Downy mildew—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 | Life
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.
spots on upper surface of leaf
growth on underside of leaf