In an experiment carried out in the field with the barley (Hordeum vulgare) cultivar BR 2 the gradient of powdery mildew incidence and grain yield were generated by the use of fungicide in seed treatment. Data was submitted to regression analysis. A critical point model was obtained that may be used to estimate barley grain yield, in relation to powdery mildew incidence, at different host growing stages. The best model was represented by the equation Y = 2,978.3 - 7.75 X (R² = 0.83) at tillering. The maximum grain yield reduction was 28.03%. This equation may be also be used to calculate the economic damage threshold for powdery mildew in barley cultivars with susceptibility similar to BR 2.
