6 Reasons Tomato Plants Get Yellow Leaves

When the conditions are right, a single tomato plant can produce dozens of fruit. But tomatoes are susceptible to a wide range of issues that can reduce your harvest and even kill your plants. 

Curling leaves on tomato plants is a common sign that something is wrong. The sooner you identify the underlying cause and address it, the better. You may even discover that there’s no problem at all—it’s simply a sign that your plant is maturing.

If your tomato plants are starting to get yellow leaves, don’t panic, we’re here to help. Let’s discuss the top five reasons it happens and what you can do to fix it.

1. Improper Light

Sunlight is essential for healthy growth but too much of it can be just as problematic as too little. Tomato plants require at least 6-8 hours of sunlight each day. Yellow leaves can sometimes be a sign that your tomato plants aren’t getting enough sun.

Here are some things you can do to increase sun exposure:

• Move tomato plants to a sunnier location if you’re growing them in containers and you can do so without too much hassle.
• Prune the trees and shrubs surrounding your garden to prevent them from blocking too much sun.
• Wait until the next growing season and plant your tomatoes in an area that gets more direct sunlight. 

The leaves on tomato plants help shade the fruit from sunlight, so if the foliage on your plants is too sparse, you might see signs of sun scald on your tomatoes. 

In some cases, intense sun exposure can lead to leaf discoloration as well. It’s usually a temporary issue that happens in extreme heat. Just make sure to water your tomato plants well and they should recover.

2. Watering Issues

Water goes hand-in-hand with sunlight when it comes to supporting healthy growth in tomato plants. And again, too much of it can cause issues just as easily as too little of it. When it comes to yellowing leaves, however, it’s usually a sign of overwatering rather than underwatering.

Tomato plants require a minimum of 1 inch of water per week. If you live in an area that gets regular rain, you may not need to do much watering yourself. In hot climates and areas that get little rain, however, you’ll need to help your plants out.

The key to watering tomato plants properly is to do it slowly. You want to thoroughly dampen the soil, letting the water really soak in so your plants grow deep roots (this helps protect them against periods of drought). But avoid pools of standing water or excessive watering that could lead to root rot and yellowing leaves.

3. Nutrient Deficiencies

Tomato plants require an array of nutrients to sustain their growth and produce healthy fruit. Even if your garden soil is rich, you may need to fertilize it from time to time throughout the growing season. But it’s important to strike the right balance to avoid fertilizer burn. 

Here’s how to identify common nutrient deficiencies in tomato plants:

• Nitrogen deficiency - If yellowing is primarily affecting the older, larger leaves on your tomato plant it could be a sign that the younger leaves are using up all the available nitrogen. Correct the issue by using a balanced 5-10-5 fertilizer or one specifically formulated for tomatoes.
• Potassium deficiency - Insufficient potassium often causes the edges of new leaves to turn yellow. Factors that can increase the risk of potassium deficiency include sandy soil, heavy rain, drought, and acidic soil. Consider switching to a liquid fertilizer over granular products to help correct potassium issues.
• Magnesium deficiency - If you notice yellow spots forming on your tomato plant leaves, it could be a sign of magnesium deficiency. Make sure your fertilizer contains micronutrients or consider using Epsom salt to boost magnesium levels in your soil.
• Iron deficiency - Insufficient iron levels typically causes the younger, uppermost leaves of a tomato plant to turn pale green or yellow. Iron deficiency happens more frequently with sandy soil and soils with high pH (tomatoes prefer a pH between 6.2 and 6.8). Test your soil and use an acidifying fertilizer if the pH is too low.

Too much fertilizer can lead to the accumulation of nutrients, salts, and ammonia in your tomato plant roots which may cause yellow leaves. Tomatoes grown in containers are more prone to excessive fertilization than in-ground plants, but it’s still a good idea to test your soil before planting so you can come up with a suitable fertilization plan.

If you suspect your tomato plants are suffering from fertilizer burn, you can flush them with water to try to remove some of the excess. This works best with container plants but may also work with in-ground tomatoes if your garden soil drains well enough.

