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Who doesn’t love to grow tomatoes in the summer vegetable garden? I surely do, but that doesn’t mean tomatoes don’t give me a run for my money each and every year. If you’ve experienced tomato growing failure before, then you may be hesitant to try again.

But I’m here to tell you to keep your head up. There are 101 ways to kill a tomato, but figuring out why the leaves of your tomato plant are yellow can easily be narrowed down. Let’s find out why your tomato leaves may be yellow!

Why are tomato leaves important?

Leaves on any plant growing in your garden are a glimpse into the overall health of your plant. This is the part of the plant most often eaten by pests, where disease begins to show, and where watering inconsistencies are visible. So how can you know exactly why the leaves of your precious tomato plant are turning yellow?

1. Monitor– This is always the first step. If you notice the leaves turning yellow, commit a few extra minutes each day to watching your plant. You’ll gain valuable information this way.
2. Check under the leaves– Pests typically do not like the heat of the day and would rather hang out on the underside of leaves. Check here to see if your yellowing leaves could be linked to a pest problem.
3. Prune– Once you’ve determined the cause for yellowing leaves on your tomato plant, don’t be afraid to prune those limbs or leaves off. Here’s a good tutorial for how to prune tomato plants.

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Yellow Leaf Causes on Tomato Plants

First plant leaves

The first leaves that develop on a tomato plant are called ‘cotyledons’ and are not true leaves. These two first tomato plant leaves will turn yellow and fall off, and this is part of normal seedling development. This will occur after the first set of ‘true’ leaves appear on the plant. So no need to worry about these.

Nitrogen Deficiency

If the plant is being grown in soil that is deficient in nitrogen it can result in the leaves turning yellow and eventually falling off. (Read up on How to Build Up Garden Soil here and learn how to have your soil tested.)

To correct a nitrogen deficiency quickly, feed the plant a general-purpose liquid fertilizer. Don’t apply too much however, because an over dose of nitrogen will result in lots of lush leafy growth and very few tomatoes. And if you’re going to put in all this work… you definitely want lots of tomatoes.

Fungal Dieseases

Early blight or late blight can turn tomato plant leaves yellow. Early blight turns leaves yellow and crates small spots that grow larger and look like a bulls-eye. Early blight is fairly common in the hot days of summer combined with heavy rain or in very humid areas (hello, south eastern gardeners!).

Late blight starts on the upper leaves and creates large, oily-looking lesions on both leaves and stems.

Fusarium wilt causes yellow tomato leaves on one side of the plant, often beginning with the older, bottom leaves. Plant growth is stunted and tomato production ends.

What can you do for fungal diseases in your tomato plants? Treat plants with a fungicide containing chlorothalonil to combat fungal diseases. Prune off thick growth to promote good air circulation.

Viral Diseases

Tobacco Mosaic Virus, Cucumber Mosaic Virus, and tomato yellow leaf curl are three of the most common viral diseases that afflict tomato plants and can cause their leaves to turn yellow. Look for curling of the leaves and/or a yellow pattern to emerge on the leaves themselves. This will give you a good indication that you may be dealing with a viral disease in your tomato plants.

There is little chance of plant survival after it becomes infected with a viral disease. Help prevent these diseases by washing hands before touching tomato plants, water properly, and maintain proper pest control. Here are guides for each of these viral diseases to help diagnose and treat/prevent these viral tomato diseases:

• Tomato Yellow Leaf Curl
• Cucumber Mosaic Virus
• Tobacco Mosaic Virus

Pests on Tomato Plants

Diseases aren’t the only causes of tomato leaves turning yellow. Garden pests can also cause the leaves to suffer and this change will take place in the leaves. Aphids, flea beetles, spider mites, thrips, and whiteflies will turn tomato plant leaves yellow.

This is mainly due to the sucking nature of most of these pests. They feed on the plant leaves with their sucking mouth parts, and this robs the plant of nutrients.

Be diligent in pest control methods to prevent pest infestation and keep plants healthy. Remember, pests and disease are usually drawn to unhealthy plants, so prevention is the always the best pest control. Here are some helpful articles for keeping pests away from your garden using natural methods:

• How to Stop Aphids
• Natural Pest Control Methods
• Using Neem Oil in the Garden

Tomato Plant Watering Issues

Too much or too little water can cause yellow tomato leaves (as well as other plants with yellow leaves). Water in the early morning and soak soil thoroughly and carefully at the base of the plant.

This keeps the leaves as dry as possible, which protects against disease. Water once every 3-5 days, allowing the soil to dry out between watering. Tomato plants do enjoy a deep watering, as opposed to a light watering daily.

End of the Season Yellow Leaves

Finally, the leaves on tomato plants will begin to yellow as the season comes to an end. If your tomato plant has had a good run, then chances are, it’s just finished and yellow, dying leaves are completely normal. You can prune away yellow leaves and limbs as they begin to die off and try to harvest those last few tomatoes.

Tomatoes are both a joy and occasional frustration in the home vegetable garden, but if you arm yourself with some plant knowledge, then figuring out why your tomato plant’s leaves are turning yellow will become easier each year. And this hopefully leads to fewer and fewer plants afflicted in this way. If you have other suggestions about yellow tomato leaves, please leave a comment below!

We’re always learning here at The Kitchen Garten and helpful gardening tips are always welcome!