How to Prevent, Identify, and Treat Tomato Diseases

When the seasons begin to change and the last frost date nears, my excitement as a gardener explodes. I love to grow tomatoes, it’s by far one of my favorite plants to grow from seed to harvest in the garden.

There is a problem, however. Tomatoes can be susceptible to a host of different bugs, pests, and diseases that have the potential to ruin your crop before you harvest a single fruit. In this article, we’ll cover the most common issues you may be having with growing tomatoes, and discuss how to prevent, identify, and treat tomato diseases that may be negatively affecting your growing season.

1. Tomato Disease
A tomato disease affecting the leaves of the plant.

Different Types of Tomato Disease

Tomato disease comes in many different forms, all of which pose unique signs and symptoms that can be overwhelming for us in the gardening community. Watching the plant that you spent so long nurturing to maturity get slowly ruined by diseases is heartbreaking and can leave you feeling helpless. Have no fear, however, as we are going to learn about all the different types of tomato diseases that can hurt our plants.

To start, let’s break down the main culprits that we can expect to find in our tomato gardens. We have…





Fungal Tomato Disease

Fungus are a common player in the garden. Most of the time, they are incredibly important for soil health as they assist the tomato plant’s root system by breaking down decaying material, making the nutrients easier to absorb by the roots.

However, certain species of fungus are destructive to tomatoes. Here are the most important ones to identify early and treat promptly.

Early Blight: This fungal tomato disease is characterized by dark spots forming on the leaves that expand and spread to the rest of the leaf. While this tomato disease can occur on all parts of the plant, the most common area that will first contract the fungal disease is the lower leaves.

This is due to the proximity of the leaves to the soil. Blight fungus lives and breeds in the soil, and gets to the tomato plant through water splashing up onto the tomato’s foliage.

The best way to keep early blight away is simple: prevention! Make sure to cut away the lower leaves and stems often, as this will be a great way to stop the ability of early blight tomato disease from taking hold on your plants.

Also, using a layer of mulch on your soil will be a game changer. Mulch comes with many benefits, but has an especially useful ability to prevent splashing from occurring during watering. By stopping the soil from splashing up to the leaves, the chances of early blight affecting your plants diminishes dramatically.

Next, let’s talk about how powdery mildew, another fungal disease, can affect your tomato plants.

Powdery Mildew: This tomato disease is a fungus that spreads through spores landing on tomato leaves through the wind. Powdery mildew is common in the garden, but can be devastating for tomato plants. It is characterized by a white dust collecting on the topside of tomato leaves, which can spread quickly and damage tomato foliage to a point of no return.

Powdery mildew can be hard to completely prevent, but one tip I’ve picked up to decrease its occurrence is related to watering. Since this fungus spreads through spores, wet leaves can be a huge vector for catching the tomato disease.

Try removing any tomato leaves that are close to the ground, and when watering, be careful not to spray the leaves of your plant. Aim directly at the base of the tomato plant, and water deeply. Avoid any misting or spraying of water on the leaves.

If you do find powdery mildew on your plant, the first step to take is determining how much of your plant is infected. For example, if you see one or two leaves of your plant has the disease, simply remove them and throw them out to best contain the spread of the tomato disease.

However, if your plant is highly infected with powdery mildew (50% or more of the leaves infected), it’s time to take action and use a remedy to clear it up. Mix together 1 gallon, 1 tablespoon of baking soda, and 1 drop of dish soap together and spray the solution on the leaves once every other day until the tomato disease is not visible on your plants.

Identifying and combating these two tomato diseases will help reduce a lot of issues with fungal infections on your tomato plants. With that covered, let’s move on to the common bacterial tomato diseases that can cause trouble.

Bacterial Tomato Disease

2. Bacterial TD
A bacterial tomato disease damaging the stem of the plant. 

There’s no way around it-bacteria is a part of life. It’s everywhere, and the garden is no different. Let’s examine some common bacterial tomato diseases that can affect our plants.

