How to Manage Pests

Downy mildew on lettuce—Bremia lactucae

Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. White fluffy growth of the fungus develops on the underside. With time these lesions turn brown and dry up. Older leaves are attacked first. Severely infected leaves may die. On rare occasions the pathogen can become systemic, causing dark discoloration of stem tissue.

Identification

Downy mildew gets its name from the downy masses of spores it produces on undersides of affected leaves. These downy growths initially may be confused with those of powdery mildew. However, the two diseases differ in several important ways. The white sporulation of powdery mildew covers much of both the lower and upper sides of the leaf. Downy mildews are favored by cool, moist conditions while powdery mildew are favored by warm, dry conditions. Downy mildew spores are produced on branched stalks that look like little trees, whereas powdery mildew spores occur in chains on a single stalk. Areas affected with powdery mildew enlarge in a circular pattern, spreading a white, dusty growth over the whole leaf; there is no clear veinal delineation as there is in downy mildew infections.

Life cycle

Damp, moist conditions and cool weather are ideal for the development and spread of downy mildew. Spores, produced only under conditions of subdued light or darkness and a near-saturated atmosphere, are dispersed by wind and, in cool, moist conditions, they can survive several days. Free moisture on the lettuce leaf is necessary for spore germination and infection, but moisture is not required for vegetative growth once the fungus is within the leaf. Areas with year-round lettuce culture and favorable climate often have the most serious outbreaks. Dry, desiccating winds and clear, warm days inhibit the growth and spread of downy mildew.

Solutions

The best way to prevent downy mildew is to avoid the environmental conditions that favor it. Freely circulating air, allowing plants to dry between irrigations, and keeping leaves as dry as possible are good ways to limit the disease. Cool, moist weather in early spring and late fall favor the disease, so adjusting planting times may help in some areas. Destroy infected crop debris and susceptible related weeds before planting a new crop. Resistant cultivars are available.

Yellow spots on upper surface of leaf

Fungal growth on underside of leaf