Tar spot and rust diseases are commonly found on leaves of polar willow in Arctic regions. However, neither taxonomic information nor epidemiological aspects of the pathogens have been reported. Purposes of present study were to characterize these two pathogens on species identification and epidemiology at Ny-Ålesund, High Arctic Norway (79°N).

On the basis of microscopic morphological observation and sequences of the rDNA-ITS regions, pathogens of tar spot and rust diseases were identified as Rhytisma salicinum and Melampsora epitea, respectively.

Epidemiological investigations were performed for fifteen colonies of polar willow at Ny-Ålesund in August 2008 and 2010 on frequencies of tar spot- or rust- diseased leaf plants, and coverage rate of mosses on soil surface of each colony. The frequency of tar spot disease was increased with increasing the moss coverage. In contrast, the frequency of rust disease was not affected by the moss coverage.

The results suggest that moss covering of soil surface increased the moisture around polar willow leaves and might enhance infection of the tar spot pathogen. The results also indicate that the both diseases have no effects on survival of the plants during the two year.