Tomato Troubles: Dealing with Black Leaves on Your Tomato Plants

Tomato plants are beloved by gardeners of all levels due to their delicious fruit and relatively easy cultivation. However, like any plant, tomatoes are susceptible to various diseases that can turn their leaves black, potentially jeopardizing the entire harvest. In this guide, we will explore five common diseases responsible for blackened tomato leaves at any stage of growth. We’ll also discuss how to diagnose and treat these diseases without sacrificing your entire crop.

Unraveling the Mystery of Black Tomato Leaves

The sudden appearance of black leaves on your beloved tomato plant may seem like a disaster. However, there is a silver lining to this unsightly symptom. Black foliage can rule out common tomato diseases like blossom end rot and mosaic virus, as well as environmental factors such as nutritional deficiencies or poor soil quality. In most cases, black leaves on tomato plants can be attributed to one of the following fungal or bacterial issues:

1. The Culprit: Septoria Leaf Spot

Septoria leaf spot, caused by the fungus Septoria lycopersici, results in grey, brown, or black spots on tomato leaves. These lesions typically start appearing after the first fruit sets, gradually spreading up the plant from the oldest leaves.

Tomato leaf spot Image source: The Gardening Fix

Causes and Treatment

Septoria leaf spot thrives during periods of prolonged dampness. Partially shaded tomato plants are more susceptible to this fungus compared to those receiving ample sunlight. To combat septoria leaf spot, remove and destroy affected leaves as soon as you notice signs of infection. Applying a fungicide containing chlorothalonil, benomyl, maneb, or mancozeb can also halt its spread. Copper-based fungicides are effective against this disease as well. Remember, fungicides cannot reverse damage to already infected leaves but can protect the healthy foliage from further infection.

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Prevention

Preventing septoria leaf spot involves keeping the tomato plant foliage dry. Allow adequate airflow by spacing out plants and avoid overhead watering. Instead, water early in the day using a soaker system. Additionally, weed control is essential as Septoria lycopersici survives and feeds on dead plant matter.

2. Early Blight: The Dark Spots Enigma

Early blight, caused by the fungus Alternaria linariae, is responsible for dark spots on tomato leaves, stems, and fruit. These spots can be up to ½-inch in diameter and often display concentric rings. Early blight tends to affect the lower leaves first, with the affected areas curling and turning black, eventually leading to significant foliage loss.

Symptoms of early blight Image source: The Gardening Fix

Causes and Treatment

Early blight is more likely to affect plants that are already diseased or stressed. Certain tomato cultivars are more susceptible than others. Fungicide applications containing chlorothalonil or copper can help manage mild infections, but their primary use is prevention. Cultural practices like ensuring good airflow and selecting disease-resistant plant cultivars can also aid in prevention.

3. Late Blight: The Water-Mold Menace

Late blight causes small, water-soaked lesions on the upper surface of tomato leaves, which gradually darken into oily brownish-purple patches. The underside of infected leaves may develop grey-white rings.

Symptoms of late blight Image source: The Gardening Fix

Causes and Treatment

Late blight, caused by the oomycete Phytophthora infestans, thrives in damp conditions and can spread over long distances through spores. While fungicides containing copper or chlorothalonil can control late blight on infected plants if applied weekly, it is best to remove and destroy any affected plants. Avoid composting infected plants to prevent further spread.

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4. Bacterial Canker: A Devastating Threat

Bacterial canker leads to dark brown or black lesions at the margins of tomato leaves, stems, and sometimes fruit. It is caused by the bacteria Clavibacter michiganensis and is one of the most destructive bacterial diseases affecting tomato plants.

Bacterial canker on tomato leaf Image source: The Gardening Fix

Causes and Treatment

Bacterial canker is typically introduced to domestic gardens through infected seeds or plants. It can also spread through wind or run-off, though the distance traveled is limited. Unfortunately, there is no effective treatment for bacterial canker. Infected tomato plants must be destroyed, and the area should remain free from tomatoes and other Solanaceae species for at least three years.

5. Alternaria Stem Canker: The Silent Killer

Alternaria stem canker primarily manifests as a dark canker at the base of the stem. These cankers can girdle the main stem, ultimately killing the entire tomato plant. The pathogen releases a toxin that causes black patches of dead tissue between leaf veins.

Causes and Treatment

Alternaria stem canker, caused by the fungus Alternaria alternata, spreads through infected soil, plant matter, or garden tools. Fungicides approved for treating black mold disease on tomato plants can also be used against Alternaria stem canker. Prevention measures include planting disease-resistant cultivars and adopting cultural practices that discourage fungal infections, such as avoiding overhead watering.

Removing Black Leaves on Tomato Plants

Managing fungal and bacterial diseases that cause black leaves on tomato plants involves removing the infected plant material. While this strategy is not foolproof, it is worth attempting whenever black leaves appear. You can safely remove up to one-third of a plant’s foliage, taking care to avoid removing leaves above the fruit whenever possible, as tomato fruit can be sensitive to direct sunlight. Remember to wash your hands and tools after handling infected plants to prevent accidental spore spread, and dispose of diseased plant matter by burying it far from the garden or burning it.

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Stopping the Spread

To prevent the spread of diseases from infected plant material, it is crucial to burn or bury the affected tomato plants. Composting infected plants is not recommended, as the composting process may not eliminate the pathogens. Additionally, pathogens can overwinter in garden soil, so adding a fresh layer of mulch at the start of the growing season can help insulate new tomato plants from soil-borne diseases. If you have experienced severe tomato diseases in a particular area, consider planting a different crop the following year, as some pathogens can affect multiple vegetable species.

Enjoying Tomatoes from Plants with Black Leaves

With regard to consuming tomatoes from plants with black leaves, it is generally safe as long as the fruit itself is disease-free. Only use firm tomatoes without any lesions, and avoid cutting out diseased sections to salvage the rest of the fruit—discard the entire tomato. Keep in mind that some diseases, like late blight, can affect tomatoes even after harvest. To prevent this, consume or preserve your tomatoes promptly and inspect stored tomatoes for any signs of disease.

Conclusion

While black leaves on tomato plants can be alarming, they serve as a warning sign of potential diseases. By promptly identifying and treating the underlying fungal or bacterial issues, you can protect your tomato plants and ensure a bountiful harvest. Follow the prevention strategies outlined in this guide and take swift action at the first sign of infection. With proper care and attention, your tomato plants can thrive and provide you with a rich and delicious harvest.

For more information on tomato diseases and treatment, please visit Ames Farm Center.