Stromata when young typically surrounded by a conspicuous bright yellow-orange region ca 2 mm wide that sometimes fades with age, black, very variable in size and shape, often coalescing to form large continuous areas that are sometimes several cm long, eventually breaking off together with the covering and underlying bark to leave bare wood.
Conidiomata preceding and accompanying ascomata on the same stroma, black, variable in extent and difiicult to distinguish externally, the surface undulating, opening by splits or wide breaks in the covering layer; embedded beneath several layers of collapsed tanned bark cells, the upper wall composed of a single layer of brown angular cells. Individual locules separated by columns to ca 50 µm diam., composed of dark brown angular cells 3-6 µm diam. Lower wall to ca 40 µm thick, also of dark angular cells. Conidiogenous cells arising directly from the lower wall, 18-24 x 2-3 µm, colourless, thin-walled, smooth, cylindrical but slightly tapering towards the apex, proliferating percurrently, with marked periclinal thickening. Conidia (3.5-)4-5(-6) x 2-2.5(3), ellipsoidal, colourless, thin-walled, aseptate, smooth, probably spermatial in function.
Ascomata black, poorly-defined externally, generally larger than the conidiomata and raising the substrate surface much more markedly, often extensive, opening by irregular longitudinal or radial cracks. Upper wall inserted below several layers of collapsed periderm cells, blackened , to ca 80 µm thick, composed of angular brown cells 3-8 µm diam., lower wall not blackened. Subhymenium 20-30 µm deep, intergrading with underlying hyphal stromatic tissue. Interascal tissue of thin-walled, septate, apparently unbranched paraphyses 1·5-2·5 µm diam., slightly wider at the apex, slightly longer than the asci, embedded in a conspicuous brown epithecium. Asci produced from croziers, 180-230 x 14-18 µm, cylindrical, short-stalked, thin-walled, not fissitunicate, the apex obtuse and without circumapical thickening, not changing colour in iodine, discharging spores through a large and irregular apical split, 8-spored. Ascospores arranged uniseriately, 28-36 x 11-14 µm, ellipsoidal, colourless, thin-walled, smooth, aseptate, when freshly collected with a group of radiating mucous hair-like appendages visible at one or both ends.
Description adapted from Minter (2001, publ. 2002). Cryptomyces maximus. IMI Descriptions of Fungi and Bacteria. Set 148 No. 1473; and Granmo et al., (2012). The secrets of Cryptomyces maximus (Rhytismataceae). Ecology and distribution in the Nordic countries (Norden), and a morphological and ontogenetic update. Karstenia, 52, 59-72.
Not formally assessed, but it has been considered as Vulnerable (provisional RDL 1992) and Vulnerable/D2 (provisional RDL2006) due to being known from a small number of known records and sites.
In recent years, this species has been found to be much more widespread than previously thought in some areas of GB&I. it is currently widespread at many sites throughout Orkney, and present in very large, albeit somewhat localised, populations in Renfrewshire (Scotland); present at six sites in Pembrokeshire (Wales); and with single sites known in Shropshire (England), and Limerick (Republic of Ireland).
Due to the size and extent of the recently discovered Scottish populations, C. maximus would probably be assessed as Least Concern in Britain based on estimated population size.
On dead attached twigs of Salix cinerea subsp. oleifolia, especially those subject to mechanical damage caused by grazing animals, in Pembrokeshire. Present on Salix fragilis (and not other co-occurring Salix spp.), and with no indication of mechanical damage, in populations centred around Glasgow,. There are old records associated with Salix fragilis and S. viminalis. Present on a number of other Salix hosts in other countries.
In GB&I, in recent years known from Wales (Pembrokeshire), England (Shropshire), Scotland (Dunbartonshire, Orkney, Renfrewshire), and Republic of Ireland (Limerick). Historical collections (pre 1980s) are known from England (E and W Norfolk, Cambridgeshire), Northern Ireland (Down) and Scotland (Peebles, W Ross). There are recent records throughout Scandinavia, from Denmark and Finland to N Norway.
Considered to be a weak pathogen of Salix bark, but does not cause noticeable damage to tree health. Apparently capable of surviving on dead twigs for several years.