Downy mildew most commonly damages crucifers, cucurbits, onion, and leafy greens, such as spinach and lettuce.
Symptoms of downy mildew vary with the host and the environmental conditions. The first symptom is usually the appearance of pale green spotsas on the upper leaf surface. These areas soon become yellow and angular to irregular in shape, bounded by the leaf veins. As the disease progresses, the lesions may remain yellow or become brown and necrotic. During moist weather the corresponding lower leaf surface is covered with downy, pale gray to purple fungal mycelium and spores. Diseased leaves soon wither, cup upward (in cucurbits), and then turn brown. With cucurbits, the fungus usually infects older leaves nearest to the center of the hill. In broccoli, the disease is systemic with limited exhibition of external symptoms. In rainy, humid weather, the disease may spread so rapidly that affected vines appear to be frosted. Downy mildew can develop during transit (for example on lettuce).
Downy mildew is caused by many different species of Bremia, Peronospora, and Pseudoperonospora. Primary infections generally come from spores produced on greenhouse-grown plants, bulbs, or sets, or spores produced on southern-grown crops and carried progressively northward on moist air currents during the spring and summer. Day- and nighttime temperatures in the low to mid-60s, along with heavy dews, fogs, frequent rains, and high humidity, favor infection and rapid multiplication of the pathogen. Extended periods of hot, dry weather tend to inhibit the spread of this disease.