There’s nothing worse than growing a big, beautiful tomato plant only to watch the leaves turn yellow.

If you’re trying to figure out what happened to your tomato plants, here are six reasons why the leaves might be turning yellow as well as how to prevent and treat them.

6 Reasons Tomato Plants Leaves Turning Yellow

1. Over or Under-watering

The common reason that the leaves of a tomato plant turn yellow is incorrect watering. Underwatering will cause it, but underwatering is more common.

Tomato plants love water and need a lot of it when they’re actively growing, especially in hot, dry weather.

Overwatering causes the soil to become heavy and compact, making it difficult for oxygen to penetrate the soil. This deprives the plant of oxygen, and the constant moisture can cause the roots to rot.

Preventing overwatering is pretty simple. Just water less! You can also invest in a rain gauge and drip irrigation system if you want the watering process to be automated.

If it’s too late and the soil is already over-saturated, remove any mulch from the surface of the soil. Exposing the soil surrounding the plant to sunlight will allow it to dry out more quickly and help oxygen reach the roots.

Tomatoes in a pot are a little different than those planted in soil. Generally, you should wait to water a potted tomato plant until the top two inches of soil are completely dried out.

As for underwatering, yellowing leaves are usually a late sign. The first thing you will notice on an under watered tomato plant is wilting. If a wilted plant doesn’t get waterer, the edges will turn yellow. The yellowing will spread to the entire leaf, and it will eventually die and fall off the plant.

To prevent underwatering, plan to water your tomato plant every other morning, aiming the water at the soil and roots, not the leaves of the plants.

The earlier you catch an under watering problem, the better. Water as soon as you notice wilting to prevent yellowing if possible. Water the plant slowly, ensuring that the water absorbs thoroughly into the soil and down to the roots.

2. Lack of Sunlight

Yellow leaves can also indicate that your tomato plant isn’t getting enough sun. Sometimes, this affects the whole plant, but it may only happen to the leaves on the bottom of the plant.

Generally, if only the leaves at the bottom of the plant are yellowing, the plant is still okay. Tomato plants will often allow the bottom leaves to die if they’re not getting enough sun so that the upper leaves and tomato fruit can continue to grow and mature.

To prevent this from happening, pay close attention to where you plant your tomatoes. Tomato plants need at least eight hours of sunlight a day. If you’re unsure about a location, use a sunlight meter to make sure the area gets enough sunlight before you plant your tomatoes in the first place.

If you notice the upper leaves are yellowing, remove any vines or other plants that might be blocking the sun. Prune back any overgrowth to make sure your tomatoes get as much sunlight as possible.

In this scenario, having a potted tomato plant is an easy fix. Just pick it up and move it to an area where it will get more sun.

3. Compacted Soil

If the soil is too compact when planting, the tomato plant may not be able to spread its roots easily. If the roots are unable to spread, they will not be able to get what they need from the soil.

It may appear that things are going pretty well at first, but once the plant reaches a certain size, the roots won’t be able to push through the firm soil anymore. At this point, the leaves may begin to yellow.

To prevent this, till the soil well before planting. If you just dig a hole for the seed or seedling without loosening the surrounding soil, the plant will only have a small area to root in. When you loosen the soil in the area around the plant, it has a better environment to grow in.

If you suspect compacted soil is causing your tomato plant leaves to yellow, you can try using a gardening fork to loosen the soil around the roots or work in organic compost to help loosen it up.

4. Nutrient-Deficiency

Yellow leaves are also a sign of a nutrient deficiency or imbalance.

If your soil is too acidic or too alkaline, your tomato plants might struggle. Tomatoes prefer soil that is slightly acidic, so you want the pH to be around 6 to 6.5.

A nitrogen deficiency is a common cause of yellowing in tomato plants. In this case, the bottom leaves usually suffer the most, while the top leaves may remain green. Overall growth, though, will slow down significantly.

If you notice the veins of the leaves are turning yellow, but not the leaves themselves, a potassium deficiency is likely the culprit. If the tips of new leaves are turning yellow and then the whole leaf dies, you might have a calcium deficiency.

A magnesium deficiency causes the growth to slow down and the edges of the leaves to turn pale and yellow. You will also see small yellow spots around the veins.

To prevent nutrient deficiencies, the best thing to do is make sure your soil is prepared before planting. This process usually involves having the soil tested and adding amendments to make up for any deficiencies.

