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 generall y exhibit morphological intermediacy in uredinial and telial traits, and mi xed virulence/avirulence on Populus trichocarpa and P. deltoides, the natur al hosts of M. occidentalis and M. medusae, respectively. Hybrids frequentl y contain ribosomal ITS sequences from both Melampsora species, but discord ant isolates were also found, like M. medusae in having a urediniospore equ atorial smooth spot, and like M. occidentalis in spine density, and interme diate in urediniospore length. Isolates with hybrid urediniospore morpholog y 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 xcolum biana was the only taxon found. Specimens from nearly a century ago indicat e that an older hybrid population is of wide distribution in North America. This 'older population' of Melampsora xcolumbiana was found on specimens o f all species of Populus in sections Aigeiros and Tacamahaca, and on many o f their hybrids, in areas outside the type localities of M. occidentalis an d 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.