The effect of Phaeosphaeria leaf spot, caused by Phaeosphaeria maydis, in the net photosynthetic rate and in the transpiration rate of maize (Zea mays) leaves was assessed under field condition, in three hybrids and one maize inbred. Photosynthetic rate was related to diseased leaf area by the model Px/Po=(1-x )beta, where Px represents the net photosynthetic rate of a leaf with disease severity x (in proportion), Po represents the average net photosynthetic rate of healthy leaves, and beta is an expression of the ratio between virtual (leaf area where photosynthesis is nil) and visual lesion (leaf area covered by symptoms). Parameter beta characterizes the effect of the pathogen on the photosynthetic efficiency for the total range of measured disease severity. Values of beta, determined by nonlinear regression analysis (R² from 0.73 to 0.85 for the different material), ranged from 2.46 to 2.69. These values indicated that the reduction in photosynthetic efficiency was detected not only in the lesioned tissue but also in part of the remaining green tissue of infected leaves. Reductions in net photosynthetic rate around 40% were observed to disease severity between 10 and 20%. The reduction in transpiration rate was proportional to reduction of healthy leaf area due to disease. There was a negative linear relation (R² from 0.59 to 0.75 for different materials) between transpiration rate and disease severity.
