Why Your Tomato Plant Leaves Are Turning Yellow—and How to Fix It, According to Experts

Whether you enjoy your tomatoes fresh in your favorite dish or have plans to preserve them for later consumption, a healthy plant is essential for producing top-notch produce. So what happens when you notice yellow leaves—and more importantly, what do you do to correct it?  While it might take a bit of sleuthing to uncover the exact cause, our experts are here to offer guidance to save your crop. 

Why Do Tomato Leaves Turn Yellow? 

Unfortunately, there isn’t one definitive cause when it comes to yellow leaves on your tomato plant. You’ll need to be a bit of a detective to uncover what’s causing your specific problem, but here are some of the most widespread culprits.

Overwatering

It might seem basic, but one of the most common reasons your tomato leaves are yellow may be due to overwatering. Tomato plants require just the right amount of moisture—too much can be as equally destructive as too little. 

Lack of Nitrogen

The primary thing that causes tomato leaves to turn yellow is a lack of nitrogen, according to Matthew Geldin, head farmer for Farmscape, California's largest urban gardening company. "This is especially true if the bottom leaves turn yellow first, and the yellowing moves up from the bottom of the plant," Geldin says. "Nitrogen is a mobile nutrient, meaning that when a plant needs more nitrogen to keep growing it can actually steal nitrogen from itself that is stored in older leaves if it can't find sufficient nitrogen in the soil."

Other Deficiencies

"There are other nutrient deficiencies that can cause yellow leaves," says Geldin. "Magnesium deficiency can also affect lower leaves, while iron and potassium will show on new growth."

Unwanted Visitors

Whether it’s aphids, spider mites, whiteflies, or other common culprits, pests can quickly lead to yellow leaves if not dealt with promptly. 

Infection

While yellowing that begins at the bottom of the plant can indicate a nutrient deficiency. when yellowing starts traveling down from the top of the plant or is associated with wilting symptoms or dark spotting, there is likely a disease infection, says Geldin. 

Whether it’s a fungal or bacterial infection or a less likely viral infection, you’ll typically see other physical signs along with yellow leaves. 

Lack of Sunlight

Your tomato plants require at least six hours of direct sunlight each day. So if they’re getting less and you see yellowing leaves, this might be the first issue you address. 

Transplant Shock

You might see a few yellow leaves if you’ve recently transplanted young plants into your garden, but this is the one instance where there’s no cause for alarm. Transitioning from a humid greenhouse to an outdoor garden might shock young plants just a bit, but they should recover fairly quickly. 

How to Fix Your Yellowing Leaves 

Once you’ve discovered the cause of your discoloration, it’s time to implement a plan of action. 

Don’t Overwater

The right amount of water depends on your growing area and soil type, but a good rule of thumb is to soak the soil every five to seven days. You never want the soil to remain soggy, however, and some suggest watering the base of the plant only—let the leaves remain dry. 

Conduct a Soil Test

If you believe the cause to be a nutrient deficiency, it’s imperative to submit a soil test to a local company. If you start applying treatments, "this can lead to tie-ups in nutrients in the soil if not properly tested first," says Warner. 

Geldin recommends sending samples to a private or university lab or checking with your county extension to assist in finding a lab in your area. Plus, many labs will provide fertilizer application recommendations based on your results, he says.

Feed Your Plants

If your soil test comes back with any noted deficiencies, Geldin recommends liquid fertilizers as the fastest-acting option. "It's best to apply liquid fertilizers early or late in the day with a pump sprayer or trigger sprayer when they will be best able to be absorbed by the plant's leaves," he says. "Be sure to coat the tops and bottoms of the leaves all over the plant."

Soil drenching is also an option. "Put the liquid concentrate in a watering can, fill it with water, and apply it to the soil," Geldin says.

Get Rid of Pests

You might need the aid of insecticidal soap or other options to get rid of unwanted pests on your tomato plants. Planting some bug-repelling companions near your tomatoes can also serve as a natural deterrent. Herbs such as basil and mint, along with flowers such as marigolds and nasturtium, repel some of your tomato plants’ greatest insect enemies. 

Stop the Spread

If you suspect disease as the cause of your plant’s yellowing leaves, and depending on what type of infection, it’s best to immediately remove the infected foliage to stop the spread—or, in some cases, the entire plant. 

Reposition Your Plants

If you’re growing tomato plants in a large container on a porch or patio, you may need to ensure it’s positioned to receive ample sunlight throughout the majority of the day. If nearby plants in your garden are shading your tomatoes, it might be time to make some tough decisions. 

How to Prevent Yellowing Leaves 

Circumventing problem areas is key to ensuring healthy plants. Here are a few things you can do to prevent yellow leaves on your tomato plants.

Conduct Annual Soil Testing

Geldin suggests investing in annual soil testing to see if you have any nutrient deficiencies or pH imbalances before planting. Depending on the results, he then prefers to use a dry, balanced organic fertilizer made from a mix of organic nutrient sources. 

"Different sources will break down at different times, providing a longer window of feeding," Geldin says. "You can mix the fertilizer into your soil before planting, and it's also nice to put an extra little handful into the bottom of the planting hole on heavier feeding crops like tomatoes."

Buy Healthy Seeds and Plants

Be sure to purchase your plants or seeds from a reputable nursery to ensure you’re getting healthy, viable plants. Pick a spot where they’ll receive ample sunlight and not be overshadowed by their garden neighbors. Consider companion plants that might help with pest control or research healthy ways to control these unwanted visitors.

Stake Your Tomatoes

You must stake the plants and prune the bottoms to keep foliage off the ground—this is key to preventing infection and infestation, as both start at the ground level. 

Measure the Soil Moisture

To avoid overwatering, Warner suggests using a probe or meter to measure soil moisture until you understand your plant’s needs. "This will measure how much moisture is in the soil and can easily help identify if there is overwatering happening," she says. "If so, hold off on watering until the plants dry out a bit, water less, and get comfortable with estimating water needs."

Feed Throughout the Growing Season

Finally, provide nutrition throughout the growing season for the best possible output, says Geldin. "When applying a dry fertilizer, be sure to scratch it into the soil so it is at least partially incorporated into your existing soil and doesn't get clumpy on top," he says. "We've bred incredibly delicious and productive vegetables over many generations, and these hungry plants require a lot of nutrition to grow and provide over the course of a long season."