Downy Mildew

The name downy mildew is somewhat descriptive of the fluffy appearance of the white, lavender, or purple sporulation that occurs usually on the undersides of diseased leaves. Downy mildew leaf lesions are often angular in shape and delimited by veins. Pale yellow, purple, or necrotic areas often are visible from the upper side of the leaf. Extensive disease can result in the death of large portions of the leaves.

In some plants, when young shoots are infected, the fungus may become systemic and the resultant growth is stunted, malformed, and discolored.

In contrast to powdery mildews, the downy mildews require very wet or humid conditions to flourish. Water is required for infection, and humidity above 90% is needed for sporulation. Growth of the pathogens is favored by cool temperatures.

Downy mildew spores are usually short-lived, although they may survive several days under cool, moist conditions. They are airborne, and when they land on a susceptible plant with free water present, germination and infection generally occur within 8 to 12 hours. Some downy mildews also produce a sexual spore (oospore) that can survive dry conditions. This enables the pathogen to survive in the absence of a host. Downy mildews are favored by moist and cool conditions (40° to 60°F).

Use varieties resistant to downy mildew. Avoid sprinkler irrigation and reduce relative humidity in greenhouses. Fungicide treatment of susceptible varieties is needed when the disease occurs on transplants or early in crop development in the field; repeated applications may be required to protect new growth. Treatment during early flowering is required on some seed crops.

Resistant varieties and some copper sprays are suitable for organically grown crops.