The spots turn black with the arrival of cool temperatures in late summer. Management is most successful with resistant hybrids, but most hybrids do not have resistance to all of the newest races present in major U.S. sunflower producing areas. The incidence of rust can be reduced by destroying wild or volunteer plants, not planting susceptible varieties near one another, avoiding higher than needed rates of nitrogen fertilizer, and planting to establish recommended plant populations. The downy mildew fungus is occasionally seed-borne. Under cool, water-saturated soil conditions the spores germinate, enter the sunflowers roots, and spread throughout the plant.
Infected seedlings have yellow leaves.
Plants infected after the seedling stage develop thickened, club-like roots, are stunted, but do not show any foliar symptoms. Wind-blown spores may adhere to leaves and cause small, localized, angular chlorotic spots. During periods of high humidity, the underside of the leaf may be covered with a white, downy growth. Downy mildew is managed by using resistant varieties, rotating crops, avoiding poorly drained fields, planting in uninfected fields, controlling wild and volunteer sunflowers, using a fungicide seed treatment, or delaying planting until soil temperatures favor rapid growth of seedlings. Most currently available hybrids are not resistant to all common races of the downy mildew fungus.