8 Reasons Your Tomato Leaves Are Turning Yellow, and How to Fix it
Are your tomato's leaves turning yellow, and you aren't quite sure why? There are actually many different reasons this can happen! In this article, homesteader and garden expert Meredith Cohrs examines the many reasons why your tomato plant's leaves are turning yellow, and how you can fix it!
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Tomatoes are an iconic home garden vegetable. They’re beautiful, delicious, and fairly easy to grow. Seeing our prized tomato plants in distress is always upsetting. Any time your previously healthy tomato plant looks off – leaf curl, yellow leaves, spotting – we tend to overreact, scour the internet for solutions, and take immediate action.
Before you panic, however, know that yellowing leaves are incredibly common on tomato plants and often represent a problem that is easy to fix. With a little TLC, in most circumstances, your tomato plants should recover in time if you apply the proper remedies.
So, what is it that actually can cause yellow tomato leaves? There are eight common reasons for yellowing tomato leaves, and we plan to cover each in detail. We’ll help you identify the problem, implement the fix, so your tomatoes can get back to their happy selves in no time!
Seed Leaves Are Turning Yellow
I’m including this on our list because it’s the first time you’re likely to see yellowing leaves on your tomato plant. The great news is that this is a completely natural part of your plant’s development so it’s not a cause for worry.
Cotyledon leaves are the first leaves a seedling develops after it germinates. These are designed for one thing – supplying the seedling the nutrition it needs to grow. Once that job is done, those leaves are no longer necessary to the plant.
At that point they will begin to yellow and drop off. By the time this happens, your tomato seedlings should have plenty of true leaves to continue providing nutrients to the growing plant.
How To Fix:
No fix! This is a completely natural part of the tomato plant’s life cycle. As your tomatoes grow, the leaves will naturally drop and new leaves will grow in their place.
Transplant Shock
If you notice yellowing leaves in a recently transplanted tomato, the cause is likely transplant shock. Tomatoes have fairly delicate roots and they can be easily damaged when moved into a new location.
A lot of new gardeners think they are supposed to ‘break up the roots’ to encourage vigorous growth. This doesn’t work for tomatoes. Even the slightest break or nick in the root ball can result in root damage. You’ll notice wilting, leaf curl and – of course – yellowing leaves if this has happened.
The good news is that mild transplant shock is pretty harmless to the plant and it should recover with a little patience. Make sure that you don’t introduce any other form of stress – pruning, poor watering habits, etc – during this time or your plant may not recover.
How To Fix:
Be gentle when transplanting. Tomato roots are very sensitive so be as gentle as you can when transplanting. Give the nursery pot a light squeeze to loosen the soil and then carefully extract the plant by pulling on the lower stem. Don’t touch the roots. Instead, place the entire root ball and lower part of the stem under the soil line. Gently fill in with soil, press down with your fingers, and water thoroughly.
Be patient. If you notice that your young tomato has already had transplant shock, baby it a little and allow the plant time to sort itself out. Avoid introducing any additional forms of stress to your plant until it recovers.
Over or Under Watering
Incorrect watering is probably the most common reason for yellowing leaves, and problems with your tomato plants in general. Even though tomatoes are considered to be fairly easy to grow, they’re pretty fussy when it comes to getting the right amount of water.
Too little and the plant won’t thrive; too much and you’ll damage the roots, which causes the plant to not thrive. You get the idea… When it comes to watering, we truly are looking for a ‘goldilocks’ situation.
A general baseline is about 2” of water a week for a plant in the ground (more if your tomatoes are planted in containers). But environmental factors like wind, heat, humidity, and soil type can all play a role in adjusting that number up or down.
You can use a water gauge if you want to be exact. Or, consider planting an ‘indicator plant’ nearby. Impatiens make a great indicator plant – especially if they are well shaded by your growing tomatoes – since they will immediately wilt when they have too little water. If your impatiens need water, your tomatoes likely do as well!