4. Pests

There are several pests common to tomato plants which can cause the leaves to yellow. In some cases, these pests feed on the plant and, in others, they cause stress or introduce disease. Here are some examples:

• Aphids - A small, soft-bodied insect typically found on the underside of young leaves. They consume sap, leaving the leaves sticky and stressed which can lead to yellowing.
• Spider Mites - Tiny pests that build fine webs and live on the underside of leaves. As they feed, it can cause the leaves to yellow and take on a webbed appearance.
• Whiteflies - Small, white flying pests that live on the underside of leaves and can spread disease which may cause the leaves to yellow.
• Thrips - Very small insects that feed on the chlorophyll in leaves, causing them to turn pale green and eventually yellow.

Getting rid of pests on your tomato plants may require a multi-pronged approach. You can start by physically removing as many of the pests as possible, either by spraying the plant with water or picking them off by hand (if they’re large enough to see). 

The next step is to kill or deter the pests to prevent reinfestation. Insecticidal soaps and organic sprays like neem oil and cedar oil can be very effective, just make sure to coat the underside of the leaves to get as many pests as possible. You can also try sprinkling diatomaceous earth, a natural material with tiny, sharp edges that damages the exoskeleton of insects and kills them by dehydration.

5. Disease

Fungal and bacterial diseases can cause tomato plant leaves to yellow, and some of them are not treatable. Fungal wilt diseases like verticillium wilt and fusarium wilt, for example, can’t be controlled chemically and can live in the soil for years. These diseases cause the leaves to yellow and quickly dry out, often killing the plant. 

If your tomato plant leaves are yellowing but not wilting or shriveling, it could be one of the following:

• Early blight - Caused by Alternarialinariae fungus, early blight starts with small, brown lesions forming on older foliage. The leaf may turn yellow around the edges of the spots and eventually die off.
• Leaf mold - Yellow or green spots forming on the upper leaves of a tomato plant could be a sign of leaf mold. It develops when mature leaves at ground level don’t get enough air circulation in humid conditions.
• Powdery mildew - Caused by fungus, this disease presents with bright yellow blotches, sometimes accompanied by powdery white spores that kill the leaves.
• Bacterial spot - This disease presents with small, irregular spots ringed in yellow with dry centers. If green tomatoes have started to grow on the plant, they may develop raised spots.
• Septoria leaf spot - A fungal disease, this form of leaf spot starts on the bottom leaves and works its way up the plant. It presents with brown or black round spots that start to yellow if not treated.

In many cases, tomato plant diseases survive in the soil over winter and are spread through watering. It can help to trim away infected lower leaves and stake tomato plants to ensure adequate air circulation. If treatment is necessary, try chlorothalonil, mancozeb fungicide, or copper fungicide.

6. Environmental Stress

You may be able to choose where you place your tomato plants but you can’t control the weather. If the conditions aren’t ideal, your plants could become stressed and that can cause the leaves to yellow. 

Sometimes young tomato plants experience transplant shock after being planted or repotted. This may cause a few leaves to yellow, but it should go away after a week or two. Just water the plant well and keep it protected if the temperature drops into the low 40s.

Other environmental factors that can cause leaf yellowing related to stress include extreme temperatures, excessive sun, poor soil drainage, or drought. It’s important to monitor your plants for signs of stress and take action when needed to correct these issues.

Final Thought: Yellow Leaves on Tomato Plants Can Be Fixed

Yellowing leaves on a tomato plant is like a flashing light—it means there’s something happening that deserves your attention. The sooner you find out what’s causing your leaves to yellow, the sooner you can address the problem and get back to business as usual.

Though many causes of yellowing leaves on tomato plants are due to environmental stressors outside your control, you can improve your chances of a bountiful growing season by starting with strong plants.

At Park Seed, we offer a wide range of tomato seeds from colorful hybrids to juicy heirloom varieties. Shop the collection to find the perfect fit for your garden.