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What Causes Black Spots on Tomatoes?

There are a number of different fungal diseases that can result in black spots appearing on your tomato leaves.

While growing tomatoes in your home garden is enjoyable and very rewarding, these plants can sometimes succumb to diseases that can affect the leaves and the fruit.

If your tomatoes are not looking as healthy as they should, you might notice the appearance of black spots on the leaves or the fruit or sometimes, both.

Let’s look at what causes these black spots and how you can deal with them.

Causes of black spots appearing on tomato leaves

There are a number of different fungal diseases that can result in black spots appearing on your tomato leaves.

Septoria leaf spot

Septoria leaf spot on tomato leaf | Fruit & Vegetables

This is probably the most common disease that can affect your tomato plants. It’s a fungal disease that is most prevalent during extended periods of wet and humid conditions. 

The fungal spores reside in the soil. When it rains, they’re splashed up onto the leaves of plants and start to spread.

To identify septoria leaf spot, look for small round spots on the leaves that have a dark ring and a greyish-white centre. Often the centre of each spot will turn black over time.

As a result of this disease, the leaves eventually turn yellow and fall off the plant.

How to prevent septoria leaf spot

You can prevent this disease from infecting your tomato plants by using good cultural practices. Make sure that your tomatoes are not too crowded so that there’s plenty of air circulation around your plants.

When watering your plants, make sure that you only water at the soil level and avoid getting the leaves wet. And, water in the morning whenever possible.

watering tomatoes | Fruit & Vegetables

Another good cultural practice is to remove the bottom leaves as the vines start to grow upwards. This makes it much more difficult for the fungal spores to take hold when it rains.

I often like to spread my tomato plants around different areas of my vegetable garden. This way, if one plant gets infected with a fungal disease, the other plants are far enough away to be safe from infection.

It’s also recommended that you don’t plant tomatoes in the same spot year after year and I try to follow this practice too. Fungal spores can easily build up in the soil and this practice helps to break the cycle of reinfection. See our guide on what to plant after tomatoes.

The best way that you can treat Septoria leaf spot

The first thing you want to do is remove the affected leaves and throw them in the rubbish. This should limit the spread of the disease.

In severe cases, you might need to spray your plants with a fungicide to control the disease. While this will not remove the disease from affected leaves, it should protect the rest of the plant.

RELATED: What’s the Correct Soil Alkalinity for Tomatoes?

Early blight or Alternaria

Early blight on tomatoes | Fruit & Vegetables

This is another fungal disease that causes black spots to appear on tomato leaves. It primarily starts in the lower leaves by showing up as black or brown spots that have dark edges. 

If allowed to progress, this fungal disease can also affect the fruit. It shows up as large black spots at the stem end that appear sunken.

Use the same preventative measures as for septoria leaf spot and remove and discard any affected leaves and fruit as soon as possible to prevent the spread.

Late blight 

Late Early blight on tomatoes | Fruit & Vegetables

This disease is caused by the fungus Phytophthora infestans. It usually shows up late in the season once the weather cools down and you experience periods of rainfall.

The leaves actually look like they’ve been damaged by frost with the appearance of irregular spots that can be green to black. 

The disease can also progress to the fruits causing large brown spots that are irregular in shape.

Use the same preventative and control methods as for septoria leaf spot.

RELATED: How Long do Tomatoes Take to Grow?

Bacterial diseases

There are also a variety of bacterial diseases that can affect your tomato plants. These include bacterial spot, bacterial canker and bacterial speck. 

These diseases can affect both the leaves and the fruits. 

Use the same preventative measures as you would for fungal diseases.

You can also spray your plants with a copper-based spray. But, this needs to be done as soon as you notice the first symptoms for it to have any effect.

Causes of black spots appearing on tomato fruits

Black spots that appear on tomato fruits are also commonly caused by fungal diseases.

Anthracnose

Anthracnose fungal disease on tomatoes | Fruit & Vegetables

This is a fungal disease that affects the fruits and can also be prevalent during periods of wet and humid weather. 

It displays as small, circular spots on the fruits that are often indented.

Anthracnose fungal disease on tomatoes 1 | Fruit & Vegetables

This causes the fruit to rot and makes affected tomatoes inedible. It can be a problem if fruits are left to ripen too long on the vine.

How to prevent anthracnose

Use the same preventive measures as described above for septoria leaf spot. Also, make sure that you don’t leave the fruits on the vine to over-ripen. 

How to treat anthracnose

Unfortunately, once your fruits are affected by this fungal disease, there’s no way to save them. Infected fruit should be removed from the plant and thrown into the rubbish.

Photo of author

Annette Hird

Annette Hird is a gardening expert with many years of experience in a range of gardening related positions. She has an Associate Diploma of Applied Science in Horticulture and has worked in a variety of production nurseries, primarily as a propagator. She has also been responsible for a large homestead garden that included lawn care, fruit trees, roses and many other ornamental plants. More recently, Annette has concentrated on improving the garden landscape of the homes that she has lived in and focused a lot of energy on growing edible plants as well. She now enjoys sharing her experience and knowledge with others by writing articles about all facets of gardening and growing plants.

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