Dew and growth chamber tests were conducted on the alfalfa cultivar Ranger to determine the effect of duration of leaf wetness and temperature on several components of the alfalfa rust (Uromyces striatus) monocycle. Duration of leaf wetness and temperature both had significant effects on pustule development. Infection efficiency (number of alfalfa rust pustules per leaf) increased linearly as duration of leaf wetness was increased from 4 to 24 h after inoculation. There was an inverse linear relationship between temperature and infection efficiency as indicated by the slope (-5.73) of the regression line relating the number of pustules per leaf to increasing temperatures between 17.5 and 28°C. Infection efficiency was approximately 20 times greater at 17.5°C than at 28°C. Inoculated alfalfa plants exposed to constant temperatures of 15, 18, 21, 24, 27, or 30°C after an initial 24-h leaf wetness period (19°C) did not significantly affect infection efficiency (P ≤ 0.05), but did affect the time (from inoculation to the time when 50% of the pustules (T50) were visible (i.e., latent period). Using this state variable definition, latent period was found to decrease with increasing temperature. When latent period was measured as a rate variable, the rate of pustule appearance (as affected by temperature) was best described by the Gompertz model. Thus, temperature and length of the initial leaf wetness period had a greater impact on infection efficiency than did postinfection (post-leaf wetness) temperatures, whereas increasing temperatures from 15 to 30°C had a significant effect on shortening the latent period (T50) and increasing the rate of pustule appearance.