Why are my tomato leaves turning yellow?

Brownwood Bulletin
Scott Anderson

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.

Yellowing leaves on tomato plants can be caused by multiple issues.

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.

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.

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.

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

Leaf symptoms of early blight are large irregular patches of black, necrotic tissue surrounded by larger yellow areas.

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.

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.

Powdery mildew is first noticed on older leaves as a yellow spotted appearance, that upon closer inspection has a whitish-gray powder on the surface.

Most fungal and bacterial diseases cause some kind of yellowing.

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.

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.

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.

Brush piles can be beneficial for wildlife

When clearing brush from land, allowing small brush piles to remain in place for a short period can create optimal wildlife habitat.

The brush piles provide temporary habitat for a variety of birds and small mammal species.

Many landowners are looking to integrate wildlife management into their operations, and brush piles can be a very effective way of doing so, specifically for quail and other upland birds. We’ve hosted a hands-on brush pile-building and burning demonstration to get people out in the field, show them different ways to build them and how to burn them safely at the end of their use.

Smaller brush piles about 10 feet in diameter and 6 feet tall are the ideal size for this purpose.

It is recommended starting with larger tree trunks and limbs stacked in a “log cabin” pattern, then topped with leafy boughs and remaining small limbs.

The brush piles can act as “rest stops along the highway” as birds move from place to place.

For optimal quail habitat, he recommends spacing brush piles about 200 feet apart.

You want to connect areas that they are foraging in and moving through. Space those smaller piles about 150-200 feet apart to help them connect the dots as they move from point A to point B.

Since the goal of clearing land and creating brush piles is to get rid of the brush by burning, Brooke reminded landowners not to place the piles too close to fences, under tree canopies or near diesel or fuel storage areas.

Deciding when to burn the piles depends on several factors, he noted.

Over time, those brush piles are going to deteriorate, and that’s when you’ll start looking at burning them. Typically [when you burn brush], you won’t see huge exoduses of animals because even though there might be some animals living in it, that doesn’t necessarily mean they’re in there year-round. It kind of comes down to the species of concern that you’re trying to manage for. Looking at birds, for example, if they’re nesting at a certain time of year, maybe wait until after that nesting period is over. The good thing is that most critters are well adapted. Once they notice fire is around, they get the memo, and they evacuate pretty quickly.

Safety should always be the priority when burning brush. Weather conditions, wind patterns and county burn bans should be considered when deciding whether or not to burn a pile, but especially when burning several brush piles at once.

There are also regulations that must be followed, Brooke noted. Burning at night is prohibited and burning brush with winds less than six miles per hour (mph) or greater than 23 mph is likewise prohibited under Texas laws.

A little commonsense goes a long way in preventing out-of-control brush fires.

Allowing brush piles to remain in place for a short duration before burning them can help wildlife flourish.

Don’t burn a pile without a way to put fires out. Have a small sprayer on hand or some shovels, just some way to help address anything that happens. Because accidents happen. Maybe you light the brush pile, and it’s creeping along the grass around the mowed firebreak you put around it. If you have a shovel, you can walk over and put it out real quick or squirt a little water on it.