Bacterial Wilt: Have you ever found your tomato plants leaning over on their side out of the blue, looking weak and wilted? You probably have encountered bacterial wilt. This is a bacterial tomato disease that lives in the soil. It infects tomato plants through cuts and scrapes in the stems or leaves, which are usually caused by bugs.

Bacterial wilt can be particularly rough for tomato plants, as it is more difficult to treat than other tomato diseases. However, you can prevent bacterial wilt from reaching your crops by doing a couple different actions.

1.) Keep your plants healthy. All tomato diseases thrive when our plants become weaker

2.) Grow flowering plants to attract beneficial insects, which naturally regulate bugs that can cause damage to tomato plants

3.) Rotate your tomato plants each season

If you find a tomato plant with bacterial wilt, immediately remove it from the garden. It can spread quickly, so taking an infected plant out will stop it from hurting the rest of your crops.

Another bacterial tomato disease that can cause problems throughout your growing season is bacterial speck.

Bacterial Speck: This tomato disease is a common one that is harmful to the quality of the tomato’s fruit. You can see the damage from the bacteria in the form of black dots forming on the undersides of leaves, as well as on the surface of the tomato fruit’s skin.

Like other bacterial tomato diseases, there is not much you can do to get rid of bacterial speck once it has infected your plant. Because of this, mitigation and prevention are going to be our greatest assets.

To prevent this tomato disease from hurting your plants, follow the golden rule of keeping your tomato leaves dry. Never spray the leaves with water, if you can help it. This allows for a plethora of fungus, bacteria, and everything in between to attach itself to your tomatoes.

Regarding mitigation, the only option that is guaranteed to work is removing the infected plants entirely. I wish there was another way, but in my experience trying to cut away leaves that are visibly infected doesn’t seem to contain the spread. Remember, prevention is key!

Bacterial tomato diseases are tough to fight once they have taken control in the garden, but can be prevented by using the principles that we discussed in this section.

Now that we’ve explored this variety of tomato disease together, let’s take a look at how viruses take their pound of flesh from our tomatoes.

Viral Tomato Disease

3. Viral TD
A viral tomato disease affecting the leaves and fruit of a tomato plant. 

Without a doubt, my least favorite of the “bads” in the tomato disease category are the viruses. They are spread primarily through human contact with the plant, instead of the external environment. This makes viral tomato disease frustrating, as it increases the risk of making simple mistakes to a vastly higher degree.

To begin, let’s start with the tomato mosaic virus.

Tomato Mosaic Virus: A withering tomato disease, the tomato mosaic virus is one that can greatly warp your tomatoes leaves, fruit quality, production, and plant height. Tomato plants that have mosaic virus look malformed, with unnaturally shaped leaves and yellow, mosaic-like splotching.

This virus cannot be cured once it has infected your plant. You have to focus your guns on preventing it from reaching your crops. The best practice for stopping your tomato plants from getting hit with mosaic is proper hand washing. Wash your hands for twenty seconds with soap to give your tomato plants the best chance of avoiding tomato mosaic.

For tobacco users, cigarettes are a vector for this virus. Make sure to use gloves when handling your plants or diligently practice hand hygiene, as you run an increased risk of infecting your tomatoes.

Next, let’s talk about the tomato spotted wilt virus.

Tomato Spotted Wilt Virus: This tomato disease, at its latest stages, makes your fruit look alien. Tomato spotted wilt virus is characterized by dark spots collecting on baby leaves, which gradually moves to other locations on the plant. Once the fruit has reached maturity, it will bear strange circular blotches of discoloration that will reduce the size and quality of the fruit.

This virus is spread through bites from infected thrips. Thrips are an insect that thrive in weeds. The best way to prevent tomato spotted wilt virus is practicing disciplined weed control, as this will cut down a major vector of the tomato disease.

With that, let’s move on to the next subject that could be causing tomato disease: Pests!

Pest Damage

4. Aphid Damage
A tomato with severe aphid damage. Look at the small holes in the leaves. 

Many tomato diseases are brought in by infected insects biting our tomato plants. We often look at pests with disdain solely because of the physical damage they cause, but when factoring how they spread disease, it becomes all the more important to naturally control these annoying critters in your garden.

