Melampsora xcolumbiana, a natural hybrid of M-medusae and M-occidentalis

Hybrids of Melampsora medusae and M. occidentalis are herein described as M. xcolumbiana. This hybrid taxon is characterized by isolates that generally exhibit morphological intermediacy in uredinial and telial traits, and mixed virulence/avirulence on Populus trichocarpa and P. deltoides, the natural hosts of M. occidentalis and M. medusae, respectively. Hybrids frequently contain ribosomal ITS sequences from both Melampsora species, but discordant isolates were also found, like M. medusae in having a urediniospore equatorial smooth spot, and like M. occidentalis in spine density, and intermediate in urediniospore length. Isolates with hybrid urediniospore morphology but homozygous for ITS sequences from M. medusae or M. occidentalis were also found suggesting that the primary F-1 hybrids have produced F-2 and/or backcross progeny. In a 1997 survey of the leaf rust population on hybrid poplar in the pacific Northwest, this 'new population' of Melampsora xcolumbiana was the only taxon found. Specimens from nearly a century ago indicate that an older hybrid population is of wide distribution in North America. This 'older population' of Melampsora xcolumbiana was found on specimens of all species of Populus in sections Aigeiros and Tacamahaca, and on many of their hybrids, in areas outside the type localities of M. occidentalis and M. medusae. These three Melampsora taxa, M. medusae M. occidentalis, and M. xcolumbiana, are the only ones that occur naturally on sections Aigeiros and Tacamahaca of Populus in North America.