Late Blight of Tomato

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Hello, this is Andrew Baucom, county director here at the Union County Agricultural Center. Even though it is only February, it’s never to early to start thinking about that first garden tomato. Today I would like to talk with you about late blight of tomato. The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions, often with a lighter halo or ring around them. These lesions are typically found on the younger, more succulent leaves in the top portion of the plant canopy. During high humidity, white cottony growth may be visible on the underside of the leaf. Spots are visible on both sides of the leaves. As the disease progresses, lesions enlarge causing leaves to brown, shrivel and die. Late blight can also attack tomato fruit in all stages of development. Rotted fruit is typically firm with greasy spots that eventually become leathery and chocolate brown in color; these spots can enlarge to the point of encompassing the entire fruit.

The pathogen is favored by cool, wet weather; clouds protect the spores from exposure to UV radiation by the sun, and wet conditions allow the spores to infect when they land on leaves. Nights in the 50s / 60s and days in the 80s accompanied by rain, fog or heavy dew are ideal for late blight infection. Under these conditions, lesions may appear on leaves within 3-5 days of infection, followed by white cottony growth soon thereafter. Although spores may also be produced on tomato fruit, they are more commonly produced on leaves. Spores can spread readily by irrigation, equipment, wind, and rain and can be blown into neighboring fields within 5-10 miles or more, thus beginning another cycle of disease.

Products containing the active ingredients copper or chlorothalonil (the trade name of one product with chlorothalonil is known as ‘Daconil’) are the best and only effective products available to home gardeners. In addition, home gardeners should grow varieties with resistance if they are worried about late blight in future years because most chemicals available to the home gardener are not sufficient to control late blight once it appears. Once plants are infected in a home garden, there is little that can be done to protect them besides weekly fungicide sprays.