Overwatering is far more common with tomatoes (I have definitely been guilty of this). We always want our tomatoes to be happy, but too much water can actually suffocate the roots and cause them to rot. Root rot can cause a host of issues and one of the first signs of this is yellowing leaves that drop off the plant.
Underwatering can also cause the leaves to turn yellow after they start to wilt. You should notice the wilting long before the color changes. But, if underwatering is the problem, you’ll see that leaves begin to yellow from the edges before they drop off the plant.
Let’s look at how to water your tomato plants correctly to avoid this problem altogether.
How To Fix:
Water slowly and deeply. Most plants don’t like a deluge of water and tomatoes are no different. Think about the difference between taking a nice sip of water from a glass vs someone opening a fire hydrant for you. In an ideal situation, you can set up a drip system before planting. This will deliver water to your plants in a consistent and manageable way. If you don’t have a drip installed, you can simply turn your hose on the lowest setting and just let it slowly stream into the soil.
Water regularly. There isn’t really a hard and fast rule to how often you should water your tomatoes. A lot depends on if you are growing in a hot, dry climate or if you’ve received rain recently and have cooler weather. It also depends on where the plant is in its growth cycle. A good rule of thumb is to water every 2-3 days during the summer months (more frequently in containers). Once the fruit is set, you can pare this back to 1-2 times per week.
Water the roots, not the leaves. A common mistake with new gardeners is to water the top of the plant rather than the soil underneath. This can actually cause a lot of problems with your tomatoes including the spread of fungal disease (caused from wet soil splashing on the underside of the lower leaves), leaf burn, and attracting pests. Always aim to slowly water the soil either with a drip system or low flow hose. Add mulch to the base of your plant to maximize water retention, keep your soil at a consistent temperature, and minimize water splash back.
Remember to always water your tomatoes in the morning and not during the heat of the day!
Soil Compaction (Poor Aeration)
Soil compaction causes a lack of oxygen in the soil. When this happens, the roots of your tomato plants begin to suffocate and they cannot transport the essentials – oxygen, water, and nutrients – to the rest of the plant. The first sign of a problem will be yellowing leaves. If the condition persists, your plant will surely die.
Soil compaction can happen in a few ways. The first is that the soil around and underneath your plants hasn’t been loosened or aerated. This creates exceptionally dense soil that can be unaccommodating to young root systems. Compaction can also be caused by stepping on the soil near your plants.
How To Fix:
The best fix for soil compaction is prevention. Start your tomato plants in good soil enriched with organic matter prior to planting. When transplanting, dig a hole 2-3 times the size of the tomato root ball and then fill it in with good soil. If you are planting seeds directly in the ground (and not transplanting), ensure you have amended and aerated the soil below and around the planting site.
Introduce worms into your soil. Worms are nature’s best aerators. If you have a healthy garden biome, worms will naturally be a part of it.
Currently dealing with soil compaction? You can try to aerate the soil by using aeration spikes or your hands. This action is likely to damage the root system, so be very cautious. If you’re concerned about people or pets stepping on the soil around your plants, consider planting them in raised beds or containers. This will remove an entire cause of soil compaction with very little effort.
Disease
There are a number of diseases that can cause your tomato leaves to yellow. We’ll talk about the 4 you are most likely to experience in this section. For any soil-borne fungal disease, be prepared to completely remove the affected soil prior to planting again. If you skip this step, your tomatoes will continue to be plagued by the same issues season after season.
Remember, when it comes to diseases, prevention is, by far, the most effective treatment. For proper prevention, follow the following steps:
- Maintain a soil pH of around 6.5.
- This is ideal for your slightly acidic tomatoes.
- It will help limit many funguses from becoming established.
- Mulch around your tomato plants.
- This will help prevent soil splash back.
- Soil splash back is a common way that soil-based fungal diseases transfer.
- Prune your tomato plants so that there are no leaves or branches touching the soil.
