Late blight of tomato is caused by a fungus which also causes disease on potato, eggplant, and other members of the potato family (Solanaceae). Gray-green, water-soaked spots appear on leaves, stems, and fruit. These quickly enlarge into dark blotches. The disease may spread to affect all aboveground portions of the plant. The brown blotches on infected fruits are firm and appear corrugated or wrinkled. These usually appear first on the upper portion of the fruit and may sometimes involve whole fruits. During moist weather, a sparse growth of whitish fungal mycelia may be seen on fruit lesions and on the underside of leaf lesions. Disease development is favored by cool, rainy weather and may be more severe under sprinkler irrigation. The fungus overwinters primarily on infected potato tubers and is spread by wind.

Management Options

Non-Chemical Management

Plant only healthy, disease-free seedlings.
Do not plant potatoes and tomatoes in close proximity.
Remove infected plants or plant parts when symptoms are noticed to reduce spread of disease.
Diseased plant materials should be destroyed or buried deeply (two feet or more). Do not compost diseased materials.
Remove plant debris from the garden in the fall.
Space plantings to provide good air circulation and minimize humidity.
Avoid overhead watering.
If possible, grow tomatoes where they will be protected from rain.

Select non-chemical management options as your first choice!

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides

Apply at first sign of late blight. Make repeat applications according to label directions.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.

Agri-Fos Systemic Fungicide
Active ingredient: mono- and di-potassium salts of phosphorous acid | EPA reg no: 71962-1-54705
Bonide Copper Fungicide Spray or Dust RTU [Organic]
Active ingredient: basic copper sulfate | EPA reg no: 4-58
Bonide Fung-onil Multi-Purpose Fungicide Conc
Active ingredient: chlorothalonil | EPA reg no: 60063-9-4
GardenTech Daconil Fungicide Conc
Active ingredient: chlorothalonil | EPA reg no: 67572-82-71004
Monterey Liqui-Cop Copper Fungicidal Garden Spray
Active ingredient: copper-ammonia complex | EPA reg no: 54705-7
Ortho Max Garden Disease Control Conc
Active ingredient: chlorothalonil | EPA reg no: 239-2522
This list may not include all products registered for this use.

Images

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Caption: Tomato late blight infected fruit and foliage Photo by: R.S. Byther
Caption: Tomato late blight symptoms on fruit and stems Photo by: R.S. Byther
Caption: Tomato late blight whitish fungal mycelia Photo by: R.S. Byther
Caption: Late blight Photo by: F. Buajaila
Caption: Late blight Photo by: F. Buajaila
Caption: Late blight Photo by: F. Buajaila
Caption: Late blight Photo by: F. Buajaila

WSU Extension

Hortsense