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AUBURN UNIVERSITY, Ala.—Late blight is a quick reproducing fungal disease primarily affecting tomatoes and potatoes. Many know this disease as the cause of the Irish potato famine in the 1840s. Late blight or Phytophthora infestans, is destructive and can eliminate entire crops in a matter of days.

This year’s cool, wet weather has provided conditions that favor the development of this disease. Dr. Ed Sikora, an Alabama Extension plant pathologist, provides information on late blight symptoms and control.

Late blight on a potato.

Late blight on potato Penn State Department of Plant Pathology & Environmental Microbiology Archives, Bugwood.org)


Unlike many other diseases that affect tomatoes and potatoes, both the foliage and fruits are susceptible to late blight at every developmental stage.

On leaves, the disease begins as greenish black, water-soaked, irregular blotches which 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 fruit skin, a delicate white web of fungal growth may develop in this area. Soft-rot bacteria often invade cracks, causing a soft water rot.


Sikora offers the following tips for controlling late blight:

  • Plant disease-free transplants.
  • Follow a weekly fungicide spray schedule when weather conditions favor development of the disease and continue at 7- to 10-day intervals until harvest.

More Information

For more information about gardening, plant pests and vegetable diseases, visit www.aces.edu. For site-specific assistance, contact your County Extension office.


Featured Image: Edward Sikora, Auburn University, Bugwood.org

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