- Just like with soil splash back, this will help prevent easy transfer of soil-based diseases.
Blight
Early blight is caused by a fungus in the soil called Alternaria solani, so if you expect this is your problem, you’ll need to replace the contaminated soil before your next planting. Luckly, blight is easy to identify.
Pale yellow spots will appear on the lowest leaves of your plant, turning into a dark brown patch with yellow at the edges. It looks a bit like a bull’s eye. Leaves will eventually turn 100% yellow and fall off. This tomato disease moves up the plant from the soil.
How To Fix:
Early blight can be managed if discovered early. Remove the affected leaves and stems and throw them away (do not compost). Apply an organic fungicide designed to treat the disease. Follow all directions on the bottle until the problem is resolved.
Septoria Leaf Spot
Septoria leaf spot is another fungal disease that looks a bit like blight in the early days. Caused by the fungus Septoria lycopersici, it is most common in areas that have extended wet periods or generally humid weather.
Brown spots will appear on the lower leaves and will eventually spread to the stems. As it gets worse, the disease causes spots to grow into large brown areas on the leaves. When left unchecked, leaves will turn completely yellow, then brown, then fall off and die.
How To Fix:
Septoria Leaf Spot can also be managed if discovered early. Similar to blight, you’ll remove the affected leaves and stems and throw them away. Again, do not compost. Apply an organic fungicide designed to treat the disease. Follow all directions on the bottle until the problem is resolved.
Fusarium Wilt
Caused by Fusarium oxysporum, Fusarium Wilt is another disease that begins in the soil. It infects the tomato plant’s roots, preventing the transportation of water and essential nutrients to the rest of the plant.
The disease is very problematic, but rarely kills the host. In addition to the yellowing leaves, you will notice a failure to thrive, poor growth, general wiltyness, and poor (if any) fruit production.
Because many gardeners can’t easily identify fusarium wilt, they continue trying to revive the plant with no success. Unfortunately there is no cure for this disease.
How to Fix:
If you encounter either fusarium wilt, there is nothing you can do to save the plant. Cut your losses and remove the plant immediately. There is no cure, and the disease can spread to other plants in your garden if left unchecked. Be careful not to let your affected plants touch others in your garden as you remove them. Do not compost.
Verticillium Wilt
Caused from a soil-borne fungus called Verticillium alboatrum, Verticillium wilt is most commonly seen in cooler Northeast gardens. Like early blight and Septoria leaf spot, yellow patches start to show up on the lower leaves and progress to brown spots, and curled dead leaves.
How to Fix:
Similar to fusarium wilt, there is nothing you can do for verticillium wilt. Cut your losses and remove the plant immediately. There is no cure, and the disease can spread to other plants in your garden if left unchecked. Do not let the infected plant touch others, and do not compost.
Nutrient Deficiency
A healthy tomato plant requires macronutrients like nitrogen, phosphorus, and potassium, and micronutrients like magnesium, calcium, and zinc. Sometimes, yellowing leaves are early indicators that your plant is deficient in one of these. Other times, it’s an early warning that the tomato roots aren’t bringing enough nutrients to the rest of the plant.
The most common nutrient deficiency that can cause tomato leaves to yellow is a lack of nitrogen. Nitrogen is a macronutrient critical to foliage growth. The plant needs different levels of nitrogen at different phases of its growth cycle, but it’s always necessary.
When there is a nitrogen deficiency in the soil, you’ll notice that the lower leaves begin to yellow while the new leaves at the top of the plant remain bright green. You’ll also notice that the growth of the plant will stall or stop completely.
A lack of certain micronutrients – magnesium, iron, sulfur, or zinc – can cause a condition called chlorosis. This is when the plant is unable to produce chlorophyll. A lack of these micronutrients (magnesium is often the culprit) cause very specific yellowing of the leaves.
When this occurs, the veins of the leaves remain green, but the rest turns light yellow. This mottled appearance is a telltale sign that you need to introduce a new source of micronutrients to your soil.
