What Causes Yellow Leaves With Black Spots on Tomato Plants?
Why Are My Tomatoes' Leaves Turning Yellow?
Tomato plants (Solanum lycopersicum) in the home garden are prone to a number of leaf diseases that cause dark spots. These diseases can also result in tomato plant leaves turning yellow, a common problem known as chlorosis.
Both fungal and bacterial pathogens are frequent causes of tomato plant diseases with these symptoms.
Fungal Tomato Diseases
Several fungal diseases of tomatoes can be the common cause of black spots on tomato leaves and can result in tomato leaves turning yellow. Let’s look at two examples.
Septoria Leaf Spot
Brown spots on tomato leaves could mean that the plant is infected with septoria leaf spot, which is caused by the fungus Septoria lycopersici. The centers of the spots often turn light brown or white. Eventually, affected leaves turn yellow before drying out and falling off the plant.
Early Blight of Tomato
Brownish-black spots on tomato leaves can be an indication of early tomato blight, a disease caused by the fungus Alternaria linariae. The spots usually show up first on the lower leaves of the plant.
The lesions are typically 1/4 to 1/2 inches wide. Yellow halos usually develop around the lesions. In some cases, the entire leaf may turn yellow.
Bacterial Tomato Diseases
A bacterial pathogen can also be the cause of yellow leaves and black spots on a tomato plant. Let's go over the two of the most common culprits.
Bacterial Leaf Spot
The disease known as bacterial leaf spot is caused by the bacterium Xanthomonas vesicatoria and related species of bacteria. This disease causes lesions on leaves that look like water-soaked areas. Older leaves are most commonly affected on mature plants, however tomato seedlings are also vulnerable to this disease.
Bacterial Speck of Tomato
Caused by the bacterium pseudomonas syringae pv. tomato, bacterial speck manifests as very small black lesions on the undersides of tomato leaves. Sometimes a yellow halo is visible around the specks. This disease can also spread to the fruit of the tomato plant.
Avoid growing tomatoes in the same soil where tomatoes were grown in the last few years.
Preventing Tomato Leaf Diseases
The pathogens that cause black spots and yellow leaves on tomato plants are capable of overwintering on plant debris and mulch. Therefore, removing plant matter and mulch at the end of the growing season can help reduce the chances of the bacteria and fungi surviving and infecting new plants in the spring.
To avoid disease, plant disease-resistant tomato varieties. In addition, practicing crop rotation in your vegetable garden in order to avoid growing tomatoes in the same soil where tomatoes were grown in the last few years is another way to stop these diseases.
Moisture and overwatering promotes the development of many tomato leaf diseases. Therefore, it is best not to water plants overhead to keep the foliage dry. Instead, use drip irrigation or water at the base of the plant, focusing on the plant’s roots. Providing good air circulation is also crucial if you want healthy plants.
Chemical control is sometimes necessary. The fungicide mancozeb works well for prevention of early blight, while chlorothalonil is the better option for septoria leaf spot. Applying a product that contains copper and is formulated for use on vegetables like tomatoes before symptoms develop can keep bacterial leaf spot and bacterial speck at bay.
- Iowa State University Extension and Research: Septoria Leaf Spot
- North Carolina State Extension: Early Blight of Tomato
- University of Wisconsin-Madison: Bacterial Speck of Tomato
- University of California IPM: Bacterial Spot
- University of Wisconsin-Madison: Bacterial Spot of Tomato
- Clemson Cooperative Extension: Tomato Diseases and Disorders
Since beginning her career as a professional journalist in 2007, Nathalie Alonso has covered a myriad of topics, including arts, culture and travel, for newspapers and magazines in New York City. She holds a B.A. in American Studies from Columbia University and lives in Queens with her two cats.