Critical-point yield model to estimate yield damage caused by Cercospora zea-maydis in corn

a model to estimate damage caused by gray leaf spot of corn (cercospora zea-maydis) was developed from experimental field data gathered during the summer seasons of 2000/01 and during the second crop season [january-seedtime] of 2001, in the southwest of goiás state. three corn hybrids were grown over two seasons and on two sites, resulting in 12 experimental plots. a disease intensity gradient (lesions per leaf) was generated through application, three times over the season, of five different doses of the fungicide propiconazol. from tasseling onward, disease intensity on the ear leaf (el), and el - 1, el - 2, el + 1, and el + 2, was evaluated weekly. a manual harvest at the physiological ripening stage was followed by grain drying and cleaning. finally, grain yield in kg.ha-1 was estimated. regression analysis, performed between grain yield and all combinations of the number of lesions on each leaf type, generated thirty linear equations representing the damage function. to estimate losses caused by different disease intensities at different corn growth stages, these models should first be validated. damage coefficients may be used in determining the economic damage threshold.