We’ve mentioned root rot before. It can be another cause of nutrient deficiency in your tomatoes. If you suspect this, let the soil around your tomatoes completely dry out. You can also gently dig around the roots of your plant to observe their health.
If root rot has progressed to a level where nutrients aren’t being transferred to your plant, your best bet is to pull it and start again.
How to Fix:
Fertilizing throughout the growing season is the best defense against nutrient deficiency. Tomatoes are heavy feeders (meaning they are water and nutrient-hungry) and they need to be fed pretty regularly for optimal growth and fruit production. Tomatoes should be fertilized when you initially plant in your garden. Once they begin setting fruit, start fertilizing again, aiming for once every 2 weeks until fruit production stops.
Test your soil to see what it is deficient in. A home soil test kit can be invaluable when dealing with a nutrient imbalance. Rather than guessing and hoping for the best, you can receive information that allows you to target the problem with confidence. Most of the time, fertilizing regularly will prevent any imbalances, but occasionally you may be deficient in just one nutrient. For instance, if your soil is deficient in magnesium, you can add epsom salts. Powdered egg shells can help with calcium deficiency.
Pests
Ah pests. They can cause so much havoc in our gardens, but sadly, they are something gardeners have to deal with season after season. Tomato plants attract a host of pests whose feasting can cause tomato leaves to yellow.
Aphids, thrips, hornworms, cutworms, spider mites, flea beetles, and whiteflies can all cause tomato leaves to visibly show signs of distress. This is most often caused because the pests feed on the sap contained within the leaves.
Unlike yellowing from disease, this type of yellowing will be localized around the area of the infestation (rather than starting from the bottom of the plant and moving up like disease).
How To Fix it:
As with most of the causes of leaf yellowing in this article, the key to avoiding a pest infestation is prevention. Creating an environment of biodiversity will encourage predatory insects like ladybugs, lacewings, parasitic wasps, and spiders to take up residence in your garden. They love to feast on the bugs that love to feast on your tomatoes! Invite them in and let them do the heavy lifting.
Companion planting your tomatoes can also be helpful in the war against pests. Plants like nasturtiums, marigolds, basil, and chives can help deter some pests and ‘trap’ others. Aphids, for example, will collect and feed on mature nasturtiums before ever noticing your young tomato plants. If you choose to give companion planting a try this year, make sure you plant these herbs and flowers several weeks ahead of your tomatoes. They need to be large enough to make enough of a difference.
Manually removing pests is another effective – but time consuming – way of overcoming an infestation. Pests like aphids and whiteflies are pretty easily knocked off stems and the underside of leaves with a hard stream of water. Spider mites are small enough that you can simply remove infested leaves if you catch the problem early enough. The key is to be in your garden daily so you can notice a problem before it becomes an emergency.
It’s the End of the Season
Just like with our first section on yellowing cotyledon leaves, it’s 100% natural for your tomato leaves to yellow and fall off at the end of the season. Remember, leaves exist to help feed the plant. Once fruit has set, that job is done.
So at the end of the summer when you see entire branches of leaves turning yellow or brown, know that it is an expected part of the tomato plant life cycle. Simply prune off those dead leaves or stems and allow the plant to send all its remaining energy to ripening the last of its fruit.
How To Fix it:
No fix! This is another normal part of the tomato life cycle and should be celebrated. Your plant did its job!
Final Thoughts
If you see your tomato leaves yellowing, my recommendation is try not to panic. As you’ve seen in this article, most reasons are fairly benign and are easily fixed. You’re now well equipped to identify the problem.
Pair this information with your own watering and fertilizing habits and use the process of elimination. No matter what issue you think is responsible, take it slow. Try to remedy the situation and allow the plant to respond. If that doesn’t work, you can move onto your second guess. The good thing is that the more serious stuff – disease and pest infestation – is very identifiable.