To begin, let’s start with everyone’s favorite tomato pest: Hornworms!

Hornworms: These are as they sound; large caterpillars with horn-like appendages on the tops of their heads. Hornworms are devastating to your tomato plant’s foliage. They can cripple your plants by eating away at leaves in short periods of time.

To prevent hornworms from forming in your garden, rotate your crops every year. This will prevent the hornworm larvae from finding ground in your garden. Also, growing marigold flowers is highly recommended. This flower attracts paper wasps, which naturally regulate horn worms by laying their larvae into them and using the worm parasitically.

While these two methods are useful, the best intervention to control hornworms once they have accumulated on your plants is by picking them off by hand. This is a foolproof way to stop the destruction of your tomato plants, and doesn’t subject your crops to any harsh pesticides.

There is a problem, though. Horn worms can be difficult to see on tomato plants, as they tend to blend in with natural camouflage to tomato leaves and stems. To combat this, use a black light at nighttime to easily identify them, as they glow under those lighting conditions.

Now that we’ve tackled the famed hornworm, let’s look at a less known gardening foe- the whitefly.

Whitefly: Whiteflies are small bugs that feast on the leaves and fruit of tomato plants. While seemingly non threatening at first, whiteflies can infest your tomato plant’s quickly. It is vital to get rid of them fast, as they have the ability to spread the tomato disease yellow leaf curl to your plants.

The best way to stop a whitefly invasion is to prevent the bugs from taking hold in your garden in the first place. We can do this in a few ways. The first, and most beneficial to the garden as a whole, is to attract natural whitefly predators to your garden by planting companion crops.

Through personal experience, I’ve found the best companion crops are those that produce high amounts of flowers. They attract a lot of bees, wasps, and ladybugs to the garden, which all love to eat the more damaging pests in the garden without hurting our plants.

Another precautionary measure you can use is applying reflective mulch to your garden. There are studies that show using aluminum sheets around tomato plants can decrease the chance of whiteflies developing in the garden.

Once they have developed, time is of the essence. We want to mitigate and stop the spread as quickly as possible. Manually remove any leaves that have whiteflies on them, and apply neem oil to surrounding leaves to ward off an infestation.

Next, let’s look at a common opponent in the garden-aphids!

Aphids: These bugs affect most plants in the garden, but can be particularly brutal to tomato plants. This is because, like whiteflies, they too can be carriers of the tomato disease yellow leaf curl. In addition to this, they do a lot of physical damage to your plant’s leaves, stems, and fruit.

Aphid control is similar to whiteflies. Selective leaf removal, use of neem oil, and reflective mulches can make a big difference in fighting back against aphid damage. I’ve also had success manually removing aphids from leaves using a washcloth and a mixture of 1 gallon of water, 1 tablespoon of baking soda, and 1 drop of dish soap to gently scrub off aphids.

A quick pro-tip to form a hedge against all types of tomato disease is stagger gardening your tomatoes. For more information on that, check out this great video!

Bottom Line

5. Tomato Damage
A viral tomato disease causing the leaves to be malformed and ineffective at photosynthesizing. 

Tomato disease can ruin your growing season. Nothing is worse than fighting an advanced disease process, and not knowing what to do when you should be enjoying fresh fruit from your plants. The good news is, this is preventable. Make sure you…

-cut the lower leaves off your plant, use a mulch to prevent water splashing, and keep leaves dry

-grow flowering companion plants to attract beneficial insects, and cut down weeds

-rotate your tomato plants

If you follow these three core principles, you should prevent most tomato diseases from ever entering the equation of your growing season. If you do find yourself stuck with a tomato disease, however, stay calm! Remember that gardening is a relaxing and fun endeavor, and working through problems is just a part of the process. Prevent, identity, and treat using the least harmful interventions first.

Above all, have fun growing your tomatoes! Good luck in the garden, friends. See you next time. 

tomato disease 1 tomato disease 3