Melampsoridium betulinum  

Spermogonia amphigenous, rather numerous, scattered, 50-65 µm diam. and 20-30µm tall, flattened-conical, subcuticular, pale yellow. Spermatia ovoid, 1.5-2µm diam.
Aecia hypophyllous, solitary or in longitudinal rows on one or both sides of the midrib, 100-150 µm wide, 0.3-1 mm long and 300-500 µm tall, subepidermal. Peridium light reddish-orange fading to white, rupturing at the apex with an irregularly torn margin, peridial cells small, 25-30 x 12-20 µm, rhomboid, the inner walls finely verrucose, 2-3µm thick, the outer of equal thickness, smooth. Aeciospores 16-24 x 12-18 µm, globose or ellipsoidal, the wall 1-1.5 µm thick, finely and closely verrucose except for a small smooth and slightly thinner area on one side.
Uredinia hypophyllous, with yellow spots showing on the upper side of the leaf, 0.1-0.5 mm diam., subepidermal, scattered. Peridia hemispherical, yellow, firm, at length opening at the apex. Peridial cells small, polygonal, 8-18 µm diam., those at the sides radially elongate, ostiolar cells ovate-conical tapering into an acute apex. Uredospores 22-38 x 9-15 µm, ellipsoidal or clavate, the wall hyaline, 1-1.5 µm thick, with distant spines but smooth at the apex.
Telia hypophyllous, subepidermal, scattered, often covering the whole leaf, orange becoming yellowish-brown. Teliospores in a palisade-like layer, 30-52 x 8-16 µm, prismatic, rounded at both ends, the wall nearly colourless, smooth, laterally ca 1 µm thick and up to 2µm at the apex.