Late Blight
Plant disease · Phytophthora infestans
Description
Late blight is a fast-spreading disease of tomatoes and potatoes caused by Phytophthora infestans, a water mold rather than a true fungus. It thrives in cool, humid conditions and can destroy foliage within days when weather favors its spread. Spores travel on wind and splashing water, while infected potato tubers and volunteer potato plants can carry the pathogen between growing seasons. On leaves, the disease begins as water-soaked patches that enlarge into irregular brown or black areas. Stems may develop dark lesions, and tomato fruit can develop firm, brown, spreading rot. Potato tubers may show reddish-brown decay beneath the skin. Infection reduces the plant’s ability to grow and can ruin a crop before harvest. Late blight is particularly concerning because new spores can be produced rapidly on infected plants during damp weather, allowing outbreaks to spread through a garden and into nearby plantings.
Symptoms
- Spots on leaves
- Wilting
- Leaves dropping
- Soft or rotting stems
- Rotting fruit
How to Identify
Look for rapidly enlarging, irregular, water-soaked leaf patches that turn brown or black, often with pale, fuzzy spore growth along their undersides during humid weather. Check stems for dark lesions and tomatoes for firm brown patches that spread across the fruit. Unlike early blight, late blight lesions usually lack distinct concentric rings. Septoria leaf spot typically causes smaller, more numerous spots, while gray mold produces conspicuous gray growth on decaying tissue. Frost damage can darken leaves suddenly but does not produce spores or continue spreading in mild, wet weather. If uncertain, ask a local extension service or plant diagnostic laboratory to confirm a suspected outbreak.
Prevention
Start with healthy transplants and certified disease-free seed potatoes. Choose resistant varieties where available, but remember that resistance may not be complete. Space plants for airflow, avoid overhead watering, and water early so foliage dries quickly. Inspect tomatoes and potatoes often, especially during cool, damp weather or when local late blight alerts are issued. Remove volunteer potato plants and destroy discarded or infected tubers rather than leaving them where they can sprout. Keep tomato and potato plants separate when practical, and clean tools after handling suspect plants. Rotate crops to help manage other diseases, but do not rely on rotation alone: late blight spores can arrive from distant infected plants. Protectant fungicides, if used, must be applied before infection and according to the label.
Treatment
There is no reliable cure for tissue already infected with late blight. First, avoid handling plants when they are wet, and promptly remove suspect leaves only if the infection is very limited; bag them without shaking and dispose of them according to local guidance. If lesions are widespread or spreading rapidly, remove the entire plant to reduce spore production. Do not save visibly diseased fruit or potato tubers for storage or planting. Keep remaining plants as dry as possible and inspect them frequently. A copper-based fungicide labeled for late blight on the crop may help protect still-healthy tissue if applied before further infection; follow the label’s timing, harvest interval, and safety directions. Copper can accumulate in soil and harm aquatic life, so use it only when warranted. In severe outbreaks, local extension guidance can help determine whether other labeled protectant fungicides are appropriate.
Plants Affected
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