Tomato Leaves Turning Black? Here’s What To Do

Tomato plants can be very easy to grow, even for beginners, and I guess this is the reason why they tend to be so popular, aside from their delicious fruit. Despite tomatoes forgiving nature, they can be particularly prone to disease. This is in part due to tomatoes and other nightshade vegetables (e.g., potatoes, eggplants, etc.) being so widespread, giving transportable diseases plenty of potential hosts to spread.

Any sign of brown or black on tomato leaves is a tell-tale symptom of disease and one that often can prove fatal to the season’s crop. Within this guide, I’ll discuss 5 common diseases responsible for tomato leaves turning black at any stage of the growth cycle. I’ll also advise you on how to diagnose and treat these diseases without losing an entire year’s harvest.

5 Reasons Tomato Plant Leaves Turn Black

The sudden appearance of black leaves on your tomato plant might seem like a disaster. But there’s actually a silver lining to this unsightly symptom.

The presence of black foliage rules out some of the most common diseases found in tomatoes, including blossom end rot and mosaic virus. Black leaves are also very unlikely to be caused by environmental factors like nutritional deficiencies or poor soil quality.

In nearly all cases, black leaves on tomato plants can be narrowed down to one of the following fungal or bacterial problems:

1. Septoria Leaf Spot

As the name implies, septoria leaf spot produces grey, brown, or black spots on tomato leaves. These lesions normally appear shortly after the first fruit sets, gradually traveling up the plant, starting with the oldest leaves.

Tomato leaf spot
Tomato leaf spot

Causes 

Septoria leaf spot is caused by the fungus Septoria lycopersici. It is most common during periods of prolonged dampness. You might notice that tomato plants growing in partial shade are more susceptible to the fungus than those that receive an appropriate amount of direct sunlight.

Septoria lycopersici can survive for a very long time in the soil or dead plant matter. In many areas, this disease can even survive winter. You will most likely see signs of this disease when temperatures are between 60 and 80°F.

Treatment

At the first sign of infection, remove and destroy affected leaves to prevent the further spread of the disease. 

You can stop the spread of septoria leaf spots by applying a fungicide containing chlorothalonil, benomyl, maneb, or mancozeb. Copper-based fungicides are also effective against this disease. 

Note that fungicidal treatments will not reverse damage to already infected leaves. They will, however, protect healthy foliage on the plant from infection.

Prevention

One of the best ways to prevent septoria leaf spots is by keeping tomato plant foliage dry. Space out new plants to ensure adequate airflow in the garden. Avoid overhead watering — irrigate early in the day with a soaker system for the best results.

Since Septoria lycopersici lives in and feeds on dead plant matter, weed control is crucial. I also recommend destroying all potentially infected plant matter via burning or burying.

2. Early Tomato Blight

Early blight causes dark spots on tomato leaves, stems, and fruit. These dark spots can be up to ½-inch across and often feature concentric rings. Early blight typically affects the lowest leaves first with the leaf curling and turning black at the edges, but can cause significant foliage loss in later stages.

Symptoms of early blight
Symptoms of early blight

Causes 

According to NC State University, early blight is caused by the fungus Alternaria linariae. This disease can also infect potatoes, so keep this in mind if you have both vegetables growing in your garden.

In my experience, early blight tends to infect plants that are already diseased or otherwise stressed. Some tomato cultivars are more susceptible to early blight than others. 

Treatment

Fungicide applications may also be effective against mild infections but are best used as preventative measures. Use fungicides containing chlorothalonil or copper to treat early blight.

Prevention

Follow cultural best practices to increase airflow and prevent tomato leaves from getting wet. Select disease-resistant plant cultivars for your garden and only purchase seeds and seedlings from trusted sources.

3. Tomato Plants With Late Blight

Tomato leaves infected with late blight develop small, water-soaked lesions on the upper surface. As these lesions progress, they grow and darken to an oily brownish-purple. The underside of infected leaves may present with grey-white rings.

symptoms of late blight
Symptoms of late blight

Causes

Late blight is caused by the oomycete, or water mold, Phytophthora infestans. Different strains of this pathogen can infect tomato and potato plants.

Late blight thrives in damp conditions and the spores can travel for several miles in ideal weather. 

