Tomato Leaves Yellowing? These Are the Reasons Why

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Tomato leaves yellowing is a common problem for a majority of gardeners. There are several causes of yellow tomato leaves. These include; herbicide damage, end of a cycle, bad watering habits as well as nutritional deficiencies.

In addition to this, infections by diseases such as powdery mildew and bacterial canker can result in yellow leaves in tomato plants.

Moreover, infestations from pests like aphids may also lead to yellowing and curling of tomato leaves leading to poor crop performance and low yields.

This article, therefore, touches on these and many more reasons for this problem and how to treat yellow leaves on tomato plants.

So, read through it and see what might be the cause.


Causes and Treatment for Yellow Leaves on Tomato Plants

End of a Tomato Plant Cycle

Towards the end of the life cycle, the weather becomes cool and frost approaches. This time your plants start to turn yellow. This shouldn’t worry you. It is the right time for the plant’s cycle to end.

Under such circumstances, trim off any new blossoms to help the plant channel all energy towards the ripening of the already developed fruits.

Additionally, cut down any dead/yellow stems to improve airflow and the plant’s health for faster ripening.


Nutritional deficiencies

Your tomato leaves might be yellowing due to a lack of important nutrients required for healthy growth.

For instance,

A shortage of nitrogen causes the older leaves to turn pale-green to yellow with symptoms gradually progressing to new growth. As deficiency persists, the old leaves turn completely yellow, then brown before they fall off the plant.

To correct nitrogen deficiency in tomatoes, top dress your plants using urea as per soil test recommendations.

On the other hand, with Zinc deficiency younger leaves become yellow and mature ones develop pits in their interveinal upper surfaces. Treat your plants with Zinc Sulfate foliar spray at 5% for correction.

Pro Tip: Apply homemade compost regularly to keep your soil rich in nutrients.


Bad Watering Habits

This is the most common cause of yellow leaves in pants. Sometimes you might forget to water your plants as per the schedule may be due to some engagements leaving them to dry out. And other times, you might overwater them.

Under-watering makes the leaves wilt and turn yellow. The yellowing starts from the leaf edges and gradually covers the entire leaf leading to leaf fall.

In the case of over-watering, the excess water suffocates the roots leading to root rot. Once the roots get damaged, and the soil is soggy, the amount of air available for the plants is too little. The leaves, therefore, turn yellow and fall from the plant.

The best way to correct this is to practice good watering habits. For example; water the plants when the top of the soil (1 to 2 inches) is dry and never leave your plants to dry out.


Transplanting Shock

When you move your seedlings from a warm spot such as a greenhouse to plant outdoors, they usually need a bit of time to adjust to the new growing conditions.

During this time (a week or two after transplanting), it is most likely that the plants develop what we call transplanting shock. The shock causes the bottom leaves to turn yellow but the new growth remains green and healthy.

This shouldn’t alarm you! The plants are just going through a short phase of adjustment.

Although transplanting shock isn’t harmful to the plant, it is good to prevent it by ensuring that the soil remains warm during the transplanting process.

And if the plants are already shocked, remove the yellow leaves at the stem to preserve the plants’ energy for new growth.


Herbicide Damage

Tomatoes are sensitive to herbicide drift especially those containing glyphosate. When applied in hot weather, herbicides can take a gaseous form and spread easily through the wind to your plants.

When this happens, the plants stop growing but remain green for several days. However, 7 to 10 days after exposure, the leaves turn yellow then into a uniform brown, and leaf distortion and dead spots follow.

It’s uncommon for tomato plants to recover from herbicide damage. All you can do is replant if there is enough time for them to produce a crop.


Diseases and pests

Fusarium wilt

This disease is caused by a soil-borne fungus Fusarium oxysporum f.sp. lycopersici. The fungus affects the plant by blocking the xylem tissues. This prevents water from moving freely from the stems to the branches and leaves of the plants.

What follows is that the plants start to wilt and the leaves turn yellow beginning with the lower ones. The wilting may start with a single shoot or leaf close to the plant’s top. At this stage, the plant usually recovers when temperatures are cooler or when watered.

