in VC90 Angus and VC77 Lanarkshire.
It was originally described from the Berwick upon Tweed area, but has not been seen there for well over a century.
Knon in GB&I from raised peat bogs, wet boggy areas on lowland heath, and woodland.