Late blight is a fungal disease that attacks tomato and potato plants. In 2009, the disease has been particularly rampant in the Northeast, Mid-Atlantic and Midwest largely because of prolonged and excessive cool, damp weather early in the summer.
Symptoms of Late Blight
Look for brown lesions on tomato stems and nickel-size olive green or brown spots on leaves. The leaf spots start out very small. White fungal growth appears if the weather is humid or wet. Firm brown spots appear on fruit.
If the lesions have a yellow border and appear on the bottom portion of the plant, the problem is more likely early blight or Septoria leaf spot.
View infected-tomato pictures.
How to Treat Late Blight
It’s important to treat infected plants because this disease is highly contagious. It spreads by wind-borne spores. Unfortunately once late blight takes hold, there is little to do but destroy the plant. Remove symptomatic plants by covering them tightly with a plastic bag (before pulling them), then pulling them up. Seal the bags and put them in the trash. Do not put the plants in your compost pile because the spores will continue to spread. Clean your tools and hands thoroughly after pulling infected plants.
Fungicides can be used to prevent infection. They must be applied before symptoms appear and be reapplied frequently. For fungicide guidelines, click here. Organic gardeners and farmers have few options for prevention. They may use copper fungicides, but copper in not very effective on late blight.