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Early blight in tomato and potato

Quick facts

  • Early blight is one of the most common tomato and potato diseases, occurring nearly every season in Minnesota.
  • It affects leaves, fruits and stems and can be severely yield-limiting when susceptible tomato cultivars are used and weather is favorable.
  • Severe defoliation can occur.  In tomatoes, fruit can be damaged by sun.
A green tomato stem with brown early blight lesions.
Tomato stem with brown lesions with concentric circles

Identification

A caged tomato plant with a green, healthy top but the bottom leaves are yellow with brown spots.
Early blight infection starts at the bottom of the plant with leaf spotting and yellowing.
  • Initially, small dark spots form on older foliage near the ground. Leaf spots are round, brown and can grow up to 1/2 inch in diameter.
  • Larger spots have target-like concentric rings. The tissue around spots often turns yellow.
  • Severely infected leaves turn brown and fall off, or dead, dried leaves may cling to the stem.
  • Seedling stems are infected at or just above the soil line. The stem turns brown, sunken and dry (collar rot). If the infection girdles the stem, the seedling wilts and dies.
  • Stem infections on older plants are oval to irregular, dry brown areas with dark brown concentric rings.
  • Fruit can be infected at any stage of maturity.
  • Fruit spots are leathery and black, with raised concentric ridges. They generally occur near the stem. Infected fruit may drop from the plant.
 A yellow tomato leaf with brown concentric circles.
Early blight lesions with brown concentric rings

Biology

Green leaves in the potato canopy with brown early blight lesions.
Early blight lesions on potato
  • Early blight can be caused by two closely related species: Alternaria tomatophila and Alternaria solani.
  • Both pathogens can infect tomatoes, potatoes, peppers, and several weeds in the Solanaceae family including black nightshade (Solanum ptycanthum), and hairy nightshade (Solanum physalifolium)
  • Disease develops at moderate to warm (59 to 80 F) temperatures; 82 to 86 F is its optimum temperature range
  • The pathogen is most likely to spread with any weather or heavy dew, or when relative humidity is 90% or greater
  • The early blight pathogens both overwinter in infected plant debris and soil in Minnesota. The pathogen also survives on tomato seed or may be introduced on tomato transplants.
  • Lower leaves become infected when they come into contact with contaminated soil, either through direct contact or when raindrops splash soil onto the leaves.
    • Spores (reproductive structures) can germinate between 47° and 90° F and need free water or relative humidity of 90% or greater.
    • Spores infect plants and form leaf spots as small as 1/8 inch diameter in as little as five days.
  • Spores can be spread throughout a field by wind, human contact or equipment, resulting in many reinfection opportunities throughout a growing season.

Managing early blight in the home garden

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Managing early blight on farms

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Marissa Schuh, horticulture IPM Extension educator, and Michelle Grabowski

Reviewed by Natalie Hoidal, Extension educator

Reviewed in 2022

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