Master Gardeners: Why are my tomato leaves turning yellow?

Published 12:34 am Saturday, May 22, 2021

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(Editor’s note: This is part one of a two-part series)

By Laura Muntean

Texas A&M AgriLife Extension Service from the weekly online newsletter Texas Gardener’s Seeds

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.”

Next week in Part 2 of 2, we will discuss diseases of the tomato plants and other causes of yellowing leaves. If you have any horticulture question, please contact our Hot-Line 409 882-7010 Tuesday and Thursdays from 10 a.m. – 2 p.m. or thru our Facebook page Orange County Texas Master Gardeners.