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Yearly, seasonal and circadian variations in airborne urediniospore concentration of birch rust, Melampsoridium betulinum (Fries) Kleb., were studied in Turku, Oulu and Kevo during 1983-1988, using the Burkard spore trap. Variation among areas and the effects of weather parameters on spore concentration were also examined. The total number of spores was much higher in Oulu than at the other locations. The yearly variation was high at all locations. The urediniospore season usually started in August and peaked in September. Urediniospores showed clear circadian variation; a peak was observed in the afternoon. High wind speed significantly increased airborne urediniospore concentration; likewise temperature affected the concentration positively.