Tomato Leaf Curl: Why It Happens and How to Treat It

Tomato leaf curl is a frustrating but fixable problem for any gardener, no matter your experience.

Thankfully, the vast majority of leaf curl in tomato plants is caused by physiological changes, transplant shock, or poor drainage, and can be fixed by amending how you water, and the location of your plants.

Some tomato leaf curl symptoms can be caused by herbicides, viruses, and bacterial problems but, in most cases, your plants will recover with some considered pruning.

To get you on the right track this year, we’ve created this symptom-by-symptom guide to prevent, manage, and treat, tomato leaf curl.

What is tomato leaf curl?

Tomato leaf curl is one of those gardening problems that doesn’t need much explaining. It’s the common name given to a range of viruses, physiological malformations, and natural plant responses to conditions, all of which cause (you guessed it) curled leaves.

There are at least twenty different viruses that can cause tomato leaf curl, but most cases are caused by overwatering, under-watering, transplant shock, nutrient deficiencies, and herbicide drift. Despite the long list, it’s actually pretty simple to fix, as long as you know what caused your tomato leaves to curl in the first place.

What is tomato physiological leaf roll?

Physiological leaf roll, or physiological leaf curl, is different to viral leaf curl. Rather than a virus, fungal infection, or bacterial problem, physiological leaf roll is caused by a plethora of environmental conditions.

If there is no sign of dampness in the soil, mold on the stem, or lesions on the foliage, your tomato plants will recover, as long as you treat them well.

Can tomatoes recover from leaf curl?

Tomatoes can easily recover from leaf curl in nearly all cases, but like any illness, in any species, speed is everything. Follow our broken-down guide to the signs and symptoms of tomato leaf curl, and follow the simple steps to treat it.

Why are my tomato leaves curling and how to fix it

Conditions

There is a long list of poor conditions that cause tomato leaf curl, and each is fixed by changing the element that’s caused it:

Overwatering can lead to root rot, and early signs are downward curling leaves that look under-watered and have a light yellow tint, accompanied by damp soil. To help your plant recover, simply stop watering for a few days, or up to two weeks, until the soil dries out.
Under-watering tomato plants causes a similar-looking leaf curl to overwatering, but without the yellowing foliage. The clearest sign of under-watering is if the soil is cracking or drying beneath the surface. In pots, soil will be coming away from the edge. Soak the root ball in a tray of water until the soil has revived, and your plants should look as good as new in a few hours.
Heat, particularly when accompanied by wet leaves, can cause foliage to shrivel and cup inwards. This is literally sunburn, or sun scorch, happening before your eyes. The wet spots on leaves intensify the heat, and like any thin organic material, when it burns, it contracts. The leaves won’t recover, but the rest of the plant will be fine. Just don’t water the leaves.
Wind has a similar effect to heat, especially in coastal areas, where salty air dehydrates the air. Those dry winds cause a cupped, often leathery leaf. This is the plant’s response to its conditions, so if you can, move it somewhere more sheltered.
Root restriction will cause your entire plant to wilt, often with downward curled leaves. The soil may be perfectly moist and drained, but if your tomato seedling hasn’t been moved into a bigger pot yet, it will struggle to grow without new nutrients and more space.
Overfeeding causing tomato leaf curl is more common at the seedling stage. Signs include curled leaves lower down the stem, while the top leaves remain fine. If you have a soil testing kit, use it, if not, avoid feeding seedlings.
Underfeeding-triggered leaf curl is more common with mature plants, but the symptoms are virtually identical to overfeeding. Lower leaves will curl, and become leathery, while higher leaves appear fine. Apply a light, balanced fertilizer twice a week until the plant looks happier.

Transplant shock

Tomatoes are tropical plants, so growing them anywhere other than the tropics nearly always requires seedlings to be grown indoors, or under cover to begin with. When it’s time to plant those seedlings into the ground, doing it too early, or too late can shock them.

Transplant shock is the result of sudden temperature changes, usually from a steady indoor temperature to a cold spring day, or warm early summer afternoon. To avoid transplant shock, harden off your tomatoes for a few hours each day, as soon as the soil temperature reaches 12ºC.

Excessive pruning

Excessive pruning won’t, in itself, cause tomato leaf curl. What it can do though, is expose the remaining leaves to more direct sun. The sudden change will result in a similar leaf curl to that of sun-scorch. Try to guard your tomatoes against direct sunlight until they recover.

Diseases

Diseases are a common cause of tomato leaf curl but are less common than physiological leaf roll. For a full guide to treating common tomato diseases, whether they’re viral, bacterial, or fungal, check the detailed list later on in this article.

Pests

One of the less likely scenarios to cause leaf curl is pests. Other than a few very clever caterpillars and moths, the only common pest that will cause leaf curl is spider mites, but that would take a huge infestation. If you see leaves curled due to excessive silk, just wash them clean to remove them, and the plant will recover.

