Why Are My Tomato Plant Leaves Turning Yellow?

Last year, I was worried when I saw that my tomato plant leaves were turning yellow. Was it serious? Would I still be able to grow tomatoes? I decided to research the problem, and I’ve compiled that research here for you in case you’re experiencing the same thing.

tomato plants

Tomato plant leaves may turn yellow for a few reasons, including :

  • fungal or viral diseases
  • pests
  • too much or too little water
  • lack of sunshine
  • lack of nutrients in the soil

Yellow leaves near the bottom of the plant are often nothing to worry about and likely won’t affect the plant’s fruit production.

How do you know when yellow leaves are a problem and when they’re not? We have a few tips and tricks to help you figure that out.

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Yellow Leaves on Tomato Plants

Are the yellow leaves on the bottom of the tomato plant? If you’re answering ‘yes!’ then you probably have nothing to worry about. This can happen as plants mature and grow larger and bushier. The leaves on top will prevent the leaves on the bottom from getting enough sunlight, thus causing them to yellow and eventually die.

If the leaves are on the bottom, go ahead and pluck them off. This should improve air circulation for the rest of the plant, and help the rest of the plant remain healthy.

In some instances, however, yellow leaves on the bottom may still be part of a deeper issue, so let’s cover that in more depth.

Location of the Yellow Leaves and Why It Matters

Yellow leaves on the top of your tomato planet could result from soil nutrient deficiency of either nitrogen or iron. This deficiency shows up in newer growth, aka the top leaves. Adding nitrogen fertilizer or manure to the soil will usually fix this.

If the yellow leaves only appear at the bottom of the tomato plant, the issue is usually a lack of sunlight. This is a natural occurrence of the plant’s life cycle after growing taller and is not a cause for concern. On the other hand, it could symbolize a fungus or virus within the soil, or a lack of adequate water.

In most instances, however, the yellow leaves are on the bottom of the plant are nothing to worry about. This often happens with large, bushy plants where the top leaves block the bottom leaves from getting enough sunlight.

Fungal Diseases That Cause Yellowing of the Leaves

Fungal diseases are noticeable by yellowing of the leaves that starts from the bottom of the plant and spread upwards over time. They usually are found dormant in the soil. Various fungal diseases can make problems for your plants, such as the following:

  • Fusarium Root Rot can make a tomato plant’s stem mushy and rotten brown, slowly spreading upwards.
  • Septoria Leaf Spot activates in high humidity and warm temperatures. It creates small, dark brown circular spots on the leaves and can easily spread from splashes while watering or from rainfall.
  • Fusarium Wilt lies under topsoil and prevents the roots from properly absorbing water. This yellows the leaves and causes them to wilt, though the plant usually does not die. It can cause confusion as to why a plant remains wilted though it has been properly watered.
  • Early Blight is a fungus that does not affect the fruit of the plant. However, it causes small blotches on the leaves, spreading upwards from the bottom of the plant.
  • Late Blight is absolutely deadly for an entire garden. This fungus can travel miles in the air or on animals and pests. It forms small lesions on leaves, stems, and flowers which grow to become quite obvious. At this point, the plant has already been defeated. One affected plant can produce hundreds of thousands of spores, each of which can infect the rest of the crop.

To remove a fungal infection effectively, it is best to toss the entire infected plant and remove the afflicted soil as well. Wash all the tools used to remove the plant and soil, and avoid using that section of the garden for at least a year or two.

tomato plant dying of disease

Viral Diseases That Cause Leaves to Turn Yellow

Viruses can also cause yellow leaves on tomato plants and can lead to disfigured leaves. Curly top virus will cause yellowing and curling up of leaves. Tobacco mosaic virus will damage the leaves and fruit while also stunting the plant’s growth. Most viruses are passed from plant to plant via pests, works tools, or hands.

There are no chemical treatments for viral infection. The only option is to discard the entire plant and find virus-resistant varieties to plant instead.

Possible Pests That Cause Tomato Leaves to Turn Yellow

Aphids, thrips, spider mites, flea beetles, and whiteflies can all create issues for a garden and tomato plants. As mentioned previously, pests can easily spread viruses around a garden in a matter of hours. Chemical pesticides are a strong method for dealing with them. Alternatively, horticultural oils and insecticide soap deal with pests just as well.

Bigger pests like cutworms and hornworms blend in with the green of your plants, making them hard to spot. A pack of these pests can devastate a crop in one afternoon! They will devour the leaves and fruits on the tomato plants. Keep an eye out for their eggs, droppings, and larvae daily, and remove these pests by hand.

aphids on plant stem

Effects of Overwatering and Underwatering on Tomato Plant Leaves

Tomato plants need a particular balance between wet and dry soil to properly grow. If you water your plant too much you risk drowning it or causing natural root rot. Avoid leaving the soil too wet and muddy, as this will damage your plants quickly.

More likely, yellow leaves are a result of inadequate watering habits. It’s preferable to water tomato plants about once every week, increasing the frequency to every 2-3 days during dry, hot months. Pour the water at the base of the plant, keeping the leaves dry. Water your tomato plants early in the day for the best results.

Lack of Sunshine Causing Yellow Leaves

If you find yellow leaves throughout the plant, your problem may be that the plant is not getting enough sunlight to properly photosynthesize. Try moving your plant to a sunnier location for about a week to see if this improves.

Tomatoes do best in full sunlight, with well-drained soil, so make sure that the plant’s new location gets at least 8 hours of direct sunlight per day. The 8 hours do not have to be consecutive. However, opt for more morning sunlight, if you can, because this will help them get the benefits of direct sunlight without being in the full sun in extreme heat.

Nutritional Deficiencies That Cause Yellowing of the Leaves

Yellow leaves can signal a lack of nutrients in the soil, specifically nitrogen and iron. If this is the case, supplement the soil with manure or nitrogen-rich fertilizer. Before doing so, check the nitrogen level of the soil to be sure that nutrient deficiency is the cause, as adding too much nitrogen to the soil can burn your plants.

watering tomato plants

Should I Remove Yellow Leaves from My Tomato Plant?

If the lower leaves are turning yellow, that means they are no longer carrying out photosynthesis effectively. They can become a sugar drain on the rest of the plant, as the top leaves would have to compensate for the yellow leaves, and this may reduce needed sugars for fruit production. So, it makes sense to remove yellow leaves at the bottom of the plant.

If the rest of the plant looks healthy, simply snap off the yellow leaves. This will improve air circulation to the rest of the tomato plant. Don’t overdo it, though. You don’t want to remove a healthy leaf that may just have a bit of yellowing. But if the leaf is yellow, dry, and brittle, you can safely remove it without worry.

As luck would have it, the yellow leaves on my tomato plants were on the bottom and turned out to be nothing serious. I simply pulled them off, and the tomatoes grew nicely. We enjoyed plump, red tomatoes (and some green ones, too—we are Southern, after all).

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