Potato: Late blight
categories: Potato Potato Diseases Vegetables
last review date: 2025-02-26 04:05
Photo by: R.S. Byther
Biology
Late blight of potato is caused by a fungus which also causes disease on tomato, eggplant, and other members of the potato family (Solanaceae). Gray-green, water-soaked spots appear on leaves and stems. These quickly enlarge into dark blotches which may be surrounded by a pale green margin. During moist weather, a sparse, whitish fungal growth may be seen on the underside of leaf lesions. Tubers may be infected either by spores washed into the soil or during harvest. Infected tubers show areas of somewhat metallic brown or purple discoloration followed by a brownish dry or wet rot. Disease development is favored by cool, rainy weather and may be more severe under sprinkler irrigation. The fungus overwinters primarily on potato tubers and is spread by wind.
Management Options
See "Using Integrated Pest Management (IPM) in the Landscape" for successful problem management techniques.
Non-chemical Management
Select non-chemical management options as your first choice!
- 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).
- Remove plant debris from the garden in the fall. Do not compost diseased materials.
- Space plantings to provide good air circulation.
- Avoid overhead watering.
- Do not dig tubers from infected vines until at least two weeks after vines are completely dead (no green stems remaining). Avoid injuring tubers.
Chemical Management
IMPORTANT: For more information on using pesticides, visit Home and Garden Fact Sheets | Pesticide Resources and Education Program | Washington State University.
- Apply at first sign of late blight.
- Make repeat applications according to label directions.
- Copper products offer limited control and their use alone to control late blight is not recommended.
- Copper fungicide applications can be alternated with other fungicide applications.
Approved Pesticides
Examples of pesticides that are legal for home use in Washington State are listed below.
The list may not include all products registered for this use.
Additional Images
Photo by: R.S. Byther
Photo by: R.S. Byther
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