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Powdery mildew is an important disease of rubber trees worldwide. To assess the effects of temperature and leaf age on conidial germination and disease development, conidia were inoculated onto leaves of rubber tree seedlings at three phenological stages (copper bronze, color changing and light green) and then incubated at six constant temperatures (10, 15, 20, 25, 30 and 35 °C). Leaf age did not affect conidial germination (<i>P</i> = 0.296) while temperature significantly affected conidial germination (<i>P</i> &lt; 0.0001) although conidia were able to germinate at all tested temperatures. The estimated optimal temperature for conidial germination was 23.2 °C. Leaf age, temperature and their interactions had significant effects on conidial infection and hypha number (<i>P</i> &lt; 0.0001). At 10 and 35°C, more time (2-5 days) was needed for infection to complete, compared to &lt; 2 days for other combinations. Sporulation and mildew symptoms were only observed on those inoculated leaves of all stages at 20 and 25 °C, and at the copper bronze stage at 15 °C only. At 15°C, the latent period on the copper bronze leaves was longer (9 days) than at 20 and 25 °C (4 days). The latent period at 20 and 25°C increased from 4 to 7 days as the leaf development stage increased from copper bronze to light green. Therefore, temperature affects germination and post-germination growth of rubber tree powdery mildew, whereas leaf age primarily affects post-germination growth of the pathogen.