Alternaria blight in sunflower, caused by the fungus Alternaria helianthi, is one of the major diseases that affects sunflower in Brazil. The effect of this organism on net photosynthetic rate of four sunflower genotypes was measured under field experimental conditions. Through a mathematical approach the diseased leaf area and net photosynthetic rate were related and used to analyze the pathosystem sunflower/Alternaria. An average decrease in the photosynthetic rate was observed in diseased leaves for all analyzed genotypes in comparison with healthy leaves. Alternaria blight reduces photosynthesis not only through a reduction in green leaf area, but also through an effect on photosynthesis of the remaining green leaf tissue. Leaves free of the pathogen lesions in diseased plants present also a photosynthetic reduction.