Peanut is affected by a number of pests and diseases that reduce yield and increase production costs. Rust, a foliar disease caused by Puccinia arachidis Speg. is one of the most important and widespread (Subrahmanyam et al. 1993). It is believed that the pathosystem rust-peanut co-evolved in Peru, where the host has been cultivated since prehistoric times. The peanut crop spread widely around the world after the Spanish and Portuguese colonization of South America, but the pathogen was largely confined to South America until late 1960s. However, it has since spread to all peanut-growing areas (Hammons 1973; Subrahmanyam et al. 1993). The pathogen is host-specific. Losses are particularly severe if the crop is also attacked by leaf spots [Cercospora arachidicola Hori and Cercosporidium personatum (Berk. and Curt.)]. Unlike leaf spots, rust-infected leaves tend to remain attached to the plant, and this, combined with a short life cycle, favor fast and prolific pathogen multiplication and rapid spread of the disease. Modern locally preferred cultivars generally are susceptible to rust and other foliar diseases. Chemical control of the fungus can be effective but is costly; timing of pesticide application also can be a challenge.