Melampsora rostrupii  

Spermogonia epiphyllous or hypophyllous, in small clusters, to 200 µm diam. and 90 µm high, shallowly domed, pale yellow.
Aecia hypophyllous and on petioles and stems, in clusters 0.5-1.5 (-2) cm diam., mostly 1-1.5 mm diam., often circinate around the spermogonia, often confluent and then elongate, pulverulent, bright orange. Aeciospores 19-24 x 18-20 (-22) µm [13-24 x 11-17 µm fide Wilson & Henderson 1966], globose to broadly ellipsoidal or broadly ovoid, pale yellow [orange en masse], the wall 1-1.5 µm thick, finely and densely verruculose.
Uredinia epiphyllous or hypophyllous, minute, ca 0.5 mm diam., pulvinate, pulverulent, bright yellow-orange. Paraphyses clavate to capitate, 14-23 µm diam. in the apical region. Urediniospores 17-28 x 14-18 µm, broadly ellipsoidal to globose or obobvoid, the wall ca 3 µm thick, with rather distant spines.
Telia mostly hypophyllous, scattered, 0.5-1 mm diam., dark brown. Teliospores 25-40 x 5-12 µm, prismatic, rounded at both ends, pale brown, the wall 1-2 µm thick, without pores.

Part of the Melampsora populnea aggregate; its status needs further investigation. Vialle et al. (2013) indicated that it is phylogenetically distinct, but collections on Chelidonium majus (often referred to as M. magnusiana but not yet recorded from GBI) were at least closely related..