11 Common Tomato Plant Diseases and How to Treat Them Organically

by Bethany Hayes

If you belong to the 90 percent of gardeners who grow tomatoes, understanding tomato plant diseases needs to be on your list of things to do. Many gardeners find growing tomatoes frustrating because the plants fall victim to a range of diseases. Knowing the common plant diseases helps you learn how to prevent them and how to stop infections before they destroy your plants. 

Plant diseases are sneaky, so prevention measures are essential. Bacteria and fungi operate on the microscopic level, making it hard to notice a disease until it takes over your entire plant. 

The last thing you want to do is treat your diseased plants with chemicals that could seep into the food you’re growing. Organic gardeners face an extra step – learning how to treat tomato plant diseases organically. Chemical control has a time and place, but in most situations, organic treatment options exist. 

11 Common Tomato Plant Diseases

We’re going to look at the most common tomato plant disease. It’s impossible to cover all of the diseases your plants could face; many diseases are regional. Instead, we’ll look at the ones you are most likely to encounter and show you how to stop the disease in its tracks. 

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Damping Off

One of the first tomato plant diseases you might encounter is damping off, a fungal disease that causes the collapse of seedlings or the failure to germinate. It can be frustrating for home gardeners to put work into starting tomato seeds at home, only to have plants fail and die because of damping off.

In most cases, damping off is due to poor drainage or compacted soil. It also can happen due to poor air circulation around your seedlings. 

Signs of Damping Off

Damping off affects seedlings more so than larger plants. The most common signs are that they fall over and die soon after germination and emerging from the soil. The stems might turn dark and start to shrivel at the soil line.

Preventing and Treating Damping Off 

There is no cure for plants that have damping off, but you can prevent the problem from happening at all. Using small fans to increase air circulation and using sterilize potting soil are just a few ways to fight damping off. 

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Bacterial Canker

Clavibacter michniganesis causes this bacterial infection. It occurs naturally; it’s easy to bring it into your garden on different tools or previously infected plants. Water splashes the bacteria onto the plant, and once the bacteria find an open sore or wound, it infects the plant. 

Signs of Bacterial Canker

It’s easy to mistake bacterial canker for cloudy spot disease. 

It starts as yellow dots on ripening red tomato fruits. If you have a magnifying glass, a close inspection of those dots will show that they have dark rims around each of the spots. That’s one way to tell bacterial canker vs. cloudy spot disease. 

Preventing and Treating Bacterial Canker 

Unfortunately, this bacterial disease is impossible to treat. Once infected, plants must be removed from the garden beds immediately, and avoid planting tomatoes in the same soil for a minimum of three years. 

Make sure you trash the plants or burn them; never put them in your compost bin.

While treating bacterial canker is impossible, preventing it is possible. 

1. Rotate Crops

It’s essential to rotate your crops to prevent diseases and bacteria from taking hold in the soil. In the following year, don’t plant tomatoes or any other nightshade plant in the same garden bed. 

2. Clear Out Garden Weeds

Bacterial canker runs rampant in the Solanaceae family. That includes many common garden weeds, so keep your garden clean and clear of weeds. That’s a simple way to reduce the spread of bacterial canker. 

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Anthracnose 

A fungus called Colletotrichum phomoides causes anthracnose. These fungal spores love hot, humid conditions, and spread easily by overhead irrigation and sprinklers, as well as heavy rain. When the water hits infected soil, it splashes the soil onto the stem and leaves, infecting the plant. 

Signs of Anthracnose

Anthracnose presents itself as the tomatoes start to ripen, forming a dark, bull’s eye on the fruits that appear on the blossom end of the tomato. The dark spots are mushy and sunken, making them easy to spot. 

In some cases, the flesh of the fruit might rot entirely off. If you slice into an infected tomato, the black spot looks rotten on the inside. Picking ripe fruit frequently helps to reduce the spread. 

Preventing and Treating Anthracnose 

1. Water Correctly

Make sure you water plants correctly. That might mean you need to switch watering methods, allowing the water to drip on the roots, not the leaves. The water should reach the roots.

