The survival of dispersed urediniospores of Tranzschelia discolor on the undersurfaces of dry prune foliage was monitored during spring and early summer under outdoor conditions in the Murrumbidgee Irrigation Areas of New South Wales. The spores were produced under standard glasshouse conditions to ensure high initial viability (generally >90%) and samples were exposed to outdoor conditions at fortnightly intervals. Early in the season, viability declined relatively slowly, and in the first four samples >60% of urediniospores were viable 20 days after initial exposure dates. From late November the decline in viability increased markedly and suddenly, and in a sample exposed on 30 November only 20% of urediniospores were viable 21 days later. Other samples exposed from late November onwards showed similar or faster rates of decline. It was shown that temperature was the main climatic factor affecting urediniospore viability and that relative humidity and solar radiation did not contribute to decline in viability. A model based on temperature was constructed to forecast decline in urediniospore viability. This showed that urediniospores of T. discolor were critically affected by temperatures in the range 25-30°C. At temperatures below this range decline in viability was shown to be negligible, whereas exposure to temperatures > 25-30°C resulted in a 34-fold increase in the rate of decline. © 1988 CSIRO. All rights reserved.