Why Are My Tomato Leaves Turning Yellow? Common Problems & Fixes

A yellow tomato leaf isn’t a cause for panic, but a sign that you need to take a closer look at your plant. Yellow leaves are the symptom, not the disease. Start by looking at which leaves are turning yellow, where the yellowing appears on the leaf, and whether you see spots, holes, wilting, or insects. 

These common issues are often the cause of yellow tomato leaves. Use this guide to help identify and treat your plants.

Both too much and too little water can result in yellow tomato leaves.

When there is insufficient water, the leaves will droop and then turn yellow if the drought continues. Tomatoes need regular and deep watering. But don’t keep the soil too wet.

Overwatering can also result in yellow leaves. If the soil stays too wet, the roots will be prevented from taking up nutrients. Heavy clay soil will hold onto moisture and prevent oxygen from being available. If your soil has too much clay, amend it with compost.

Yellow tomato leaves can be a sign of plant disease. Each disease displays a unique pattern on the leaves, which can help determine what disease may be affecting your tomato plants.

When leaves turn yellow due to fungal diseases, you will also see small dark spots, usually with concentric rings. When fungal diseases are the culprit of yellow leaves, taking action quickly to save your tomato plants is crucial.

Powdery mildew starts out as white spots on the leaves, and as it progresses, it kills the plant tissue, and the leaves turn yellow. Powdery mildew is quite common in many climates, and there are helpful tips for prevention and treatment.

Verticillium and Fusarium wilt diseases are both caused by pathogens in the soil. When tomato leaves turn yellow from wilt, you will first notice that, even with consistent watering, the plants droop. 

A note for next season: If these pathogens are present in your soil, consider planting yellow mustard as a cover crop. Yellow mustard has chemical properties that can help kill the wilt pathogens.

Tomatoes are considered “heavy feeders” and need a sufficient amount of nutrients to stay healthy. Nutrient deficiencies result in yellow leaves, stunted growth, and less fruit production. Getting the right nutrient mix will help your plants thrive. 

When diagnosing the cause of yellow tomato leaves, first check whether older leaves are turning yellow or new leaves. This piece of information is important in diagnosing nutrient deficiencies.

Insufficient nitrogen is the most common issue for yellow leaves. When there’s not enough nitrogen, the older leaves turn yellow first. Following a healthy fertilizing schedule for tomato plants will help ensure the right amount of nitrogen at crucial growth times.

An iron deficiency is called iron chlorosis. One sign is that the new leaves are turning yellow. The leaf veins will stay green, but the tissue between veins will turn yellow. Like many nutrient deficiencies, there may be plenty of iron in the soil; it just isn’t getting to the plant because of the soil pH. Tomatoes grow best in acidic soil. If the pH is too high, the plant can’t absorb iron. A soil test will determine if there is enough iron, and a pH test will help determine if you need to add amendments to change the pH.

Magnesium is required for chlorophyll formation, which gives leaves their green color. Similar to iron deficiency, magnesium deficiency in tomato plants results in green veins and yellow leaf tissue. The difference is that magnesium deficiency affects the older leaves first. The interveinal chlorosis pattern can look like spots at first. Magnesium deficiency is more common in sandy soil.

Check tomato plants carefully for the presence of insects. Pests are often a cause of yellow tomato leaves. Many pests can be small and hard to see. A helpful method to identify pests is to hold a white sheet of paper up to the plant and shake the leaves.

Any pests will fall onto the paper, where you can see them more closely. A sticky trap can also catch roaming pests to help you see what’s munching in your garden.

Pests suck on the leaves, feasting on the juices, and often spread disease. When pests attack tomato plants, they damage plant cells, preventing the leaves from photosynthesizing. Identifying the specific pest can help you target your defense.

Aphid damage on tomato plants can be difficult to see until it is advanced. You may first notice sticky, shiny leaves. Aphids don’t often destroy a whole tomato plant but can affect the leaves. Encouraging beneficial predator insects is one natural way to get rid of aphids.

Damage caused by adult flea beetles leaves a scattering of various-shaped holes on the leaves. The damage is usually not fatal on mature plants, and the beetles can be controlled with natural methods. As with other insect damage, flea beetles can spread disease.

Thrips are hard to see, but their damage is easy to spot. As these minute insects suck out the green chlorophyll, the leaves will look yellow. Thrip damage to young seedlings is especially harmful. Waiting to plant out larger seedlings and using companion planting can help combat thrips in your garden.

Spider mites on tomato leaves will suck out the chlorophyll, lay eggs, and cover the plants in webbing. This can happen quickly since these insects have a rapid reproduction rate. Spraying tomato plants with water can wash off the spider mites and their eggs.

Cold temperatures can damage tomato plants and turn the leaves yellow in the early spring and fall.

Transplant shock can also result in yellow leaves.  If a newly transplanted tomato seedling has yellow leaves, it could be a sign of transplant shock. With proper care, the plant will recover. On an older tomato plant, yellow leaves can indicate the end of its life cycle.

1 - Check the soil moisture

   Stick your finger several inches into the soil. Soil should be moist but not overly wet or dry.

2 - Look at where the yellowing starts

   Older leaves or new leaves?

3 - Examine the pattern

   Entire leaf? Between veins? Edges? Spots?

4 - Check underneath the leaves

   Small pests like aphids and mites are hard to see.

5 - Check the stem and entire plant

   Look for wilting, lesions, discoloration, or one-sided symptoms.

6 - Remove damaged leaves

   Discard diseased leaves in the garbage.

7 - Correct the underlying problem

   Remember, yellow leaves are a symptom, not the disease.