Description contributioned by Graham Kinsey (CABI).
Anamorph: Asteroma alneum (Pers.) B. Sutton (1980). Conidiomata 100-260 µm diam, acervular, subcuticular, on both sides of the host leaf. Conidiogenous cells 7-10 x 2-2.5 µm. Conidia 8-12 x 1.5-2 µm, cylindrical, hyaline, aseptate.
Teleomorph: stromata absent. Ascomata 350-450 µm diam, somewhat flattened, the neck 250-400 x 80-100 µm and straight or curved, brown, immersed with the neck emergent, appearing as a low rounded brown pustule on both sides of the host leaf, thickly scattered or grouped. Peridium 13-15 µm thick, composed of 3-4 layers of cells. Interascal tissue absent. Asci 55-80 x 12-18 µm, 8-spored. Ascospores 12-16 x 5-7 µm, ellipsoidal to lemon-shaped, sometimes inequilateral with one side curved and one side near straight, containing many small guttules.