2. Try Liquid Copper Sprays and Sulfur Powders

You can apply liquid copper sprays and sulfur powders weekly, starting when the foliage appears in the early spring. It’s best to spray your plants in the early morning.

3. Use Neem Oil Spray

Neem oil is always good to have on hand because it’s multi-purpose and organic. It prevents fungal attacks on plants, and when applied early, it can stop the fungus right in its tracks. 

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Early Blight

Chances are you’ve heard of blight. There are two forms: early and late. Alternaria solani is the fungus that causes early blight.

Early blight lives in the soil throughout the winter, which is why it’s best to change locations for your tomato plants. If they had early blight in the previous years and didn’t rotate crops, they’ll contract it again from the soil’s fungus. It lives for years!

Signs of Early Blight

Early blight leads to black or brown spots on the tomato leaves; the disease starts with the oldest leaves first and works its way up. Each spot develops a ring as the disease progresses, similar to a target. The leaves with the spots turn brown and eventually fall off of the plant.

The fruit stems might be attacked as well, developing sunken black areas. If early blight takes over, it leads to the plant having few, if any, leaves. If high humidity and temperatures take place at the same time, most of the foliage will die. 

Preventing and Treating Early Blight 

Getting rid of early blight is more challenging than you might imagine. Here is what you can try.

1. Use Proper Plant Care Recommendations

Using proper care and gardening techniques is the best way to prevent late blight. Rotate crops, clear away debris and weeds, and space tomato plants appropriately for air circulation. Always water tomato plants at the base, not on the stem. 

2. An Organic Fungicide

Look for a brand of fungicides that is compliant with organic gardening. One option is called Safer; look at your local garden center for other choices. Garden fungicides can treat plants infected with early blight. 

3. Use Liquid Copper Fungicide

Another option is to spray the plant with liquid copper fungicide concentrate. It’s an organic treatment that needs to be done on a dry day. The next time, remove the lower branches and repeat the treatment in one to two weeks. 

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Late Blight

In contrast, another one of the most common tomato plant diseases is light blight, caused by the fungus Phytophthora infestans. Late blight appears during cool, rainy periods that most often happen at the end of a growing season. 

This plant disease is no joke; it affects all parts of the plant. Complete defoliation of the tomato plant happens within 14 days after the first symptoms, so watching your plants closely is key to stopping the disease before it gets too bad. 

Signs of Late Blight

As the weather starts to cool or periods of rain happen, look for signs of late blight. It might look as if there is frost damage on the leaves of your plant, leading to irregular green-black splotches. The fruits could have enormous, irregular-shaped brown blotches that rot quickly. 

These leaf spots enlarge quickly, and white mold might appear at the margins of the affected areas. Depending on the severity of the disease, the fruits could develop shiny, dark lesions. Complete browning and wilting of the leaves and stems happen within two weeks after the first symptoms. 

Preventing and Treating Late Blight

Getting rid of late blight is tricky, so the first steps are to prevent the disease. Purchase certified disease-free plants and seeds and keep your garden beds cleared out to remove debris that might harbor disease. 

1. Water Properly

It’s wise to water adequately. Keep the foliage dry and water the base of the plant only. Overhead watering spreads diseases even more. When planting, provide extra room between the tomato plants; air circulation reduces diseases. 

2. Pull Out Plants

Plants infected with late blight often needed to be pulled out of the garden and destroyed. Don’t compost them; they should be trashed or burned. 

3. Use Copper Fungicide

In some cases, a copper fungicide can be applied and used to treat late blight effectively. 

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Septoria Leaf Spot 

Here is another fungal disease that might take over your plants. A fungus called Septoria lycopersici leads to this infection that takes over and infects the plant’s foliage. 

Like many fungal infections, Septoria Leaf Spot primarily happens when watering the plants. The water spreads the spores across the leaves on the plant, and the disease begins. 

It also appears in warm, wet weather; humidity and moisture are primary spreaders. This fungus is most active when temperatures range between 68 and 77℉ and when humidity levels are high. 

Signs of Septoria Leaf Spot

As the plants grow and develop, the lower leaves start to develop yellow spots. Look closely, and you’ll find dark grey centers with dark borders appearing as the fungus progresses. 

