Tomato Leaves Curling Up, Down or Inwards – Why?

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When you notice tomato leaves curling, it’s frustrating to see the plant lose vigor, but it’s more worrisome knowing the plant is losing energy. 

The leaves on tomato plants are like solar panels, harvesting the energy the plant needs to thrive. 

An issue is the energy your tomato plant loses from leaves curling upward, downward, or cupping inward.

There are numerous causes for curling leaves, but only a couple are sinister problems.  Sometimes, it’s just a coping mechanism. 

I studied all possible reasons, as my garden tomato plants and hydroponics system in the grow tent often had curled leaves.

Tomato Leaves Curling

Common causes of tomato leaf curling are micronutrient deficiencies (molybdenum and boron), Curly Top Virus, Tomato Mosaic Virus (ToMV), or insects such as broad mites and the tomato pinworm. Leaf discoloration and the direction of curling (upward curl or downward curling) help identify the cause.

Symptoms and the Reasons for Tomato Leaves Curling

Leaf curling up

This is the most common version of tomato leaf curl you’ll encounter. This happens particularly during summer and spring as the temperatures rise. 

You’re more likely to notice upward leaf curls when it’s too hot and humid for the plant. 

To confirm that this is what’s happening, look at new growth. 

When it’s caused by dry, hot weather, it’s the mature leaves that curl. New growth will be smaller so they won’t need to curl to conserve energy. 

Mature leaves are bigger, and those are the ones to curl because of their size. 

You see the plant protecting itself from drought, even if it has enough water in the soil. 

You’re witnessing a battle of respiration versus transpiration, which is just a part of how plants grow.

• Respiration is when the plant transforms the carbon dioxide it absorbs into oxygen during photosynthesis. 
• Transpiration is the water movement inside the plant, after which excess water escapes through the stomates on the underside of the tomato leaves. 

When it’s too hot for the tomato plant (in the high 90 degrees Fahrenheit), the upward leaf curl is a survival technique because it reduces the surface area, lessening the water lost to transpiration. 

When transpiration is happening faster than respiration is, leaf curl reduces the surface area, helping the plant store water until it can convert it to energy. 

It doesn’t mean, though, that the plant needs more hydration. When temperatures are high, it’s just the plants’ way to use better the moisture it has access to. 

That being said, check the soil moisture level because too high a heat will cause the soil to dry faster. Tomatoes like the soil to be moist but not wet. 

Leaf Curling Down

Downward leaf curl is a problem to address ASAP. Take this as a sign of the plant telling you it can’t get a drink. 

The confusing part of this type of curling is that it tends to happen after a heavy watering, such as after deep watering the soil or when there’s been a heavy downpour.

Traditional wisdom tells you that the plant can’t be thirsty, but the reality is that if the soil is waterlogged, the roots can start to rot, and when that happens, the plant will defensively try to prevent the rot from spreading. 

The noticeable signs of root rot above the soil are leaves drooping and curling downward. 

Catch it early enough. You’ll only see leaves wilting with a downward curl around the edges. The plant struggles to take up water and nutrients when the rot worsens. 

The lack of nutrient uptake – particularly nitrogen – causes yellowing leaves. 

A nutrient deficiency with overwatered tomato plants results in downward leaf curl with yellowing leaves. 

The yellowing is the nitrogen deficiency, and the leaf curling downwards is the lack of water uptake. 

Before you reach for the fertilizer, know that the tomato plant is tricky with fertilizers. Just because its leaves are yellowing doesn’t mean it needs a nitrogen fix. 

Often, it’s not a nitrogen top-up it needs, but rather a trace amount of molybdenum, a micronutrient that helps plants convert nitrates in the soil into nitrogen that can be used. 

Keep reading below, and you’ll learn to read your tomato plant like a book, picking up on its subtle clues…

Macronutrient Deficiencies

Different plants have different needs, so the best fertilizer for tomatoes varies by the variety you’re growing. 

The one thing they all share is the need for similar macronutrients. These are NPK, magnesium, and calcium. 

At the micronutrient level, is where some fertilizers can let you down. 

Two common macronutrients that present problems for tomato plants are molybdenum (Mo) and Boron (B), both essential for healthy tomato plant growth. 

Here’s how tomato plants react when starved of macronutrients.

