What Causes Black Spots on Tomato Plants?

Black spots on your tomato (Solanum lycopersicum) plants don't always signify the end, but they are certainly a sign something is amiss. Identifying the cause is the first step to treating your existing plants and preventing future problems.

Poor growing conditions are among the simplest problems to treat. As long as you have a reasonable amount of healthy tissue left on the tomato plant, you can usually expect a full recovery. In these cases, since there is no resident pathogen to battle, the plant will normally recover and start growing again as soon as you identify and correct the problem.

Frost: If your previously healthy plants are suddenly blackened and shriveled following a bout of cooler weather, frost is the likely cause. Foliage will at first be deep green and wilted, particularly near the top and on the west-facing side of the plant, and will gradually turn black and crispy. Frost damage is easily avoided by covering your plants any time there is risk of freezing or near-freezing temperatures.   

Drought Stress: At the other end of temperature extremes, drought stress brought on by hot, dry conditions can cause leaf tissues to die back, turning brown or black. Cornell University reports that many gardeners mistake this kind of leaf discoloration for late blight, a far more complicated problem than drought stress.

For any of these pathogens, you can follow the same general protocol to reduce infection. Scrupulously remove all old plant debris from the garden, plant tomatoes in a sunny area with good airflow to prevent dampness, practice crop rotation and if necessary, treat with a food-safe garden fungicide such as copper sulfate. But it's worth trying to narrow down the culprit so that in the future you can choose cultivars that are resistant to that particular fungus. Common fungal infections include:

Early blight**:** Look for irregular concentric patches of black or brown spots on leaves and fruit, creating a rough bull's-eye pattern. 

Late Blight: Slimy gray-to-black areas that first appear on mature leaves and stems indicate late blight. In moist weather, fuzzy white patches may appear on the underside of leaves. 

Septoria leaf spot: Numerous small round lesions occurring only on the leaves and stem, but not the fruit, point to Septoria leaf spot. Lesions are white to dark gray with a brown or black border. 

Fungal infections tend to manifest as rounded lesions, whereas bacterial infections usually follow the outline of the leaf vein, giving them an angular appearance.

Since both bacteria and fungi can live in the soil and thrive in moist environments, the same garden hygiene strategies used to treat and prevent fungal infections are also effective against bacterial infections. Two common bacterial infections are:

Bacterial Speck: Look for dark brown or black lesions near the leaf margin. Leaf tissues near the lesion are often yellowed. Specks appear on fruit as tiny raised black spots that usually do not penetrate into the flesh of the tomato.

Bacterial Spot: On young leaves, bacterial spot may only manifest as barely noticeable yellowish lesions that cause the leaf to twist and deform. On mature leaves, lesions can be large, dark and greasy in appearance. 

While several commercial preparations are available to treat fungal infections, most sprays are only effective as preventatives against bacterial infections. If you suspect a bacterial infection, cut off the affected parts and dispose of them far away from your garden.