8 Signs Your Tomato Plant Has a Disease - Backyard Boss
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8 Signs Your Tomato Plant Has a Disease

Tomatoes are a kitchen staple, so it’s no wonder so many gardeners grow them. There are so many varieties to choose from and if you need to save space, they are easy to grow in containers. However, there are also lots of diseases that kill tomato plants. If you’ve lost tomato plants to disease, you know how hard it can be. Especially when you don’t realize it until the fruit has been infected.

As a preventative measure, always plant disease-resistant varieties. Unfortunately, “resistant” doesn’t mean “immune,” so they are still susceptible to some types of diseases. When disease strikes, it’s important to act fast before you lose the whole crop. Here are eight signs your tomato plant has a disease.

1. Spots on the Leaves

Leaf spot disease of tomato
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Spots on the leaves of your plants are never a good sign. With tomato plants, it’s a good idea to rule out the possibility of heat damage first. Burnt leaves are pretty easy to spot as the edges will usually be crispy and brown. Scorched leaves affect the top leaves of the plants, whereas a lot of diseases appear on the bottom leaves of the plant first. If you’ve ruled out heat damage, and the spots are brown or black, it’s likely a disease.

Spots on the leaves could be many different diseases including early or late blight, septoria leaf spot, bacterial spot, or tomato spotted wilt. Unfortunately, these diseases present similar symptoms, making some hard to differentiate.

Early blight and septoria leaf spot, for example, both start as brown spots on the bottom leaves. Early blight spots are much larger whereas septoria leaf spots present as small bullseyes. With bacterial spot and tomato spotted wilt, you will also notice spots on the fruit. Remove the infected leaves or plants immediately.

2. Yellowed Leaves

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Yellowed leaves may be another sign that your tomato plant has a disease. Leaves also yellow due to nutrient deficiencies, herbicide use, and when excess minerals aren’t flushed out of the soil. If you’ve ruled out these problems, it’s probably a disease.

Most tomato plant diseases present with some yellowing of the leaves. Unfortunately, this doesn’t help you narrow it down. As a preventative measure and to treat infected plants, spray with a fungicide every seven to 14 days.

3. Mold on the Leaves

Powdery Mildew Tomato
Image credits: Taew Jari via Shutterstock

Mold, a type of fungi, is a sign of tomato disease. This is more common in wet, humid, or cool climates. Common diseases are late blight and powdery mildew. The leaves will look whitish-gray and may include some yellow discoloration. 

Mold can get out of control quickly, so it’s best to remove any infected plants. Do not compost them as you could pass the disease on to future seasons. Use a fungicide to treat and prevent. Or for a natural method, try baking soda.

In addition, do not get water on the leaves of your plants and avoid overwatering. This is especially important in cooler weather. Provide adequate airflow, making sure your plants have an appropriate trellis. Tomatoes should also be placed in full sun.

4. Leaves Curling

Leaf curl tomato/Thrips symptoms destroying tomatoes/Crinkled leaves from thrips destroying tomatoes
Image credits: Kittisak Chysree via Shutterstock

When you see your tomato leaves curling, you may start to panic, but there are many reasons this can happen. The most common is over or underwatering. It could also be nutrient deficiencies, heat damage, or over-pruning.

If the curling leaves are accompanied by yellow leaves and stunted growth, it could be tomato yellow leaf curl virus. This disease is transmitted by whiteflies. While some other diseases don’t damage the fruit, this one can take out whole crops. Remove and burn all affected plants. Weeding and spraying with horticultural oil to reduce the whiteflies is the best way to fight this disease.

5. Wilting Leaves

Tomatoes wilted in the garden.
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Wilting leaves are often a sign of dehydration and are normal for tomatoes during hot weather. However, wilting leaves can also be a sign of bacterial wilt, fusarium wilt, or southern blight. With these types of diseases, you will usually also notice a browning, decaying, or molding at the root of the plant. Proper crop rotation and clean-up at the end of each season can help to prevent the spread of many diseases. 

6. Spots on the Tomatoes

The fungus buckeye rot of tomato caused by the pathogen Phytophthora parasitica badly affected a tomato plant. Late stage of tomato disease.
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Hopefully, you catch diseases before they affect the fruit. If you don’t, it will be pretty easy to spot the disease, but much too late. Many diseases cause the fruit to rot, such as buckeye rot and anthracnose. They both start as small spots, but as they progress the spots indent and mold making the fruit inedible.

Black spots on the bottom of tomatoes can also be blossom-end rot. BER is actually not a disease, but a result of a calcium deficiency.

7. Blackened Stems

Backyard greenhouse with tomato growing. Ripening tomatoes
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If you notice the stems on your tomato plants are turning black, this is a sign of disease. If it’s early in the season, it’s likely tomato pith necrosis and is common in greenhouses. If the fruit is able to form, it will rot similarly to late blight and buckeye rot. Too much nitrogen can be a factor, so be sure to use a tomato-appropriate fertilizer.

8. Stunted Growth

person holding a green tomato
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Stunted growth of the leaves or fruit could be due to several reasons such as nutrient deficiencies or inconsistent watering. If you’ve ruled out the basics, stunted growth could be a sign of disease. Many of the above-mentioned diseases cause stunted fruit growth. In addition, seedling disease is a fungal disease that uniquely affects a seedling’s ability to emerge and grow. This can happen when seedlings are planted too early and the earth is still cool and damp.

Check Your Plants

When it comes to growing tomatoes, it’s important to check your plants regularly. It’s a good habit to look over your plants every time you water. At the first sign of disease, remove or treat the affected plant(s). If you can catch it early enough, you still may have a good harvest. While you’re at it, keep an eye out for pests. They cause many of these diseases and can severely damage tomato plants.

Have you ever had to deal with a disease on your tomato plants? Share in the comments!