Early Blight Fungus Disease

Early blight
Early blight on potato leaf
Early blight on potato leaf

Early blight is a fungal disease that targets tomatoes in particular but also hits potatoes, peppers, and eggplant.

Early blight can infect plants at almost any time during the growing season. It is most prevalent in regions where there is high humidity and when temperatures are greater than 75°F.

Fungal spores carrying the disease are spread on the wind or by insects. Spores germinate and grow when plants are moist from heavy dew or frequent rain.

Target Plants: Potato, tomato, eggplant, pepper, weeds in the tomato or nightshade family.

Specific Damage:

Leaves: symptoms appear first on older, mature leaves near the base of the plant–one or two irregular spots per leaf, spots ¼ to ½ inch in diameter; spots have tan centers with concentric ridged rings and yellow halos around the edges, target-shape appearance with defined border; spots enlarge and coalesce. Large portion of leaf becomes a diffuse yellow and may drop. No mold appears. Disease spreads slowly.

Stems: dark, sunken lesions at or above the soil line, sometimes called collar rot.

Fruits of tomato, eggplant, pepper: sunken spots appear on the stem end of fruits; concentric ring pattern or target-shape appearance around spots; disease moves from stem to rot fruit.

Tubers of potatoes: tuber lesions are brown or black sunken spots less than ½ inch wide.

How Spread: Early blight fungi spores are wind- and rain-borne. Sometimes the disease can be transmitted by flea beetles who have fed on diseased plants. Hyphae enter the plant through cracks and wounds in plant tissue.

Life Cycle: Fungi overwinter on decomposed plant debris and form new spores each spring. Several generations of spores can occur during summer when the disease is wide spread.

Organic Control and Prevention:

  • Use seed potatoes certified disease free.
  • Plant resistant varieties; long-season potato varieties are most resistant.
  • Do not plant tomatoes and potatoes close together to prevent cross infection.
  • Rotate tomato and potato crops that do not get early blight.
  • Avoid overhead watering.
  • Space and thin to allow light and air to reach plants.
  • Space potatoes widely.
  • Stake or cage tomatoes, eggplants, and peppers.
  • Keep plants healthy; side-dress plants with a balanced fertilizer.
  • Remove infected leaves from plants as soon as you see them.
  • Mulch around plants to prevent fungal spores from splashing onto plants.
  • Spray plants with compost tea or Bacillus subtilis to prevent blight from spreading.
  • Keep garden free of dropped or diseased plant material during the growing season.
  • Clean garden of all plant debris at the end of the season.
  • Eliminate tomato family weeds such as black nightshade that can harbor fungal spores.

Non-Organic Control: Drench soil with Trichoderma harzianum at planting time. Spray with fungicide containing chlorothalonil. Consult your nearby cooperative extension for regionally-approved fungicides.

Disease Range: United States and Canada; severe in humid regions, Midwest, humid eastern states.

Scientific name: Alternaria solani

Prevention of Fungal Diseases:

  • Remove diseased leave as soon as they appear.
  • Improve air circulation around plants.
  • Mulch around the base of plants to reduce water splashing.
  • Do not use overhead watering.
  • Control weeds.
  • Rotate crops.

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. has more than 10 million visitors each year.

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