The rust fungus Melampsora populnea causes galls on the alternate host plants of Dog's Mercury and (rarely) Annual Mercury.  These are pale orange-yellow, white edged swellings up to 2 cm across, on leaf tips, distorted petioles and sometimes on stems.  The swellings carry spermogonia and orange aecia.  

The primary hosts, bearing uredinia and telia, are White Poplar (Populus alba), Aspen (Populus tremula) and the hybrid between the two, Grey Poplar (Populus x canescens).

This is complex of species and taxonomy is unclear, with no established consensus.  We have retained Melampsora populnea as the name for the rust causing galls on Dog's Mercury.  Within the broader complex are a wide range of aecial hosts - see 'Further Information' below.

Other species of Melampsora produce uredinia and telia on Grey/White Poplars and Aspen - see M allii-populina

Dog's Mercury Populus

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.  If you are recording uredinia/telia on Populus, microscopic examination will be needed.

Alternating between White Poplar/Grey Poplar/Aspen and  Dog's Mercury

Several species found in the UK are part of the complex of species collectively known as Melampsora populnea. Uredinia and telia are formed on Populus alba, P x canescens and P tremula, but aecia are on a wide diversity of plant species.   See Plant Parasites of Europe website for more details.

• Melampsora laricis-tremulae - aecial host = Larch (Larix)
• Melampsora magnusiana - aecial host = various species in poppy family (Papaveraceae) 
• Melampsora pinitorqua - aecial host = Pine (Pinus) - the species causes galls
• Melampsora pulcherrima - aecial host = Annual Mercury (Mercurialis annua)
• Melampsora rostrupii - aecial host = Perennial Mercury (Mercurialis perennis)