Tomato Powdery Mildew

Fungi including Leveillula taurica and Erysiphe lycopersica

Powdery mildew on tomato [Credit: Dollymoon]

Powdery mildew on a tomato plant [Credit: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org]

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

Host Plants:

On Crops: Tomatoes

Where Found:

Worldwide, mostly in warm, dry climates

Description:

Tomato powdery mildew begins with pale yellow spots on leaves. The spots soon become covered with white spores, which makes the leaves look like they have been dusted with flour. As this fungal disease advances, the whitish parts of the leaves turn brown and shrivel, becoming dry and brittle. Powdery mildew is most likely to occur in late summer. Old, stressed plants are more susceptible than healthy young ones.

Damage:

Powdery mildew fungi clog up leaf pores and block light to photosynthetic cells, so the plants are weakened in their ability to use light as an energy source. New growth stops, old leaves fall off, and the plants struggle to stay alive. Fruits produced by infected plants often lack flavour.

Preventing Problems:

Tomato powdery mildew is most likely to develop on stressed plants, so provide ample nutrition, water and mulch to satisfy their needs. If tomato powdery mildew is common in your areas, spray plants preventatively with a mixture of one part milk to five parts water weekly, starting in midsummer.

Managing Outbreaks:

Pick off individual leaves that show powdery mildew as soon as you see them. Promptly harvest tomatoes from mildewed plants. Compost old tomato plants in an active compost pile so that all infected plant material will be gone by the following summer.

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