A rust on leaves, petioles and stems of Circuta virosa (cowbane), visible as yellow-orange (aecidia), cinnamon (uredosori) or brown-black (teliosori) pustules on leaves and stems.
Spermogonia on both sides of leaves, between the aecidia, rounded, almost colourless, subepidermal, 100-125 µm diam., with projecting paraphyses. Aecidia on all sides of leaves, petioles and stems, on yellowish spots which become darker, in rounded ellipsoidal or elongated groups, up to 1.5 cm long, pustular, peridium cupulate or hemispherical, with entire margin, rather short, golden-yellow; aecidiospores globoid to ellipsoid, 17-26 x 10-20 µm, wall finely puctate, subhyaline. Uredosori generally on leaf undersides, scattered, minute, pulverulent, cinnamon; uredospores subgloboid to ovoid, yellow-brown, 18-28 x 14-22 µm, wall echinulate, with 3 equatorial pores. Teliosori similar, but blackish-brown; teliospores ellipsoid, or oblong, rounded at both ends or rarely attenuated downwards, not thickened above, generally constricted, wall 28-46 x 18-30 µm, wall somewhat verruculose or distinctly verrucose-reticulated, occasionally nearly smooth, brown, pore of upper cell apical, of lower cell subequatorial, pedicels hyaline, thin, short, falling off when mature. Auteu-form (all life stages associated with one host plant), aecidia developing in June, and uredospores and teliospores developing from July-October
Description adapted from Wilson, M., & Henderson, D. M. (1966). British rust fungi.