What Do Black Spots on Tomato Plant Seedling Leaves Mean?

Early Blight

• Black spots on the leaves of young tomato plants may mean early blight. The fungus Alternaria solani causes this condition, which produces brown to black target-like spots on the leaves. Later, the leaves may yellow and drop. In severe cases, the fungus may infect stems and fruit, as well. Remove dead or diseased leaves and destroy them, rather than composting them. Treat severe infections by dusting the leaves with sulfur. To prevent early blight infections, water the plants with drip irrigation or a soaker hose, rather than overhead sprinklers, because wet leaves spread disease. Plant tomatoes in a different location every year, in full sun with at least 2 feet between each plant.

Septoria Leaf Spot

• Septoria leaf spot causes brown or gray spots with black margins to develop on leaves. As the disease progresses, leaves may turn yellow and drop, leaving the fruit susceptible to sunscald. Septoria leaf spot responds to the same treatments used for early blight, such as careful watering practice and proper air circulation. Remove and destroy diseased plant material because the disease can overwinter in the soil.

Bacterial Spot

• Bacterial spot causes small dark brown to black spots to form on the surface of the leaves. The spots may be surrounded by a yellow margin or have a greasy feeling. As the spots grow, they may engulf the entire leaf, causing it to turn brown. Fruit may also be affected with raised, black pimples. The pimples grow and soften, becoming gray and scabby. Treat bacterial spot using the same methods recommended for Septoria leaf spot and early blight. Bacterial spot can be spread through infected seed, so it is more likely to occur in young seedlings. Buy certified disease-free plants.

Bacterial Speck

• Bacterial speck is similar to bacterial spot, but as the name implies, the lesions are tiny, less than 1/16 of an inch. The specks are often surrounded by a yellow margin. Fruit may develop small, raised pimples. Bacterial speck often develops after a period or rainy, cool weather. Treat it as previously described.