Spermogonia amphigenous, 70-80 µm diam., 30-45 µm high, scattered or aggregated, pale yellow.
Aecia usually hypogenous, scattered or in small groups on yellowish spots, rounded or oblong in outline and then to 750 µm long, pale yellow, erumpent. Aeciospores 14-17 x 12-16 µm, subglobose, ovoid or angular, hyaline, aseptate, the wall ca 1 µm thick, finely verrucose.
Uredinia hypogenous, to ca 500 µm diam., pulvinate, yellowish-orange, pulverulent. Paraphyses 40-60µm tall, the apex clavate to globose, 8-17 µm diam., the wall 3-6 µm thick. Urediniospores 14-23 x 10-16 µm, broadly ellipsoidal, ovoid or obovoid, the wall 2-3 µm thick with stout, rather distant spines about 2 µm apart.
Telia hypogenous, subepidermal, often extending over the whole leaf, 500-1000 µm diam., often irregularly shaped and frequently bounded by host veins, dark brown. Teliospores in a single palisade, 40-60 x 7-12 µm, cylindrical or slightly broader towards the apex, sometimes angular in horizontal section due to compression from surrounding spores, rounded at both ends, yellow-brown, the wall 1-2 µm thick, smooth, without germ pores.
Not formally assessed. More studies are needed to confirm species boundaries before an assessment is appropriate.
Uredinia and telia are difficult to distinguish from other cryptic species in the Melampsora populnea aggregate; M. laricis-tremulae and M. magnusiana (with spermogonia and aecia on Chelidonium, Corydalis and Papaver) tend to have longer and slightly broader teliospores than M. pinitorqua (aecia on Pinus) and M. rostrupii (aecia on Mercurialis). M. magnusiana has uredinial paraphyses that tend to be broader and more thick-walled than those of M. laricis-tremulae (14-22 µm versus 8-17 µm broad at the apex and with a wall ca 3 rather than 2 µm thick).
Spermogonia and aecia formed in needles of Larix decidua, uredinia and telia on leaves of Populus alba, P. tremula and their hybrid P. x canescens.
Not well understood in GB&I due to confusion with other cryptic species in the Melampsora populnea aggregate.