A systemic smut fungus visible as black masses in flowers of Rhynchospora spp. (particularly R. alba) and causing dwarfing of the plant and growth of congested flowers. Possibly more visible in older inflorescences. Common name: small spored white beak-sedge smut.
Sori in inner part of all flowers of an inflorescence as black, semi-agglutinated to powdery spore mass protected by the glumes. Infection systemic, infected plants often dwarfed, with more congested flowers and shorter, more pointed glumes and bristles than the healthy ones. Spores single, flattened, in side view elliptic, slightly irregular or reniform (kidney-shaped), 6.5-9.5 µm wide, rarely with one, flat, hyaline view rounded subangularly or angularly irregular occasionally subcircular or elliptic, 9-13 x (11-)12-17 µm, yellowish brown; wall even or slightly uneven 0.5-1 µm thick, thickest at the angles, finely densely foveolate-reticulate (with small pits or depressions), spore profile smooth, in SEM the wall between the shallow foveolae (small pits) very finely verrucose (warty). Spore germination of hibernated spores results in 4-celled basidia bearing lateral and terminal, ellipsoidal basidiospores on sterigmata which conjugate in pairs, or two basidial cells conjugate and develop two, large, elongate, dikaryotic conidia.
Description adapted from Vánky, K. (2012). Smut fungi of the world. St. Paul, Minnesota: APS press.
Not formally assessed. Considered as potentially extinct in the provisional RDL (Evans et al., 2006), but recent collections from England demonstrate that this is not the case. Very infrequently recorded, with only three historical records in the FRDBI.
Ustanciosporium gigantosporum, a morphologically similar species also specific to Rhynchospora alba ovaries, has also been previously recorded in GB&I but was also feared to be extinct (Evans et al., 2006). U. montagnei is known from R. alba in Europe, but is not considered authentically British. Historical collections under this name in the Kew Fungarium have been redetermined as a mixture of U. majus and U. gigantosporum.
Ustanciosporium gigantosporum, U. majus and U. montagnei are primarily differentiated by their different teliospore sizes (largest measurement 18-21 µm, 14-17 µm and 9-13 µm respectively). However, mixed infections on R. alba are considered common, and there is little available DNA evidence to indicate whether these represent distinct species concepts, or are one single species with different teliospore phenotypes.
Ustanciosporium majus is a specific biotrophic parasite and pathogen of Rhynchospora alba, infecting host plant ovaries, and in GB&I is only known from this host. Elsewhere (in Europe, Asia, N, C & S America) it has also been recorded on R. fascicularis, R. glomerata, R. marisculus, R. stipitata, and R. nana. Sori are found in all the spikelets of an infected inflorescence, usually with all the stems of one tuft infected and dwarfed. The fungus is visible as swollen black masses still enclosed in the glumes. Infected inflorescences may be more easily detected later in the flowering season by inspecting visibly stunted and dwarfed plants, or stunted spikelets and flowers. Sori should be well-developed and more apparent in older infected inflorescences.
Recent collections are known from Dorset VC9 and W Sutherland VC108, with older records from Dorset VC9, West Sussex VC13, and West Galway VCH16.