To improve simulation of epidemics of peanut rust, information on the effects of leaf age and pustule eruption dynamics is needed. An analysis was made of the change of leaf susceptibility of rust with leaf aging using plants exposed to field inoculum. The youngest leaf was most susceptible to rust infection. As leaves aged, susceptibility decreased quickly. The relationship of infection frequency (Y, relative number of pustules) to leaf age (X, leaf position downward on the main stem) was described by a mathematical model Y = 2.17X^(-0.83). The dynamics of pustule eruption also was studied using artificial inoculation. Analysis showed that after inoculation, pustule eruption over time was distributed in a logistic pattern. Rust eruption began at 130 and reach a peek at 250 rust degree days—that is, between 7 and 15 d after inoculation. Mathematical models of leaf age-related susceptibility and rust eruption dynamics will be incorporated into simulation model of rust epidemics.