UT Extension News: Early Blight in Tomatoes

July 13, 2021

By Chris Hicks, County Director – UT Extension Smith County

With the fair going on I had slacked off on checking my garden vegetables. When I did get around to giving them a close look, I found that early blight had hit my tomatoes pretty hard. Caused by the fungus Alternaria solani, early blight is the most common disease of tomatoes in Tennessee, but with proper planning in the home garden, it can be avoided. 

Like many fungi, early blight thrives under the extremely humid weather conditions we typically face in Smith County. Unlike some diseases, early blight can affect the leaves, stem, and fruit of the plant. Lesions usually appear first on older leaves as irregular, dark brown necrotic areas. 

As the disease progresses, the lesions will develop concentric black rings which look like targets or “bull’s eyes” on the leaves. This is often surrounded by a yellow chlorotic area. In conditions that are favorable for disease development, the plant may become completely defoliated. The lesions can also be on the stem in the form of long dark brown areas that are slightly sunken. 

The early blight fungus is spread by wind and splashing rain, and outbreaks are favored by warm, rainy weather. The fungus overwinters in crop debris and on seeds and can survive between crops on solanaceous crops and weeds. This makes sanitation and crop rotation an integral part of the control strategy. 

Removing as much plant debris as possible in the fall and only growing tomatoes in the same ground every third year will often keep the fungus under control by allowing infested plant debris to decompose in the soil. Rotations with small grains, corn, or legumes are preferable to solanaceous crops such as potatoes and peppers. 

Like many plants, you can lessen the chance of bringing this disease into your property by purchasing healthy plants from a reputable source such as a local greenhouse or by growing your own. Avoid using sprinkler irrigation which will cause leaves to remain wet for long periods of time and instead water only at the base of the plants. Staking and pruning the plants will also provide good drying conditions that will slow the spread of the fungus.

Although there are no highly resistant varieties currently available, some tomatoes do show partial resistance to early blight. These include large fruited determinate types such as Defiant and Mountain Merit, as well as small fruited indeterminate types such as Mountain Magic and Juliet. Check your seed catalog or ask the nursery operator about other varieties with partial resistance to early blight. 

Although these measures are important to minimize infection, it may be necessary to apply fungicide sprays to fully protect plants from early blight. Garden fungicide products containing chlorothalonil or mancozeb are most affective at preventing early blight. These can be found under many trade names and will have to be applied every 7-14 days to slow the spread of early blight. As with any chemical, read and follow all instructions on the label. For help with diagnosing tomato diseases and disorders, contact the University of Tennessee Extension office at 615-735-2900.