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Extension > Garden > Yard and Garden > Vegetables > Late blight of potato and tomato

Late blight of potato and tomato

Kathryn J. Bevacqua

Late blight on potato plants
Photo: University of Minnesota Plant Disease Clinic

Late blight of potato and tomato is caused by the fungus Phytophthora infestans. Although late blight can occur at any time during the growing season, it 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. The fungus becomes inactive during dry periods.

Late blight of potato was responsible for the Irish potato famine of 1845. When the fungus was introduced into Ireland in the 1840's, the cool, moist growing conditions so favorable to growing potatoes were also favorable to disease development. The famine was particularly devastating because a large population of people was dependent on only a few susceptible varieties of potatoes.

Potato infected with late blight.
Photo: University of Minnesota Plant Disease Clinic

The late blight fungus survives winter in living potato tubers remaining in the ground or in cull piles. The fungus can also survive winter in perennial weeds, such as nightshade. As infected tubers and perennial weeds germinate and grow, the fungus becomes active and reproduces on the young plants. Spores produced on infected volunteer potato plants and perennial weeds are disseminated by water and wind to healthy leaves and tomato fruits. Spores are also carried to potato tubers in the soil by moving water.

Late blight initially appears as irregularly shaped, dark green (water-soaked) lesions on the lower leaves of the plant. During periods of high humidity, a white cottony growth (fungal mycelia) may be visible on the underside of the leaf, near the margin of the lesion. 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.

Tomato fruit infected by late blight develop brown to dark brown greasy spots. These spots or lesions may eventually engulf the entire fruit. White cottony fungal mycelia often develop on the fruit during wet conditions.

Late blight infected tubers develop irregular shaped, firm, sunken, dull brown to purple lesions around the eyes. The infection can penetrate into the tuber, creating a reddish-brown dry rot. Sometimes tubers are infected during harvest but show no symptoms until they have been placed in storage. Once in storage, the fungus can spread quickly from late blight infected tubers to healthy tubers. In addition, severely infected tubers are often infected by secondary organisms that cause soft rot.

Late blight on tomatoes
Photo: Beth Jarvis

Currently there are two different strains of the fungus (US 7 and US 8) found in Minnesota. This development is significant because these strains, US 7 and US 8, are more genetically diverse than the original strain, US 1. These new strains may develop resistance to commercial fungicides and have been able to overcome formerly resistant plant varieties.

Late blight can be prevented using the following cultural controls:

Controlling late blight is difficult once the disease is established. Remove and destroy diseased plant parts as they appear. Destroy diseased potato tubers. Apply a fungicide such as chlorothalonil (Daconil 2787) or mancozeb as a preventative at the first sign of disease or when conditions are favorable for disease development.


Hooker, W. J., ed. Compendium of Potato Diseases. St. Paul: The American Phytopathological Society, 1981.

Howard, Ronald J. et al. Diseases and Pests of Vegetable Crops in Canada: An Illustrated Compendium. Ottawa: The Canadian Phytopathological Society and Entomological Society of Canada, 1994.

Jones, J.B. et al. Compendium of Tomato Diseases. St. Paul: The American Phytopathological Society, 1991.

Koepsell, Paul A. and Pscheidt, Jay W., eds. Pacific Northwest Plant Disease Control Handbook. Extension Services of Oregon State University, Washington State University, and the University of Idaho, 1994.

Sherf, Arden F. and MacNab, Alan A.. Vegetable Diseases and Their Control. New York: John Wiley & Sons, 1986.

Representative trade names may be included along with generic names. This information is supplied with the understanding that no discrimination is intended and no endorsement is implied.


Revised by Chad Behrendt and Crystal Floyd 2000

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