Why are my tomato leaves turning yellow?

As a tomato plant grows, it is often thought that it is in the plant’s nature for the lower leaves to turn yellow and die off. However, that is simply not true according to Joe Masabni, Ph.D., Texas A&M AgriLife Extension Service vegetable specialist in Dallas.

Masabni, an assistant professor in the Department of Horticulture in Texas A&M University’s College of Agriculture and Life Sciences, explains that a healthy plant that is well maintained and not stressed by disease or nutrition should have green leaves from the bottom to the top.

Typically, yellowing leaves are a result of a nutritional imbalance or disease outbreak, but other causes can play a part.

Nutrition can be a cause for yellowing leaves on tomato plants

“Nitrogen is the most common cause, because people generally don’t fertilize tomatoes enough,” Masabni said.

Tomatoes are heavy feeders, meaning the plant requires twice the amount of fertilizer that a cucumber needs, and even four times the amount as beans, he explained.

If you don’t fertilize enough with nitrogen, the older leaves will begin turning yellow and, in many cases, may fall off. The older leaves turn yellow because they are providing their nitrogen to the younger leaves to survive.

Yellowing of leaves can also be the result of an iron deficiency in the plant, but this will be most prominent in the youngest leaves. A magnesium deficiency however will produce yellowing that looks more like speckles or spots on the older leaves.

“Those three – nitrogen, iron and magnesium – are the most common nutritional deficiencies growers should pay attention to and fertilize regularly for,” Masabni said.

It is good to keep in mind, that with the use of a lot of fertilizer, the plant will also require a lot of water.

“There is no perfect recipe for how much water your tomato may need, but a good rule of thumb is to do a moisture test where you place a finger several inches deep in the soil to test for moisture near the roots,” he said. “If it feels dry, it’s time to water, and as the tomato plants get closer to full maturity, they will require more and more water. Better yet, buy a soil moisture meter and use it regularly as a guide on when to water.”

Diseases may present with yellow leaves

Texas is a prime location for fungal diseases in tomatoes, simply due to the heat and humidity that are common in the state. Because these conditions are ideal for spreading diseases, Masabni suggests using a fungicide protectant on a regular basis, once every seven to 10 days, and up to 14 days in a dry year.

“Spray on a schedule whether you think you need it or not,” he said.

Fungicides are typically used as protective and not as a curative measure for fungus. So, this is a proactive approach that gardeners will want to start before seeing signs of disease to protect the plants from developing one. Once you can see the disease, it is often too late.

Most fungal and bacterial diseases cause some kind of yellowing, he explained.

The most common fungal disease seen in Texas is powdery mildew or early blight, which starts from the bottom of the plant and moves up as the leaves die off.

Physiological disorders can produce yellowing of the leaf

Salt damage – not just table salt or sodium chloride, but any excess mineral – can result in yellowing.

If you are growing tomatoes in a container and your water contains a heavy amount of salt, once in a while water the container until it leaches out, so the salt can run through the soil and flush out of the container. This will help in preventing buildup of those salts within the container itself.

Use caution with herbicides

“Gardeners should avoid Roundup near the vegetable garden because tomatoes are super sensitive to Roundup,” Masabni said.

Roundup injury to tomatoes creates a bleaching effect from the inside to the outside of the leaf and affects the newest growth of the plant such as the youngest leaves and shoots.

Vegetable problem solver and maintenance

On the Aggie Horticulture website, the vegetable resources link provides a vegetable problem solver where you can look at different common problems you may encounter in Texas.

“The bottom line—any form of yellowing is not good,” he said.

Even if you don’t know the cause, remove any yellow leaf and throw it away in case it is diseased so it will not spread and infect others. Remove that leaf, spray a fungicide and hopefully the problem will be resolved by early diagnosis. When removing leaves, be sure to remove them with a clean hand and properly dispose of the leaf. Wash your hands thoroughly before you continue working on other healthy plants to avoid spreading any disease between plants.

Also, ask yourself if you have been fertilizing regularly. Does the plant look tall enough or is it the same height as a month ago, which may mean you need more fertilizer?

Placing a fertilizer solution on the end of your hose and washing off your plant from top to bottom on occasion will also simulate a rainfall situation, he explained. This will be especially helpful in a dry year, when mites may become a bigger issue. Washing the plant with water will wash off the mites, and clean and cool the plant, all while fertilizing it.

For more information on vegetables and gardening resources, visit the Aggie Horticulture website.