Downy mildew of lettuce has been a significant challenge to lettuce production for many years and occurs wherever lettuce is produced. Although the incidence of the disease is higher in cool humid areas, it can occur even in desert production areas if conditions are favorable. Symptoms include light green to yellow angular spots on the upper surface of the leaves, with white fluffy growth on the undersurface of the leaf below each spot. The disease typically infects older leaves first and continues to spread to younger tissue. In some cases, the disease can become systemic causing necrosis of the plant stem.