Many seedlings will be infected early, resulting in systemic infection and death of seedlings or in severely stunted plants that fail to grow. These severely stunted plants may die, or may survive, but not produce seed. The earlier the infection, the greater the damage. If only a low percent of the plants are systemically infected, normal plants can compensate yield. However, yield compensation may result in larger heads, slower dry down and lodging.
Systemic infection occurs through the roots when the soil is saturated.
It causes stunted plants with yellow veins on the upper leaf surface and a downy white growth on the lower leaf surface, opposite the yellow veins. Later, secondary spread may occur from wind borne spores, resulting in local lesions, which are yellow spots on the upper leaf surface with a downy white growth on the lower leaf surface opposite the yellow spots. The later infections usually are of no importance. Downy mildew was formerly controlled very well by seed treatment with Apron fungicide, but now a substantial portion of the downy mildew population is resistant to Apron. We hope to have a new fungicide available in time for seed treatment of the 2001 sunflower crop.