Technical Abstract: Field experiments were conducted in 2002-2006 to characterize yield potential and disease resistance to Cercospora arachidicola (early leaf spot) and Cercosporidium personatum (late leaf spot) in the Bolivian peanut (Arachis hypogaea) cultivar, Bayo Grande, and breeding lines developed from crosses of Bayo Grande and the U.S. cultivar, MDR-98. Bayo Grande, MDR-98 and three breeding lines along with the U.S. cultivars C-99R and Georgia Green were included in split-plot field experiments in six locations across the U.S. and Bolivia. Whole-plot treatments consisted of two tebuconazole applications and a nontreated control. Genotypes were the sub¬plot treatments. Area under the disease progress curve (AUDPC) for % defoliation was lower for Bayo Grande and all breeding lines than for Georgia Green at all U.S. locations across years. AUDPC for disease incidence from one U.S. location indicated similar results. Severity of leaf spot epidemics and relative effects of the genotypes were less consistent in the Bolivian experiments. In Bolivia, no improved disease resistance was observed among the breeding lines compared to Bayo Grande. In the U.S., yield of Bayo Grande and the breeding lines was improved compared to the other genotypes in one of two years. In Bolivia, low disease intensity resulted in the highest yields in Georgia Green, while high disease intensity resulted in comparable yields among the breeding lines and MDR-98 and C-99R. Two applications of tebuconazole significantly suppressed leaf spot in Bolivia compared to the U.S., possibly indicating a higher level of fungicide resistance in the U.S. Overall, data from this study suggest that Bayo Grande and the breeding lines may be desirable cultivars for U.S. and Bolivian production.