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late blight on tomato

Alabama Extension professionals recently detected late blight on a potato sample from a relatively large commercial field in DeKalb County. Late blight can be a devastating disease in tomato and potato crops, destroying entire fields in a short period of time.


On leaves, this disease begins as greenish-black, water-soaked, irregular blotches. These blotches rapidly develop into large purple-black, papery lesions. The lesion margin is often thin and pale yellow. During moist conditions, white, glistening, web-like, fungal growths often appear on the lower leaf surface at the lesion’s edge. If cool, moist conditions persist, blight will spread rapidly and kill the plant.

On fruit, gray-green, water-soaked, greasy spots appear near the stem end. As lesions develop, they become brown and wrinkled. When cool, moist conditions exist, lesions quickly expand, covering up to half of the fruit’s surface. Decay may extend several inches deep into fruit. When cracking occurs on the fruit skin, a delicate white web of fungal growth may develop in this area. Soft-rot bacteria often invade these cracks, causing soft water rot.


  • Plant disease-free transplants.
  • In tomatoes, plant late blight resistant varieties.
  • Follow a weekly fungicide spray schedule when weather conditions favor development of the disease. Continue this schedule at seven to 10-day intervals until harvest.

More information on fungicides appropriate for late blight can be found in the Southeast Vegetable Crop Handbook. 

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