An open-top chamber experiment was performed which highlighted the interactive effect of ozone (O-3) on the diffusion and incidence of the peach rust parasite. Such effect was linearly related to the O-3 cumulative dose received by the exposure variants, while the best fit with the daily 8-h O-3 average concentration was found to be multiplicative. Filtering ambient air reduces in a dramatic way (more than 50%) the rust severity. Stomatal conductance was neither correlated to O-3 levels nor to rust infection. The possible mechanisms involved in the above interaction are discussed herein.