Puccinia menthae  

Spermogonia scattered or arranged in little groups among the aecidia, honey-coloured, with flexuous hyphae and periphyses. Spermatia 2.5-3.5 x 3-4.5 µm.
Aecia hypophyllous and often on the stems, arranged on the leaves in clusters on orange or purplish spots, or forming elongate patches on the thickened and deformed stems and petioles, opening irregularly, margin scarcely torn, erect or slightly incurved, peridial cells elongate and compressed, not firmly connected. Walls rather thin, striate, outer ca 2 µm thick, finely punctate, inner ca 3 µm thick, verrucose. Aeciospores 24-40 x 17-28 µm, pale yellow, ellipsoidal, the wall colourless, verruculose 1.5-2.5 µm thick.
Uredinia hypophyllous, on yellowish or brownish spots (or without spots), ca 1.5 mm diam., roundish, scattered or aggregated, surrounded by the ruptured epidermis, sometimes confluent, cinnamon brown. Urediniospores 17-28 x 14-19 µm, ellipsoidal, rarely subglobose, the wall 1.5-3 µm thick, echinulate, pale brown, with 3 equatorial pores.
Telia similar but dark brown. Teliospores 26-35 x 19-23 µm, subglobose to obovoid, rounded at both ends, not or scarcely constricted at the septum, dark brown, the wall indistinctly verruculose, sometimes smooth, pore of upper cell apical, of lower cell close to the septum, each with a broad hyaline papilla.

Description adapted from Wilson & Henderson (1966).