Abstract: Uromyces striatus is the causal organism of alfalfa rust. The effects of humidity, temperature and time on disease severity and the effects of incubation temperature on the latent period of alfalfa rust were investigated using growth chamber and greenhouse experiments, respectively. Alfalfa plants were inoculated with U. striatus and incubated at 19℃ with 100% relative humidity (RH) for different periods before being transferred to 25℃ conditions for 15 days. Subsequently, spore densities on the leaves of inoculated plants were assessed. The results showed that spore density of treatments incubated at 19℃ for 12 h to 48 h was significantly higher than those incubated for 4 h to 8 h. Plants were also inoculated and incubated at 19℃ with 100% RH for 24 h and then transferred to different temperature environments; 15, 20, 25, 30 and 35℃. Plants transferred to 35℃ conditions did not display any alfalfa rust symptoms. The disease latent period on inoculated plants grown at 15, 20, 25 and 30℃ was 11, 10, 7 and 7 days, respectively. The spore density on leaves in the 25 and 30℃ treatments was significantly higher than the 15 and 20℃ treatments. There was no relationship between incubator temperature and disease latent period.