Field trials were carried out in 2002/03, with soybean (Glycine max) cultivars BRS 133 and BR 16, to evaluate the effect of foliar diseases on yield. The experimental design was complete randomized blocks with six treatments (different levels of disease) and four replications with plots of 18 m². Different times and numbers of applications of fungicide (difenoconazole + propiconazole) were used to generate a disease gradient. Disease severity and leaf area index were assessed weekly. Yield was estimated on central rows of the plots. Powdery mildew and cercospora blight were the predominant diseases in these trials. Disease severity at R6 (pods with full sized green beans) and R7 (pods yellowing, 50% of leaves yellowing) presented a significant negative linear correlation with yield. The relationship between the area under the disease progress curve (AUDPC) and healthy leaf area duration (HAD) and yields were investigated. The AUDPC presented a negative linear correlation with yield (R²=0.67 for BRS 133 and 0.69 for BR16) and a positive correlation with HAD, when a monomolecular model was used (R²=0.76 for BRS 133 and 0.62 for BR 16).
