Sunflower stem canker caused by Diaporthe helianthi Munt.-Cvet.et al., is an important disease in the state of Tamaulipas, where more than 50% of infected plants have been observed. A field study was undertaken to determine the influence of stem canker on oil content and weight of seed and yield, at different stages of seed development. The lowest percentage of diseased plants was registered during flowering. Oil content, seed weight and yield were significanty reduced when stem canker occurred at any stage of seed development. These effects became progressively less pronounced with later disease appeaance.