Ustanciosporium majus  

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.

Map of known distribution in Great Britain and Ireland.