As the infection progresses, the entire plant wilts, and the yellowing gradually moves up the plant. This time, not even watering or cooler temperatures will help the plant recover.

For effective control and management of this disease, always stick to tomato varieties that are resistant to Fusarium wilt and, use a sterile potting mix/soil when starting the seedlings.

Apart from this, make it a habit to clean and disinfect your garden tools regularly to curb the spread of the pathogen.


Powdery mildew (Leveillula)

Leveillula is a common tomato disease in most parts of the dryer western states. It is characterized by yellow lesions on the upper side of the leaves. On the other hand, the underside side of the leaves has a light powder of fungal sporulation.

Control powdery mildew in your tomatoes by proper spacing, use at-soil watering methods such as drip hoses (avoid overhead irrigation) and apply systemic fertilizer feeds so that there is no spike in nitrogen levels (high levels of nitrogen encourages mildew growth).

Also, remove and destroy affected plants at the end of the season and, practice regular weed control.

Furthermore, use sulfur dust and sprays for prevention. If the plants are already infected, using neem oil or horticultural oils at the first sign of infection helps reduce and eradicate mildew in tomatoes.


Bacterial Canker

This is an infection caused by the bacterium Clavibacter michiganensis. The bacterium is introduced primarily through infected seeds or transplants. It is therefore important to use pathogen-free seeds. Also, make sure to carefully inspect the transplants for any infections.

Infected tomato plants start to show signs of brown marginal lesions on the leaves. With time, these lesions become interveinal and irregular.

After this, the plant wilts, and the yellow leaves remain attached to the stems.

To control this disease, use disease-free seeds and transplants, exercise regular weed control and, use resistant tomato varieties.


Leaf mold

Another common disease in tomatoes. Leaf mold is caused by the fungus Passalora fulva. The pathogen first affects the older leaves leading to pale greenish-yellow spots on the upper side of the leaves.

While on the lower side of the leaves just below the spots some olive-green to brown velvety molds form. As the infection continues, the infected leaves turn brown, wither and, die but, remain attached to the plant.

For management, plant resistant tomato cultivars, avoid splashing water on the foliage and, space the plants properly.

Plus, remove and discard all crop residues at the end of the season and, sterilize all your garden ware.

Should you go for chemical control, make sure you repeat the fungicide applications according to the instructions on the leaflet.

Furthermore, alternate between different chemical families to avoid pathogen resistance and improve efficiency.


Whiteflies

Whiteflies are winged insects that suck sap from the underside of the leaves using their piercing mouthparts.

Just like aphids, these white bugs deposit honeydew on the leaves after feeding for several days leading to fungal infections.

What’s more, is that these insects affect the photosynthesis process and this leads to yellow leaves and withering of the plants.

Get rid of whiteflies using reflective mulch and control the existing infestations with insecticidal soaps.


Aphids

Aphids are small sap-sucking insects that affect tomato plants early in the season. They suck sap from the leaves by piercing the veins with their needle-like mouthparts.

This causes the leaves to yellow and wilt with drought-stressed or diseased plants experiencing severe damage.

These insects also affect the plant’s health by depositing honeydew on the leaves thus attracting ants leading to sooty mold infections. Aphids are also responsible for the transmission of various viral infections.

Use reflective mulch such as aluminum to repel winged aphids or use other measures, for example, foliar sprays with insecticidal soaps to get rid of aphids organically.


Conclusion

As you have seen, the causes of yellow leaves on tomato plants are many. Surprisingly, most if not all of these causes are avoidable.

Below is a summary of how to avoid the yellowing of tomato leaves.

  • Practice good watering habits
  • Avoid using weed killers or herbicides close to your tomatoes
  • Keep your plants healthy by fertilizing
  • Protect your plants from pests and diseases
  • Transplant tomato seedlings at the right time.

By doing this, you are sure of having healthy and bountiful tomato fruits at the end of the season.

How is your experience with tomato growing?

Let us know in the comments.


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