Brassica Downy Mildew

Hyaloperonospora parasitica

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

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

Downy mildew on the underside of a cabbage leaf [Credit: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org]

Host Plants:

On Crops: Turnips, broccoli, Brussels sprouts, cabbage, cauliflower, kohlrabi, kale

Where Found:

Worldwide, wherever host crops are grown, especially in cool climates

Description:

The early evidence of downy mildew consists of light green irregular spots on leaves, which quickly turn brown and become more numerous. The spots have a damp appearance, and often merge to become patches. On leaf undersides, you may be able to see a fuzzy mould with the help of a magnifying glass. 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:

Leafy greens are not appealing to eat when covered with spots. When almost-perfect heads of broccoli or cauliflower become infected, dark sections of rot are often invaded by other microorganisms, making a bad situation worse.

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 in heavy rains. Avoid using sprinklers or other overhead irrigation methods after cabbage, broccoli or cauliflower heads have formed.

Managing Outbreaks:

Clip off and compost badly affected leaves. With the return of dry weather, leafy greens will usually produce a new crop of sound leaves.

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