Treatment

When it comes to blight, fungicides containing copper or chlorothalonil can be an effective treatment available to home gardeners. However, these fungicides are most effective before symptoms occur. Weekly applications may control late blight on already infected plants, but in truth, I always remove any signs of infected plants and burn them or place them in the waste. Never compost infected plants.

Prevention

I highly recommend planting blight-resistant tomato cultivars in your garden. Starting tomatoes earlier in the season will reduce the risk of infection before harvest. 

Avoid situations where your tomato plant’s leaves will be wet for long periods. Water using a soaker hose instead of from overhead.

4. Bacterial Canker

Bacterial canker infections can cause a variety of symptoms affecting the leaves, stems, and sometimes the fruit of tomato plants. Infected leaves often develop dark brown or black lesions at their margins.

bacterial canker on tomato leaf
Bacterial canker on tomato leaf

Causes 

Bacterial canker is caused by the bacteria Clavibacter michiganensis and it is the most devastating bacterial disease that affects tomato plants.

In domestic gardens, bacterial canker is typically introduced via infected seeds or plants. It can also spread by wind or run-off but rarely travels great distances in these ways.

Treatment 

Unfortunately, there is no effective treatment for bacterial canker. If symptoms appear, infected tomato plants should be destroyed. Avoid planting tomatoes and other Solanaceae species in the area for at least 3 years.

Prevention

Planting only certified disease-free seeds and seedlings is the best way to keep bacterial canker out of your vegetable garden. If your tomatoes are grown nearby other garden plots — such as in a community garden — avoid sharing tools and supplies that may lead to cross-contamination.

Many common garden weeds can carry this pathogen. Weed control can go a long way in preventing its spread. Do not compost plant matter that displays symptoms of bacterial canker.

5. Alternaria Stem Canker

The primary symptom of an Alternaria stem canker is a dark canker that forms at the base of the stem. These cankers can girdle the main stem, killing the entire tomato plant. 

According to the University of California, a toxin released by the Alternaria stem canker pathogen causes black patches of dead tissue to form between leaf veins.

Causes 

This disease is caused by the fungus Alternaria alternata. Spores can spread from infected soil, plant matter, and garden tools. 

Treatment 

Fungicides approved to treat black mold disease on tomato plants may also be used on those infected with Alternaria stem canker. 

Prevention

Alternaria stem canker is rare in the home garden. However, you can reduce the risk of infection by selecting disease-resistant cultivars.

Cultural practices that prevent other fungal infections — e.g., not watering from overhead — are also effective against Alternaria stem canker.

Removing Black Leaves On Tomato Plants

Fungi and other pathogens that affect tomato leaves can be managed by simply removing the infected plant material. This control strategy is not 100% effective but is definitely worth trying anytime you see black leaves appear on your tomato plants.

It’s safe to remove up to one-third of a plant’s foliage. Keep in mind that tomato fruit is sensitive to the sun — you should avoid removing leaves above the fruit whenever possible.

I recommend washing your hands and tools after handling infected plant matter. Otherwise, fungal spores could be accidentally spread to healthy plants. Destroy diseased plant matter by either burying it far away from the garden or burning it.

Stopping The Spread

Many diseases can easily spread from infected plant material to new, healthy plants. It’s strongly recommended that you burn or bury infected tomato plants to destroy pathogens. You should never compost infected tomato plants as the average composting environment is not guaranteed to kill off these diseases.

Plant diseases can also overwinter in garden soil. Laying down a fresh layer of mulch at the start of the growing season can insulate new tomato plants from pathogens in the soil.

If you’ve had a particularly bad year in terms of tomato diseases, consider planting a different crop in the area the following year. Keep in mind that some pathogens affect multiple vegetable species.

Eating Tomatoes From Plants With Black Leaves

It is generally safe to eat tomatoes from plants with black leaves as long as the fruit itself is disease-free. 

Only use tomatoes that are firm and free of lesions. It would help if you never cut out diseased sections in order to use the rest of the fruit — throw away the entire tomato.

Note that some diseases, like late blight, can affect tomatoes even after harvest. I recommend eating or preserving your tomatoes immediately to prevent this. Be sure to inspect tomatoes kept in storage as they may have developed symptoms since harvest.

FAQ Tomato Leaves Turning Black

Citation

NC State University – Early Blight in Tomato

Cornell CALS – Bacterial Canker of Tomato

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Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.