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Why tomato leaves turn yellow

Radek Štěpán 13. 06. 2025 1 views

There are many reasons why leaves may turn yellowish. One cause is a lack of light. So, if we talk about tomatoes make sure your tomato plants get enough sunlight. Another cause is a severe lack of water, but also too much water. Too much water followed by dry soil creates a compacted soil. If you have that, you need to loosen the soil.

There are other reasons and we shall discuss them below. These include viruses, fungi or bacteria but also an imbalance between nitrogen, minerals and alkalinity. If old leaves are turning yellow it means that soil is lacking nitrogen, but if young leaves are turning yellow, it means that your soil is lacking sulphur. If you notice yellowing between leave veins then your soil is probably lacking potassium. And finally, yellowing around edges indicates low magnesium content. Below we discuss various diseases and how to fight them.

Iron chlorosis

Iron chlorosis is caused by a variety of diseases, nutrient deficiencies, insects or mites, and even by environmental issues, such as too much water. An unbalanced soil pH is another culprit. The ideal pH range for tomatoes is from 6.2 to 6.8, but slightly acidic environment should not be a problem.

Tomato or tobacco mosaic virus

This virus is tricky because it “only” reduces the number of fruits and it rarely kills the plant. Common symptoms include leaf curling or green or yellow spots on leaves. Besides tomato plants, this virus also affects other plants and it is pretty common in ornamental gardens because it is spread by insects, plant remains and even by contaminated tools. If you notice it during one season, remove infected plants and do not plant in the same location in the following year. Plant tomatoes at a different location.

Photo: Radek Štěpán

Wilting

Yellow spots often signal a disease called fusarium wilt or verticillium wilt. Fusarium wilt affects seedlings. Verticillium wilt is less common and does not affect seedlings but it appears much later – when the colder weather comes. Both of these diseases have one thing in common, and that is that they are caused by fungi that can survive in soil for many years, even if you plant tomatoes elsewhere…

Fusarium wilt is especially dangerous during warm summer days. It is caused by a fungus called Fasurium oxysporum. First, you notice dropped leaves while lower leaves often turn yellow and wilt. If you do not apply proper treatment, your plants will die eventually. The “good” news is that you can spot the first sings pretty quickly as leaves turn yellow on one side of the stem and leaf.

First, the stem does not show any signs of a soft decay, but if you cut it lengthwise, you should see a brownish colour inside. The fungi enter the plant through roots spread through the water distribution system in the stem. This will block veins which can no longer carry nutrients.

Treatment

This is unfortunate, but we have a really bad news for you, because you cannot suppress the spread of fungi, not even with chemicals. So the only thing you must do is to dig out all diseased plants and burn them! Do not put them in your composter as you will spread the disease everywhere! If you are shopping for tomato seeds, look for seeds marked with letters VF. These should be resistant to these fungi. However, the level of resistance may be questionable.

Early blight

This fungus or mould is known as Alternaria solani and causes early leaf spots. The first sign you can see is usually the appearance of small brown spots on older leaves. Over time, the spots get bigger and form concentric circles that look like a bull’s eye. A yellow webbing may form around the spots. Heat and humidity accelerate the disease and leaves will die eventually.

The infection starts at the ground and works its way up. You will see lesions on fruit that are very large and hard to miss. The fruit will fall and rot. This fungus will kill some leaves, but not all of them, but fewer leaves mean that fruit will be exposed to sunlight and unfortunately, heat supports this disease. The fungus survives in infected debris, seeds, in the soil and even in self-pollinating plants that will grow in the following spring.

Care

Cut off and discard lower leaves and other infected parts of the plant. If the infection is really bad, use a fungicide.

Tomato yellow curl virus

The virus is transmitted by a flying insect called Rabdophaga, which gets the disease from other infected plants. Unfortunately, these plants may grow even several kilometres from your garden. The first signs of infection appear after 2-3 weeks and include yellow edges around leaves or curling upwards. If the infection is severe, the leaves will curl. If you ignore these symptoms the plant’s flowers will die and the plant would not grow. This virus also suppresses fertility of the infected plant.

Treatment

Remove infested plants and place in a bag for disposal. To repel butterflies and reduce further damage, you need to spray plants at weekly intervals. The treatment should consist of 0.5% oil, 2-4 teaspoons of rapeseed oil and dishwashing fluid.

Walnut toxicity

Some types of walnut trees produce a toxic substance that is harmful to tomatoes. If you notice yellowing and wilting or suppressed growth, check the surrounding area for walnut trees.

Protection

Unfortunately, tomatoes affected in this way cannot be saved. Remove sick-looking or dead plants – do not plant tomatoes near walnut trees.

Septoria leaf spot

This disease affects the entire plant. The infection is caused by a fungus called Septoria lycopersici. It appears at lower part of stems right after plants start producing fruits. It also creates beige circles as big as a thumb and dark edges on older leaves. Spots have a black spot in the middle. Infected leaves turn yellow, die and fall off. The fungus usually survives in weeds and spreads during warm days.

Care

With a bit of luck, you can prevent the spread of the disease by applying fungicides repeatedly. You may also uproot and burn infected plants.

Photo: Radek Štěpán

Leaf mould

Leaf mould usually occurs where there is not enough airflow and high humidity. The first visible signs are yellow and light green spots on upper leaves. In very high humidity, gray velvety growths may appear. Sometimes you can see these signs on flowers, fruits and stems. Spores are spread by infected garden tools and by wind.

Treatment

Remove all infected plants. Trim and cut branches to improve air circulation.

Tomato pulp necrosis

This bad disease appears early in the season during cool weather or cloudy and rainy periods. It usually gets into greenhouses and where there is a high concentration of tomato plants so, large garden colonies are mainly attacked. You can tell that your tomato plants got pulp necrosis if you notice dark spots on stems. These black spots later merge into one large one. The bacteria multiply and can split the stem. If you let the disease spread further the flesh will crumble, destroying the stem.

Check often

If you do not catch the disease early, you will have to remove and destroy all affected plants. All means all and totally. You need to dig and pull out roots and sprinkle soil surface with a lime in the fall. You also need to reduce fertilizes. Too much nitrogen activates the disease. However, chances are that this disease will not trouble you throughout the entire growing season, because it needs rather cold temperatures.