Molybdenum Deficiency

Molybdenum deficiency causes the strongest inward curling, which you ought to be careful with because it shows symptoms similar to a nitrogen deficiency. 

The difference between the two is that nitrogen can remain in the soil, but only when molybdenum is present can nitrates be converted into a soluble form that the plant can use to take up nitrogen. 

It only needs a minuscule amount of this to form nitrates. 

If only this is deficient, adding in a nitrogen-rich fertilizer can do more harm than good. 

Molybdenum deficiencies are more prevalent in acidic soils. The nitrogen can be there, but the tomato plant can’t use it without molybdenum. 

Only 0.1 to 1.0 PPM (parts per million) is needed. 

Most soils have molybdenum or at least the molybdate particles that plants can use to make it. The more acidic the soil, the less molybdenum is available. 

Most fertilizers for fruiting plants contain this, but when it lacks, such as when the soil is too acidic for it to take, a foliar spray solution is a practical way to increase absorption. 

In farming applications, this would be treated by liming the soil.

However, a spray application will be more cost-effective and yield the same results.

These formulations stop the leaves from yellowing and curling without overdosing the plant with nitrogen. 

If you mistake a molybdenum deficiency with a nitrogen deficiency, what’s likely to happen is you’ll get a burst of deep green leaf growth at the expense of fruit production. 

With more leaves soaking up water, there’s a higher likelihood of stressing the plant, resulting in the older leaves curling due to environmental stress. 

Boron Deficiency

Boron is another macronutrient, but tomatoes do not need much of it. Most fertilizers have sufficient amounts. 

When the soil is deficient in boron, symptoms on the tomato plant include stems growing shorter and thicker, uneven ripening, cracks on the tomato skins, and leaves yellowing and curling inward. 

Boric Acid has up to 17.5% of the boron compound. If applying this, go easy because tomatoes do not need much. 

Micronutrient Deficiencies

Phosphorous Deficiency

Phosphorous deficiency causes backward curling (leaf roll), the opposite of upward curling. 

A downward curl doesn’t roll all the way around. When it does, that’s referred to as a backward roll, indicative of a phosphorous deficiency. 

Magnesium Deficiency

Magnesium deficiency will eventually lead to leaf curl, but you’ll see the symptoms earlier, when leaves’ edges turn yellow. 

If it isn’t corrected, leaves will curl. 

Potassium Deficiency

Potassium deficiency causes the tips on the leaves to brown and curl forward. 

Calcium Deficiency

A calcium deficiency causes new leaves to grow in pale green, turning yellow and curling inward. 

If left uncorrected, this leads to blossom end rot, a common goji berry plant problem, which is (unsurprisingly) part of the same Nightshade family that tomato plants belong to. 

All tomato fertilizers will have sufficient micronutrients. The two macronutrients of concern with tomatoes are molybdenum and boron, which can be topped up with a foliar spray rather than treating the soil.

Viruses that Cause Leaf Curl in Tomatoes

Curly Top Virus

The Curly Top Virus is a systemic virus infecting only the host plant. It doesn’t spread to neighboring plants. 

The virus is introduced by a beet leafhopper (Circulifer tenellus) bite. If your tomato plant gets it, it’s just bad luck because not every leafhopper carries the virus. 

It’s prevalent in adult leafhoppers that pick it up when feeding on other plants. 

Once infected, they carry it for the rest of their lives. It only takes one leafhopper with the virus to sample your tomato plant to infect it. 

The signs of Curly Top Virus (as the name implies) are the leaves at the top of your tomato plant curling. 

The direction will be an upward curl, like cupping inward toward the center.

The “Top” part in the name is important because the upward curl this causes is similar to the damage inflicted by the tomato pinworm. 

The difference is that the Curly Top Virus only causes leaf curl on the top leaves. Tomato pinworms will cause an upward curl on every leaf they feed on from the base of the plant up. 

Unlike with leaves curling upwards caused by heat stress, Curly Top Virus causes leaf discoloration, too, and new growth becomes leggy. 

If you suspect this, particularly if your area has a lot of leafhoppers, look at the underside of the leaves for discoloration. 

Purple veins on the underside of leaves are confirmation of Curly Top Virus. 

Other signs include stunted growth, particularly any upward growth. 

On indeterminate tomato plants (the ones that don’t stop growing upwards), you’ll notice more suckers branching out from side shoots, causing the plant to grow bushier with very little fruit production. 