Eventually, black dots appear in the center of the spots. Those black specks are spore-producing bodies and spread the disease further. The foliage starts to die and falls off the plant after it turns yellow. It takes time for this to happen as the fungus spreads throughout the plant. 

Defoliation weakens the tomato plants, reduces the tomatoes’ quality, and leaves the plant vulnerable to other diseases. 

Preventing and Treating Septoria Leaf Spot 

Getting rid of Septoria Leaf Spot is a bit tricky; it’s best to focus on preventative measures, such as proper watering techniques and leaving room between plants for air circulation. If your plants end up infected anyway, here are some suggestions.

1. Rotate Crops

Always practice crop rotation to reduce the inoculum. Removing crop debris is an essential part of crop rotation and decreases the risk of the disease living in the soil for years. 

2. Use a Garden Fungicide

With this fungus, you also can use an organic fungicide as a treatment. Spray the foliage and entire plant at least one time to kill off the spores. Copper fungicides or those containing chlorothalonil are most effective. 

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Fusarium Wilt 

If your plants look fine then suddenly start to wilt, you might have fusarium wilt. Fusarium oxysporum f.sp. Lycopersici is a fungus that attacks your tomato plant’s vascular system, which is similar to the veins throughout your body. 

As this fungus progresses through your body, it destroys the xylem tubes, essential for transporting water and nutrients from the roots into the leaves. That’s devastating to tomato plants and often leads to their death. 

Signs of Fusarium Wilt

Fusarium wilt develops rapidly. One day, the plant looks healthy and fine, and then suddenly, it starts to wilt even if appropriately watered. The wilting might begin on one side, but quickly, the entire plant is wilting. Within a few days, the whole plant is dead. 

Preventing and Treating Fusarium Wilt 

When it comes to fusarium wilt, preventative measures are most important because there is no proven treatment method. It happens so quickly that most gardeners say that their plants are dead within a week. 

Focus on proven methods to prevent fusarium wilt. Here are some suggestions.

1. Crop Rotation

Rotating crops is a vital trick to preventing this fungus from taking over your garden. Make sure you don’t plant tomatoes in the same part of the garden the following year. 

2. Purchase Resistant Varieties

Purchase wilt resistant tomato varieties if you’ve experienced these problems in the past. The fungus overwinters in garden soil, so you need plants that can fight it off. 

3. Try Actionvate

One product that might work against Fusarium Wilt is Actinovate, which is a pesticide and fungicide that targets the diseases that cause these problems. It introduces beneficial microbes into the soil to help the roots and foliage fight back.

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Verticillium Wilt 

Here is another fungal disease that is a common tomato plant disease. Verticilliurn albo-atrum is the fungus that causes this infection. It attacks the roots and goes up the xylem tube along with water. Once it enters the xylem tube, it stops the flow of water and nutrients to the leaves. 

Signs of Verticillium Wilt

This fungal disease starts when yellow blotches appear on the lower leaves. As the disease progresses, the veins in the leaves turn brown, and eventually, the leaves fall off of the plant. 

In the end, the disease prevents water and nutrients from spreading throughout the plant, causing it to die. At first, it seems as if its growth is stunted, but soon, the entire plant dies. 

Preventing and Treating Verticillium Wilt

Like Fusarium Wilt, treating Verticillium wilt isn’t possible. Once infected, saving the plants is impossible, so the best solution is to remove them from the garden and toss them into a trash bag. 

1. Keep Your Plants Healthy

You might be able to help your plants survive verticillium wilt. Water regularly, fertilize on a schedule using a low-nitrogen, high-phosphorus fertilizer and prune off dead branches. 

2. Soil Solarization

Since verticillium wilt lives in the soil, soil solarization can get rid of it in the ground. This method heats the top six inches of the soil high enough to kill any fungus present.

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Mosaic Virus 

The Mosaic Virus attacks tomato plants, as well as a range of other vegetable plants. This virus doesn’t kill most plants, but it can reduce the number of fruits produced and their quality. That means all your hard work might go down the drain.

Open wounds in the leaves and stems allow the virus to enter, so avoid handling the plants. That’s especially true if you smoke because tobacco harbors the mosaic virus. 

