Sori surrounding rudiments of host tissue at the tip of the axis of sterile spikelets, with stunted glumes. Sori spherical to ovoid, approximately 1 x 1-1.5 mm in size; when young, hidden in the glumes and exposed when old, without a peridium or sterile stroma, with spores developing in a hyaline matrix on the surface of the host tissue. Spore mass agglutinated at the base, powdery in the upper part, dark. Ustilospores (teliospores) 11-14(-15) x (15-)17-19(-20) x (16-)18-21(-24) µm, single (not aggregated into clumps), subglobose to flattened, in plane view circular to ± bluntly angular; light to medium reddish to olive brown, sometimes with hyaline appendages. Walls approximately 1 µm thick, thinner on both flattened sides, finely and densely foveolate-reticulate.
Description derived from Piepenbring (2000).
Not formally assessed. Considered as potentially extinct in the provisional RDL (Evans et al., 2006), but two recent collections from Scotland and England demonstrates that this is not the case. Very infrequently recorded, with only two historical records in the FRDBI, both from the pre-1900s.
Ustanciosporium majus, a morphologically similar species also specific to Rhynchospora alba ovaries, has also been previously recorded in GB&I but has not been recorded for the past 50 years and is therefore 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 gigantosporum is a specific biotrophic parasite and pathogen of Rhynchospora alba, infecting host plant ovaries, and is only known from this host. 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 VC 9 Dorset and VC108 W Sutherland, with older records from VC22 Berkshire and VC29 Cambridgeshire.