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Growing Green: Look Out for Tomato Blight

Many of our area gardeners are finding themselves with more tomatoes than they know what to do with. Tomato harvesting is under way; unfortunately so are tomato diseases. Noticing any discoloring on you leaves? Or funny spots on the fruit? Fungal diseases are common this time of year, particularly early and late blight. With proper care and a careful eye, you too can end up with an abundant amount of tomatoes this season and for many seasons to come.

Early Blight

Symptoms of early blight usually appear near the end of the season. Brown lesions first appear on older, lower leaves, and spread up toward new growth. Lesions are small (1-2 mm), dry, and papery. The leaf tissue often turns yellow at the edge of the lesion. As the disease progresses, the entire leaf can become yellow and then brown. Severely infected tomato plants may become completely defoliated exposing fruit to sunscald.

Late Blight

Late blight of potato and tomato is caused by the fungus Phytophthora infestans. Late blight is more likely to be seen in late summer and early autumn. This disease can spread rapidly during cool, rainy weather, killing plants within a few days. Daytime temperatures between 60°-70° F, night temperatures between 50°-60° F, and relative humidity near 100% create ideal conditions for infection and spread of the disease.

Late blight initially appears as irregularly shaped, dark green lesions on the lower leaves of the plant. During periods of high humidity, a white cottony growth may be visible on the underside of the leaf. Lesions eventually turn brown and infected leaves die. Spores produced on leaves during the growing season continue to infect other portions of the plant such as petioles and stems. As the disease progresses, lesions enlarge causing leaves to brown, shrivel, and die. Eventually, the entire plant will collapse.

Preventing these fungal diseases may prove to be a challenge. It is impossible to control the temperature and weather conditions, but here a few things that can be done:

  • Remove and destroy any infected plant material to prevent the fungi from overwintering in the debris and creating new infections
  • Plant resistant varieties
  • Keep your garden weed-free since many weeds are hosts for disease
  • Keep plants off the soil to prevent disease from spreading through water splashes
  • Apply mulch around the base of plants to minimize water splash
  • Rotate your garden by planting tomatoes where no tomatoes, potatoes, peppers or eggplants have been for the past 3 to 4 years.
  • Water tomatoes early in the day and at the base of the plant so leaves are able to dry in the sun
  • Stake plants to improve air circulation around the plant
  • Apply fungicides early in the season to prevent the spread of fungus

For more information on tomato plant and other plant diseases, visit:

Robin Trott is a Horticulture Educator with University of Minnesota Extension.