Lettuce Downy Mildew

Bremia lactucae, a fungus-like microorganism

Downy mildew on lettuce [Credit: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org]

Host Plants:

On Crops: Lettuce and a few flowers

Where Found:

Worldwide, wherever lettuce is grown, especially in cool climates

Description:

When otherwise healthy plants are exposed to a period of cool rain, yellow patches may develop on leaves, between veins. On leaf underside, a fuzzy mould often can be seen under the yellow patches. When dry weather returns, the patches turn brown. This fungus grows best when temperatures range between 10-15C (50-60F. A prolonged period of damp weather is needed to trigger an outbreak of downy mildew.

Damage:

Lettuce leaves with brown spots are not appealing to eat. Frequently the inner leaves of infected plants escape damage.

Preventing Problems:

Choose resistant varieties when they are available. Make sure plants get good air circulation and plenty of sun, and keep weeds controlled to promote prompt drying after rains. Use mulch to keep soil from splashing onto plants during heavy rains. Avoid using sprinklers or other overhead irrigation methods after lettuce heads or hearts have formed.

Managing Outbreaks:

Clip off and compost badly affected leaves. With the return of dry weather, leaf-type lettuce plants will usually produce a new crop of sound leaves from the plants’ centres.

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