Side shoots are the plant trying to outgrow the virus, which it may do. That’s rare, though. 

Most likely, the plant is going to die. 

Thus, it’s always a good idea to have backup plants, especially tomato plants, because it only takes one bite from a nuisance leafhopper to kill your harvest. 

Preventative measures are the only safeguard to protect your plants from leafhopper damage.

Take solace in the fact they prefer feeding on leaves in direct sunlight. Knowing that fact, you can grow your tomato plants in a shady location or put a canopy over them. 

Also, know that any tomatoes already on the vine, as will the seeds, will be healthy. 

You can use the seeds of ripened tomatoes to grow new plants. It’s the same process used to grow goji berry plants from seed.

The Tomato Yellow Leaf Curl Virus

Similar to how the beet leafhopper transmits the Curly Top Virus, it’s the whitefly that’s the carrier of the Tomato Leaf Curl Virus. 

Once the whitefly has the virus, it’s contagious for a few weeks. During that window, they can infect several plants. 

While this is one of the rarer tomato viruses, it is important to know the symptoms because if you get a whitefly infestation, they can spread the disease to other plants. 

The symptoms to look for are yellowing leaves curling downward but also drooping too, shriveling up, and stunted growth. 

Like Curly Top Virus, you’ll also notice purple veins on the leaves. 

The only way to protect your tomato plants from this disease is to protect them against whiteflies. 

Mosaic Virus

The Mosaic Virus affects hundreds of crops, mostly fruiting types of plants. Whichever strain of this virus afflicts the tomato plant, they still cause the same symptoms.

Streaky discoloration, yellow spots on tomato plant leaves, and a mosaic pattern. 

Don’t compost your plant if there’s even a hint of a virus infection. Most viruses can overwinter in the soil, infecting new plants the compost is used on.

Insects that Cause Tomato Leaves to Curl

Broad Mite

The broad mite is a sap-sucking insect that feeds from the edges of the leaves. 

These are even smaller than the menacing spider mite measuring only 0.0078 inches (0.2 mm), and if that’s not enough, they’re translucent, so even if they were bigger, you’d still struggle to see them. 

You will certainly see the damage they inflict on your tomato plants.

Broad mites feed on young leaves first, avoid light, and they feed on the underside of leaves.

The damage results in the leaf margins turning brown and curling downward. 

Tomato Pinworm

The tomato pinworm is a tiny caterpillar from a micro moth species, only a few millimeters in size. 

They feed only on Solanaceous plants, which are all part of the nightshade family, including eggplants and potatoes. 

Much of the damage they do is similar to that of the eggplant leaf miner. Only on tomatoes, it’s worse. 

At the early stage of infestation (which you’re unlikely to notice), the moths lay their eggs close to the stem near the base of the plant. 

When the eggs hatch, tiny caterpillars emerge and start burrowing to feed on the leaves. 

Signs of damage by the tomato pinworm are brown blotches and streaks along the leaf edges. 

At first, it causes the leaves to fold inwards, creating a cocoon-like a shelter. 

As the larvae mature, they begin burrowing their way through the leaves, into the stem, eventually reaching the fruit where they bore into the tomato, then feed on the inside out. 

These will ruin crops. 

Early signs of tomato pinworm are leaf-fold on new leaves.

Repeat applications of neem oil can eliminate a tomato pinworm infestation. 

Frequently Asked Questions

What’s the best way to prevent leaf curl on tomato plants?

High temperatures and high humidity are often the underlying cause of tomato leaves curling, whether that be environmental stress or insects. In areas where the afternoon sun reaches scorching temperatures, you could try growing taller plants to shade your tomatoes or use a shade cloth with 30% to 50% sunlight reduction to reduce the radiant heat under the canopy. This helps protect from heat stress and insect damage.

Will curled leaves flatten out?

Curled leaves on tomato plants will only recover from the heat, water, or humidity-related environmental stress. Suppose a virus or insect damage causes it. In that case, the leaves should be removed. Treat the plant insecticide or miticide. Adjust the growing conditions, such as soil amendments, or placing the plant in a more suitable location.

Takeaways

Leaf curling in tomatoes is caused by the following:

• Micronutrient deficiencies (molybdenum and boron)
• Curly Top Virus
• Tomato Mosaic Virus (ToMV)
• Pests (broad mites and the tomato pinworm)