Melampsora populnea  

A species aggregate with uredinia and telia on white poplars (Populus alba, P. tremula and hybrids), containing the following cryptic species:
1. Melampsora laricis-tremulae (spermogonia and aecia on Larix decidua)
2. Melampsora magnusiana (spermogonia and aecia on Corydalis and Chelidonium)
3. Melampsora pinitorqua (spermogonia and aecia on Pinus spp.)
4. Melampsora rostrupii (spermogonia and aecia on Mercurialis).

All probably occur in Great Britain.