Leaf area index (LAI), light interception (LI, %), and frequency of two stem diseases (Phomopsis helianthi, Phoma macdonaldii) were assessed for sunflower over two years (1994 and 1995) at Auzeville (southwestern France). The field experiments compared different crop management methods varying in genotype, crop density, N fertilization, fungicide application, and irrigation. A relationship between LI and LAI was proposed and discussed in regard to crop density and growth stages. A close relationship was found between the rate of infection by Phomopsis and LI at anthesis in unsprayed plots. This relationship could be used to predict the potential infection as a function of foliar development. The relationship was less obvious with Phoma.