If you’ve already planted and thought this might be the problem, the treatment is the same. Have the soil tested and mix in any necessary amendments to correct the issue.

Some common fixes are urea to increase nitrogen, potash to increase potassium, and Epsom salts to increase magnesium.

5. Pests

Many pests can also turn the leaves of a tomato plant yellow. Here are some common pests that you might have to deal with:

Leafhoppers: Mature leafhoppers are triangular in shape and are ½ inch long or less. They may be brown or green or a mix of different colors. They will fly away when you approach them. In addition to yellowing leaves, leafhoppers cause spotting and leaf curling.

To prevent leafhoppers, remove trash and debris and cover the plants when necessary. Ladybugs eat both the eggs and the larva and are great for prevention. You can spot treats with insecticidal soap or apply diatomaceous earth to the plants.

Mealybugs: Mealybugs are small, oval-shaped insects that have a distinct white wavy coating. They are very small and appear to be wispy spots of cotton. They are slow movers, and when they find a place they like, they stop moving and group together, forming clusters on the stems and leaves.

While unsightly, mealybugs are pretty easy to treat. All you have to do is mix two teaspoons of Dawn dish soap with water in a spray bottle and spray a few leaves on the top and bottom.

Wait a day or two and come back to reassess. If the leaves are still healthy and untouched, spray the whole plant. Some varieties of tomato plants don’t tolerate the dish soap treatment.

After covering the whole plant, come back after about four hours and rinse it thoroughly with a hose. Repeat every three or four days until the bugs are gone.

Aphids: Aphids are a common garden pest that can do significant damage to your tomato plants. These small, soft-bodied insects multiply quickly, so stopping an infestation quickly is key to your plant’s survival.

Aphids are under ¼ inch long and can be gray, yellow, green, brown, pink, black, or white. They may be wooly or waxy and have long antennae and tubes projecting from their rear.

If you only notice a few aphids, you can brush them away by hand. If there are more, use a strong spray of water to remove them daily or mix a solution of castile soap and water and spray all over the plant to suffocate them.

Ladybugs also eat aphids and can help lower the risk of infestations.

6. Disease

Many diseases can cause yellow leaves on tomato plants.

Fusarium wilt comes from a fungal soil pathogen called Fusarium oxysporum. It gets into the plant through the roots and stops the plant from absorbing water. Leaves will yellow and eventually fall off.

This fungus can only get into a plant through a damaged root, so the best thing you can do to prevent it is to maintain a healthy plant and avoid root rot.

Fusarium wilt is difficult to treat, and the plant is unlikely to survive, though it may take awhile to die.

Septoria leaf spot is a fungus and is common in areas where the weather is generally wet and humid. Spores can survive the winter and can even stay dormant on your garden equipment in the off-season.

This fungal infection is somewhat easy to identify. It appears to be brown or grey splotches between 1/16 and an inch in diameter, surrounded by yellow. These spots can also run together, forming larger yellow splotches with outer rings.

To treat, remove any disease leaves, but be careful how you dispose of them because this is an easy way to spread spores.

One way to try to prevent Septoria leaf spots is to place mulch on the soil to protect the plant from splashes from rainwater and watering. Keep your plants’ leaves from touching the ground to stop them from coming in contact with spores on the ground.

Ralstonia solanacearum is caused by bacteria found in the soil. It’s common in areas that are humid, moist, and sandy. While yellowing leaves are an early sign that there is a problem, this disease is incurable. The best thing you can do is remove the plant and burn it so it doesn’t affect anything else.

A fungus called Alternaria solani causes early blight. This disease presents as small brown spots that form a bull’s eye shape. The leaf will eventually turn yellow and die. To treat, remove the disease leaves and dispose of them carefully. Then, water as you would when treating Septoria leaf spot.

Finally, another soil-borne fungus that can cause leaf yellowing is Verticillium albo-atrum. This fungus is common in colder areas, like in the northeast US. Yellow patches appear first. These soon progress to brown spots and leaf death. This is another disease that is impossible to treat. The best way forward is to remove the plant and treat the soil with an antifungal to prevent problems in the future.

Conclusion

As you can see, there are many reasons why the leaves on your tomato plant may be turning yellow. Some of them are pretty easy to fix while others are a little more damaging. Whatever the cause, we hope we helped you figure out what’s wrong with your plant and some easy ways to treat or prevent future problems.