If, however, you unwrap a curled leaf to find a moth cocoon, or a caterpillar, just remove the leaf and move it somewhere else. Moths are vital to the garden, but if a caterpillar hatches inside the curled leaf, it will devour the surrounding leaves too!

Herbicide drift

Due to their thin leaves, tomatoes are easily damaged by most herbicides. While chemical herbicide drift does the most damage, even organic herbicides can cause serious issues. If you notice bent stems that have lost both vigor and color, with inward cupping leaves, it’s most likely caused by herbicides or chemicals.

Even if you’ve not directly treated your tomatoes or the area with herbicides, they can drift for miles on the wind, with farming and industrial herbicides like 2,4-D and dicamba being the worst.

Even common garden weed killers that include glyphosate can completely kill tomato plants, and shop-bought compost can sometimes accidentally contain traces of pasture herbicides like aminopyralid, picloram, and clopyralid.

Diseases that cause tomato leaf curl, and how to treat them

If you’ve tried everything else, and you’re still having problems with tomato leaf curl, it can also be caused by some common fungal, bacterial, and viral infections. The signs of these infections are often much clearer than physiological leaf roll.

Distinguishing between each viral leaf curl, and each fungal leaf curl can be challenging but, thankfully, most are treated in a similar way. Follow the guides below to treat each different type of tomato leaf curl disease.

Fungal problems that cause tomato leaf curl

Fungal infection is a common cause of tomato leaf curl, and the brown, yellow, or moldy lesions across the foliage are the first sign. The two most common fungal leaf curls on tomatoes are late blight and septoria wilt.

Septoria Wilt causes very slight curling around the very edge of a leaf, accompanied by yellow patches (often half a leaf) and brown lesions with white centers.
Late blight starts with sooty, or moldy spots on tomato leaves usually toward the edge. As it develops the entire leaf will fold in on itself and being to droop. Eventually, the whole truss will curl up, or drop from the plant, infecting other parts of the plant as it passes.

To prevent these common fungal causes of leaf curl on tomatoes, simply remove them as soon as you see them, and burn the infected material. The prevent reinfection, use an organic fungicide like neem oil for indoor crops, and reduce humidity and watering for outdoor crops where possible.

Viruses that cause tomato leaf curl

Tomato leaf curl is a common symptom of many viruses, but the three to look out for are tomato yellow leaf curl virus (TYLCV), tomato mosaic virus, and curly top virus.

Tomato yellow leaf curl virus (TYLCV) is usually carried from plant to plant by whiteflies and aphids. Symptoms include marginal leaf yellows, cupped leaves (both up and down), dropped fruits or un-dropped flowers, and deformed or stunted growth of fruits and foliage.
Tomato Mosaic Virus literally causes a mosaic pattern across the leaves it affects. If caught quickly, you can just remove the infected materials. If left without treatment, mosaic virus will spread and stunt the plant’s growth, and ruin fruits (turning the inside brown).
Curly Top Virus is probably the most frustrating viral disease for any tomato grower, not because it ruins fruit, but because it’s so hard to diagnose. The signs are essentially the same as sun scorch, with dry, curly top growth, affecting the new growth only. Eventually the virus will spread to other plants, and down the plant. Removing any affected plant is key to next year’s success, but if you don’t plan on growing tomatoes again next year, they will continue to crop, but yield less.

For any viral problem causing tomato leaf curl, the best course of action is to remove the entire plant and burn it. Viruses spread through every cell in the plants they infect, and can only be eradicated by complete removal. Thankfully, tomato viruses are pretty rare and are generally imported on shop-bought seedlings, or by using infected compost.

Bacterial infections that cause tomato leaf curl

There are two common bacterial problems that are clearly identifiable from the way tomato leaves curl. They are; bacterial canker and southern bacterial wilt.

Bacterial canker causes the new leaves to develop slowly, and with brown edges. The lower leaves will curl up, similar to over or underfeeding. The clearest sign of advanced bacterial canker is brown streaks up the stem of the plant, leading up from lower leaves.
Southern bacterial wilt looks pretty identical to late blight. When advanced, every single leaf can turn brown, and curl downwards before dropping off and rotting. Southern blight starts from the ground up so lower leaves will turn first. Prevent it by removing lower leaves before it forms, and maintaining a bare stem for the lowest 6” for the entire growing season.

Treat bacterial leaf curl in tomatoes in the same way as viruses. If you catch it early, you can often limit the spread by removing infected growth. If it’s too late, remove the entire plant and burn it to prevent the spread to the rest of your crop.

Takeaway

Tomato leaf curl might seem complicated, but it’s really quite simple: check if it’s a viral, bacterial, or fungal cause, and if not, amend your care routine. If your tomato’s curling leaves are caused by disease or infection, treat them quickly before it spreads.

If you just follow that basic order, it’s pretty safe to assume your curling tomato leaves are a sign of something as simple as hot weather, or wet summers. Keep an eye on them, do what you can to manage their conditions, and in no time at all, tomato leaf curl will be a thing of the past in your garden.