Yellow Leaves on Tomato Plants? 8 Causes and How to Fix Them

Problem: Too Little Light

Tomato plants require at least six to eight hours of sunlight each day to thrive and produce fruit. Your plant may be showing yellow leaves due to a lack of direct sunlight.

How to Fix It:

If your plant is in a container and has yellowing leaves, move it to a sunnier location. Prune trees and shrubs around garden plots to allow more sunlight to reach the plants.

Problem: Watering Issues

Yellow leaves can appear if you water too much or too little. Watering issues often occur in compacted soil in the garden or a container.

How to Fix It:

Once outdoors, tomato plants will need at least 1 inch of water each week either from rainfall or watering. Watering should be done slowly and deeply so the plants form deep roots. If the soil has become compacted around the root system, aerate the soil. Aeration is easily done with a garden rake and the addition of mulch.

Problem: Lack of Nitrogen

Proper fertilization is a requirement for a healthy plant that produces fruit. Tomatoes require lots of nutrients to be healthy. Yellow leaves are often a sign that the plant is not receiving enough nitrogen. The older, larger leaves will turn yellow or fall off because the younger leaves are using so much nitrogen to survive.

How to Fix It:

Even though tomatoes are heavy feeders, fertilization must be balanced. After planting in well-nourished soil, tomato plants benefit from an application of a balanced 5-10-5 NPK fertilizer a few times throughout the growing season. Avoid fertilizers high in nitrogen (the first number) or you'll have lots of bright green leaves and fewer tomatoes.

Problem: Iron Deficiency

Iron deficiency in a tomato plant is most often characterized by a pale green or yellow color in the youngest, uppermost leaves. The veins in the leaves may appear dark green, but the rest of the leaf is pale. Imbalanced soil pH is often a cause of iron chlorosis. The ideal pH level for tomatoes is between 6.2 and 6.8, but plants may grow in more acidic soils with a pH as low as 5.5.

How to Fix It:

Iron deficiency is most common in sandier soils and soils with a high pH. Conduct a soil sample test, amend the soil with mulch, and improve drainage. Apply a premium fertilizer that contains micronutrients.

Problem: Magnesium Deficiency

A magnesium deficiency in a tomato plant will produce yellowing that looks more like speckles or spots on the older leaves. The element magnesium is required in the chlorophyll molecule, which enables the plant to synthesize sunlight into starches and sugars for food.

How to Fix It:

Look for a fertilizer that contains micronutrients like iron and magnesium. Or, feed the plant with common Epsom salts. Dissolve one-half ounce of the salts in a pint of warm water. Apply in the morning before the temperatures rise by spraying generously from top to bottom, wetting the leaves so that the undersides absorb the feed.

Problem: Tomato Yellow Leaf Curl Virus

Whiteflies may transmit yellow leaf curl virus from nearby infected weeds such as nightshades and jimsonweed. Symptoms may include yellow leaf margins, upward curling of leaves, and stunted leaf growth. Flowers may drop and the plant may become stunted lessening fruit yield.

How to Fix It:

To reduce the whitefly feeding that causes tomato yellow leaf curl virus, use reflective mulches (aluminum or silver-colored) between rows. To repel whiteflies and reduce feeding, mix a solution of 0.25 to 0.5% oil spray (2 to 4 teaspoons of horticultural or canola oil with a few drops of dish soap per gallon of water) and apply weekly. At the end of the growing season, remove plants and bag them for disposal. Consider rotating crops the next growing season.

Problem: Leaf Spot, Leaf Mold, or Early Blight

In the early stages, it is nearly impossible to distinguish between leaf spot, leaf mold, or early blight. Fortunately, they are all treated using the same methods. Most tomato leaf diseases overwinter in the soil and then splash onto the lower leaves of the plant during watering. Most thrive in areas with high summer humidity.

Septoria leaf spot rarely affects the fruit. The infection begins on the bottom leaves and can spread upward if not treated early. Look for brown to black round spots that are the size of a pencil tip or larger that leads to yellowing.

Bacterial spot forms small, irregularly shaped spots with a yellow outer ring and dried-out centers that tear on the leaves and can affect green tomatoes with slightly raised spots on the tomatoes themselves.

Leaf mold affects older leaves near the ground where the air may not circulate as well and humidity levels are high. The first symptoms include yellow or pale green spots on the upper leaves, turning yellow and, under especially humid conditions, covered with gray, velvety spore growth.

The fungus Alternaria linariae causes early blight. The first sign is small, brown lesions on aged foliage. Around the spots, the tissue may become yellow. Early blight can cause much if not all of the foliage to die off, which exposes fruit to sunscald and results in less production.

How to Fix It:

To help control all of these problems, trim and dispose of lower leaves and branches that have become infected. Stake and prune plants to improve air circulation. If the disease becomes severe, use mancozeb fungicide, chlorothalonil, or copper fungicides. At the end of the season, remove all of the plant material and consider rotating crops the next growing season.

Problem: Fusarium Wilt and Verticillium Wilt

Yellow leaves may be a sign of fusarium wilt and verticillium wilt. Fusarium wilt is a warm-weather disease that can occur in seedlings, but the less common verticillium wilt does not affect seedlings and usually occurs later in the season in cooler soils. These diseases are caused by fungi, which can survive for many years in the soil, even if tomatoes are not planted in the same spot every year.

How to Fix It:

Unfortunately, it is not possible to chemically control these fungal wilt diseases. Remove and discard diseased plants (but not in a compost pile where disease could also spread).