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The difference between late and early blight and how to prevent it

In the Garden

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Several years ago, I received a call from a gardener who was concerned that his potatoes and tomatoes were not doing very well. He said he rotated them every year. When I looked at his garden, I saw that he was only growing tomatoes and potatoes, rotating them back and forth. This created a perfect storm for soil and crop residue-borne diseases.

Cooler, fall weather brings conditions that favor the development of late blight and early blight. These are diseases of potatoes, tomatoes and eggplant, but can also be found on solanaceous weeds, such as hairy nightshade and cut leaf nightshade. LB and EB can infect leaves, stems, fruits, and tubers.

Late Blight

LB is caused by a fungus-like microorganism that can overwinter on crop residue, cull potato piles, volunteer plants, or solanaceous weeds. It can also be brought in by infected seed potatoes or blown in on the wind and then spread by rainfall or overhead irrigation. Environmental conditions that favor disease development are high humidity greater than 80% with cool temperatures over a period of 10 hours or more, several days in a row. Researchers monitor these conditions and there have been very few days with favorable environmental conditions in Eastern Idaho so far in 2021.

The symptoms of LB begin on the younger leaves. Large, sunken, dark green or brown lesions develop on leaves, and can eventually kill entire leaves. The margins of the lesions may be green-yellow or have a water-soaked appearance. The lesions on the stems will be brown to black.

Early Blight

EB is caused by a fungus that overwinters as spores and mycelium in infected crop residue or on solanaceous weeds, such as hairy nightshade and cut leaf nightshade. It can also be carried on the seed. Reduced fertility, along with wet leaves (or high humidity greater than 90%) at temperatures of 75- to 84-degrees favor the development of this disease. EB shows up on older leaves that are touching the ground first, then spread to the rest of the plant.

The symptoms of EB start out as small, dark spots. As the spots get larger, they have definite concentric rings, looking like a target. The tissue around the spots is usually yellow. The spots will grow and eventually fuse together, forming irregular dead patches on the leaves. The stems develop brown, dry, sunken lesions.

blight stems01
Early blight, left, and late blight in stems. | Courtesy Robert Wick, University of Massachusetts, for photo on left and Howard F. Schwartz, Colorado State University, for photo on right

Integrated Pest Management (IPM) Control Options for Both


  • Plant resistant cultivars.
  • Sanitation—clean up crop residues, especially if they have diseases. Compost crop residues to temperatures above 140oF for at least a week, then turn the pile and compost again.
  • Use clean seed, or don’t save seed from infected plants.
  • Inspect transplants for symptoms of disease before planting.


  • Rotate ground out of solanaceous crops for at least two years.
  • Remove volunteer potato or tomato plants and solanaceous weeds.
  • Use drip irrigation, or run sprinklers in early morning so leaves can dry quickly.


  • Apply fungicide listed for LB or EB as soon as the disease shows up to protect uninfected plants.

Late blight and early blight may appear at the same time and there are other blights that will infect potato and tomato plants. If you are unsure, bring a fresh sample to the University of Idaho Extension office in Idaho Falls and we will get it to the lab if necessary.