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Current taxonomy places all rust fungi that occur on willow (Salix spp.) in North America in one species complex, Melampsora epitea Thüm. Characteristics of M. epitea isolates from the Canadian arctic were compared to M. epitea isolates from temperate regions of North America. Sequences from internal transcribed spacer (ITS) regions of rDNA were obtained from urediniospores from rust-infected Salix leaves collected in the Canadian arctic and in Minnesota and compared. Phylogenetic analysis of nuclear ribosomal ITS regions indicated that arctic M. epitea samples were divergent from temperate M. epitea isolates, perhaps in part because all rusts examined diverged according to host species. Four urediniospore characteristics were examined: area, circularity (shape factor), major axis length and spine density. Statistically significant (P < 0.05) differences were observed for spine density among all host species except S. nigra and D. bebbiana. However major axis length differed between these species. These results represent the first evidence that arctic and temperate Melampsora species on Salix hosts in North America have evolved distinct molecular and morphological characters. [ABSTRACT FROM AUTHOR]