Signs of Mosaic Virus

The virus gets its name because the markings resemble a mosaic with light green and yellow marks on the leaves. It also causes mottling on the fruits, and some leaves might become distorted, resembling ferns instead of tomato plant leaves. 

Preventing and Treating Mosaic Virus 

Since this is a viral disease, there is no cure for a virus, and every effort should be made to prevent it from taking over in your garden. Using fungicides will be of help treating this virus, and don’t save seeds from infected crops. 

Here’s how to prevent the mosaic virus.

1. Treat with Neem Oil

Several insects carry the mosaic virus, so pre-treating your plants regularly with neem oil and other natural pest control products can help ward off the insects. 

2. Use Row Covers 

Another trick to keep insects off of your plants is to cover your tomato plants with row covers. These will keep the pests off your vulnerable plants. 

3. Always Clean Garden Tools

This virus often spreads by garden tools. Always disinfect garden tools, ties, pots, and any other equipment with a solution that is one part bleach hand four parts water. 

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Leaf Mold

Here is another fungal disease that infects tomato plants. Fulvia fulva is the fungus that causes leaf mold, and it loves to grow in places with poor air circulation and high humidity levels. That’s why it’s most commonly found on tomatoes grown in greenhouses. 

Signs of Leaf Mold

The fungus starts on the older leaves closest to the soil, where air circulation is the lowest. It begins with pale green or yellow spots on the leaf surface that gradually get larger. Over time, the spots turn a distinctive yellow color. 

If humidity levels are high, these spots turn grey and develop a velvety texture, the fungus spores. Severe cases cause the spots to join and kill the foliage entirely. 

Leaf mold affects more than the leaves; the fungus also likes the stems, blossoms, and fruit. Both green and mature fruits develop black, leathery spots on the stem end. 

Preventing and Treating Leaf Mold 

Preventing leaf mold is like many other fungal diseases. Improving air circulation and using proper watering techniques are essential. Always stake your tomato plants to increase airflow and avoid getting water on the leaves. 

1. Use an Appropriate Fungicide 

One of the most common leaf mold treatments is to use a preventative fungicide that contains chlorothalonil or copper fungicide. 

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Bacterial Spot

Bacterial spot is caused by the bacterium Xanthomonas vesicatoria. Instead of attaching red tomatoes, this bacteria targets green tomatoes, as well as peppers. It’s more common to experience bacterial spot during wet seasons with excess rain and humidity. 

The bacteria survive winter on infected plant debris and other plants, so you must remove as much plant debris as possible at the end of the growing season. It loves and thrives in moist weather. Most bacterial spot outbreaks are due to heavy rainstorms that cause the bacteria to find open wounds on the leaves. 

Signs of Bacterial Spot

As the bacteria take over, bacterial spot causes damage to the plant such as spots on the leaf and fruit, reduced yields, and defoliation. Small, irregular, water-soaked spots might develop on the leaves, and scabby spots appear on the fruits. 

The leaf spots might have a yellow halo with a dry center. Over time, the leaves tear open and start to die off of the plant. Defoliation is common with these bacteria. 

Preventing and Treating Bacterial Spot

It’s hard to prevent and control bacterial spot when it appears in your garden. The bacteria spread by water, splashing from one plant to another. Avoiding overhead watering is the first step; using drip irrigation is essential around tomato plants. 

1. Prune Your Plants

It’s essential to prune your plants to encourage good air circulation. 

2. Spray a Copper Fungicide

The best way to treat bacterial spot organically is to spray your plants with copper fungicide. It’s effective at controlling this bacterial disease. 

Work to Prevent Tomato Plant Diseases

Unfortunately, a common theme that you might notice is that getting rid of tomato plant diseases is hard, if not impossible, in many cases. That’s why the key is preventing these diseases from happening by using proper watering and gardening techniques. Those will keep your plants as healthy as possible.

Learn More About Tomato Plant Diseases

• http://vegetablemdonline.ppath.cornell.edu/DiagnosticKeys/TomWlt/TomWiltKey.html
• http://extension.msstate.edu/publications/common-diseases-tomatoes