Thekopsora minima is a heteroecious rust, with spermogonia and aecia occurring on the needles of hemlock (Tsuga spp.) and uredinia, telia, and basidia occurring on the leaves of ericaceous genera, including species of Rhododendron. The effect of temperature was determined for urediniospore germination, germ tube growth, and infection efficiency on Rhododendron ‘White Lights’. Percent germination and germ tube growth were assessed at 10, 15, 20, 25, and 30°C after 3 h of incubation on 1.5% water agar in the dark. Polynomial regression analyses revealed a significant effect of temperature on both germination (P < 0.001, R2adj = 0.936) and germ tube growth (P < 0.001, R2adj = 0.933), with predicted optimum temperatures of 21.5 and 22.0°C, respectively. Germination and germ tube growth were reduced greatly at 30°C and below 15°C. Temperature also was found to have a significant effect on infection efficiency, as measured by incubation period (P < 0.001, R2adj = 0.808) and uredinia produced (P < 0.001, R2adj = 0.866). On excised leaf disks of Rhododendron ‘White Lights’ maintained under a 14-h photoperiod, the shortest mean incubation periods of 10.7 and 10.0 days were at 20 and 25°C, respectively, with a predicted optimum of 23°C. The mean incubation period at 15 and 30°C was approximately 4 and 3 days longer, respectively, than at the predicted optimum temperature. The number of uredinia produced was similar at 15, 20, and 25°C, but was reduced sixfold at 30°C. The predicted optimum temperature for uredinial production was 19.5°C, with a 5% variation in uredinia production between 17.5 and 22°C.