Spotty Tomato Leaves

How to Diagnose This Common Condition

Septoria lycopersici (Septoria Leaf Spot). Ascomycota. Common fungal leaf disease of tomatoes. Tomato leaf showing small brown spots characteristic of the disease and some chlorosis or yellowing. Franklin County. Ohio. USA
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No one wants to walk out to the garden and see disease spots on their tomato plants. Unfortunately, tomato plant diseases can infect plants being grown by the most experienced gardeners as well as novices. But what, exactly, are you dealing with? Read on about the three diseases that could be causing spotty leaves on tomato plants.

Early Blight

Early blight begins on the lower branches of a tomato plant. Foliage will exhibit one or two spots per leaf, approximately 1/4 to 1/2 inch in diameter. Eventually, the spots grow larger and run together. Spots have tan centers with concentric rings in them and yellow halos around the edges. The stems will develop dark, sunken cankers at or above the soil line, and some fruit will start to show dark, sunken spots on the stem end of fruits.

There is no treatment for early blight. Once you discover it on a plant, the best course of action is to remove infected branches immediately. You can still harvest unaffected fruit and the plant should continue to grow and blossom. Remember to sanitize your pruners when you have finished removing infected branches. If the disease overruns the plant, pull up and dispose of the plants in the trash—do not compost or burn blighted plants. Next year, plant your tomatoes in a different area of the garden.

Late Blight

Late blight was responsible for the Irish potato famine in the 1840s. If your tomato plants are infected, the symptoms are leaves with large, dark brown blotches with a green gray edge. Stem infections are firm and dark brown with a rounded edge. Spots can become mushy as additional bacteria invade. In periods of high humidity, thin powdery white fungal growth appears on infected leaves. Fruit damage in the form of dark, leathery patches that appear on the top and sides of unripe fruit is yet another sign of late blight.

Late blight is a terminal disease, and the method for dealing with it is to pull out and destroy the plants. Do not place the plants in your compost pile. For the next growing season, purchase late blight-resistant varieties, and grow your them in a different part of the garden.

Septoria Leaf Spot

Septoria leaf spot is aptly named because the primary symptom of the disease are the numerous brown spots that appear on the leaves, approximately 1/16 to 1/8 inch in diameter. The spots lack a yellow halo, and, upon close inspection, have black specks in the center. Unlike early and late blights, there is no stem or fruit damage.

If your tomatoes are infected, they will continue to produce fruit. Remove infected foliage immediately and disinfect your pruning tools when working from plant to plant.

To prevent the disease next year, plant your tomatoes in a different part of the garden because the fungus overwinters on infected tomato debris and in the soil, Purchase disease-resistant varieties, do not water overhead, use mulch to prevent soil from splashing up on foliage, and increase air circulation between plants.

Article Sources
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  1. Early Blight. University of Maryland Extension

  2. Late blight of tomato and potato. University of Minnesota Extension

  3. Septoria Leaf Spot - Vegetables. University of Maryland Extension