10 Tomato Diseases and How to Treat Them

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Whether you’ve been growing tomatoes for a while or you’re a beginner gardener, you need to be aware of the most common diseases that can affect your plants. Despite tomatoes being considered fairly easy to grow compared to some other vegetables, there are still lots of bacteria, viruses, and fungi that target tomato plants. You need to understand the potential threats when it comes to plant diseases and how to apply the best treatment methods for a healthy harvest. Here’s how to identify and deal with the most common tomato diseases.

1. Septoria Leaf Spot

A fungal disease that tends to appear during warm and wet weather, Septoria leaf spot can be recognized by characteristic circular spots on the leaves. The disease can affect other plants but it’s frequently found on tomato plants. The spots have gray-white centers and brown-black edges.

Although it may not appear like a serious disease at first, Septoria leaf spot can severely damage the foliage of your tomatoes. Leaves will eventually wither and fall so it’s best to catch it early to prevent plant-to-plant spreading and ruining your fruit harvest. The best solution to deal with Septoria leaf spot is to spread out your plants and avoid overwatering. Remove infected leaves and add fungicide for persistent infections. This multipurpose fungicide spray found on Amazon works like a charm.

2. Southern Bacterial Wilt

Southern bacterial wilt is an extremely deadly disease that can only be treated through prevention. Once infected, the plant can’t be cured. It’s essential to quickly discard it before the bacteria spreads to your other tomato plants. Southern bacterial wilt thrives in tropical regions but can also be found in greenhouses.

Your tomato garden can get infected if you purchase a contaminated plant from another place. The disease debuts with the wilting of some of the plant’s leaves. It will progress further and turn all the leaves yellow but it appears not to affect the stem. If you cut some stems, southern bacterial wilt can be recognized easily by observing a slimy substance that becomes milky when in contact with water.

3. Anthracnose

If you see little circular areas with a slightly indented appearance on your tomato fruits, that’s one important sign of a disease called anthracnose. It’s caused by a fungus that spreads using spores splashed through the water. The diseased spots become larger while the infection goes deeper into the tomatoes.

When the center of the circular areas turns darker, it’s safe to say that the anthracnose infection is spreading further. The disease doesn’t affect exclusively tomatoes but other closely related vegetables as well such as peppers and eggplants. Treatment of anthracnose involves the use of fungicide to reduce spread. It’s also important to never irrigate tomatoes overhead and maintain sufficient air circulation between your plants. Weed removal is also recommended for prevention.

4. Early Blight

One very common disease affecting tomatoes, early blight is called by a specific Alternaria fungus pathogen. Identification involves checking for circular brown spots on the leaves closer to the soil. The diseased areas on the leaves will sometimes turn yellow once the infection spreads. Early blight isn’t a particularly severe disease because fruit production continues in most cases without major problems.

That being said, it’s important to implement some disease management solutions to avoid further spread and more notable damage. Copper-based organic fungicides can work quite effectively against early blight for tomatoes. Prevention matters a lot to keep this disease in check. It’s recommended to add mulch to your tomato garden or some organic compost for protection.

5. Late Blight

Late blight is another important tomato disease to watch out for. The infection is made by the Phytophthora infestans fungus. It’s more likely to affect your tomato garden during rainy periods when the temperatures start to drop such as the beginning of autumn. Late blight can be a severe disease because all the parts of the tomato plant can be infected.

To recognize this disease, check out for green-black splotches on the leaves that resemble frost damage. More serious cases of late blight will affect the fruits quite visibly. The tomatoes show brown blotches that rot very fast. You might also notice some shiny, dark lesions. If left unchecked, late blight will kill the plant within two weeks after initial signs. When it comes to treating this disease, you should try a powerful fungicide and take some preventive measures such as keeping good ventilation between plants and watering only directly to the root.

6. Tomato Pith Necrosis

More commonly appearing in tomatoes grown in greenhouses, this is a bacterial disease that can be easily identified by characteristic blackened areas on the stems. These necrotic blotches will eventually spread further to other parts of the plant such as the leaf blades. The stems are the most affected parts, however. Tomato pith necrosis cracks and splits the stems making them hollow inside as the bacterial infection progresses.

This disease isn’t considered very dangerous in most cases. Given the right environmental conditions, plants may recover but it’s important to keep high soil moisture because tomato pith necrosis can reduce the flow of water moving through the stems. Excessive amounts of nitrogen fertilization can make this disease worse. It’s best to control the bacterial infection through better maintenance strategies such as improved air circulation and avoidance of overhead watering.

7. Mosaic Virus

The mosaic virus is a threat to many kinds of vegetables, tomatoes included. As its name suggests, the disease can be identified by looking for characteristic markings on the leaves of the plant that look very similar to a mosaic. Other signs to look for include strange leaf distortion and yellowing.

The disease caused by the mosaic virus isn’t particularly deadly as plants will usually survive an infection. However, fruit production and quality are affected. The bad news is that you can’t cure this tomato disease. You can only limit infection to other plants and apply some good preventive measures to avoid contracting the virus. Neem oil and fungicides can help in that sense because some pests can carry the virus to your plants. Garden tools should be disinfected to ensure that you’re not spreading the mosaic virus further.

8. Fusarium Wilt

Caused by the Fusarium oxysporum pathogen, this tomato disease spreads fast and can be very difficult to control. It’s more often encountered in warm climates and can be identified by checking for some specific symptoms. Fusarium wilt affects the leaf stems and spreads to the entire branch of a tomato plant from its base upwards causing it to wilt. A clear sign of infection can be discovered by cutting the main stem and searching for black streaks spreading through it.

There’s not much you can do in terms of managing this disease. Fusarium wilt can destroy entire tomato crops so your best bet is to implement solid protective measures such as soil solarization or fungicidal drenches. The disease is hard to control because spores persist for years in the soil and spread in multiple ways.

9. Leaf Mold

If your tomato plants live in an area with high humidity and poor ventilation, there are increased chances of the appearance of leaf mold. This is another pesky fungal disease that requires preventive treatment to control. As you can probably guess from the name, leaf mold can be recognized by the appearance of specific spots on the surface of your tomato leaves.

Many diseases affect the leaves and cause similar symptoms. To differentiate leaf mold from others, it’s recommended to check for some distinctive signs such as the characteristic look of the yellowish spots. Lower leaves will typically show gray growth of the spores with a texture that resembles mold. Severe infections can kill the foliage and the entire plant once the disease spreads to all the parts of your tomato. The best treatment method for leaf mold is to get rid of crop residue and include some advanced staking and pruning maintenance.

10. Tomato Yellow Leaf Curl

Caused by the virus with the same name, this disease is brought into your tomato garden by whitefly pests. This is a tricky disease to deal with because symptoms can appear weeks after the initial infection. Leaves become curled and infected plants experience stunted growth. Tomatoes affected by the yellow leaf curl virus will look bushier and often produce very little fruits.

In some severe cases, such as infection at an early growth stage, the tomato garden can become completely unproductive as flowers can’t develop and fall off. This is an extremely damaging tomato disease but the good news is that it’s commonly found in restricted areas such as California and certain warm climate zones. If you plant tomatoes together with peppers, the virus can also damage them. To treat the tomato yellow leaf curl, it’s essential to keep whiteflies out of the garden. Control weeds and apply some horticultural oil to repel the pests spreading the virus.


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