Gardening
Twisted or curled leaves on tomato plants can be caused by environmental stress, chemical exposure, or biological factors.1 While curling leaves do not necessarily kill the plant, determining the cause, or combination of reasons, as early as possible is crucial.
First, take a close look at the tomato plants and observe the plants over a few days to see if there is any progression:
How many leaves are curling, just a few or all the leaves on the plant?
Are the curling leaves mainly old leaves, new growth, or both?
In what direction do the leaves curl, upwards or downwards?
Are other parts of the plant, including fruit, showing any symptoms?
Here are seven reasons why your tomato plants are having trouble with curling leaves and what you can do to fix it.
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01
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Problem: Planting Tomatoes Too Early
Tomatoes can suffer from leaf curl if they have not been properly hardened off or were planted when the weather was still too cool. It usually starts with the lower leaves curling upwards, then rolling inwards lengthwise. The leaves also become thick and leathery, but typically remain green.1
How to Fix It:
Make sure to plant tomatoes only when daytime temperatures consistently reach between 70 and 79°F, and nighttime temperatures are no lower than 61 to 65°F.
02
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Problem: Heat, Drought, and Wind
In extremely hot, dry, or windy weather, tomato leaves twist and curl upwards to protect themselves from further water loss.2
How to Fix It:
If you live in a hot or warm climate, plant tomatoes in a location where they are sheltered from strong, drying winds and receive some late afternoon shade.
Regardless of your climate, always keep tomato plants well-watered. Once outdoors, tomato plants will need at least an inch of water each week from rainfall or watering. Watering should be done slowly and deeply, so the plants form deep roots. Mulch around their base, so the soil stays evenly moist.
03
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Problem: Nutrient Imbalance
Tomatoes overfertilized with nitrogen may show upward curled leaves that are thicker and greener than usual and also lots of foliage instead of setting fruit. Low phosphorus might also lead to leaf curl.1
How to Fix It:
Even though tomatoes are heavy feeders, fertilization must be balanced. After planting in well-nourished soil, tomato plants benefit from applying a balanced 5-10-5 NPK fertilizer a few times throughout the growing season. Avoid fertilizers high in nitrogen (the first number), or you'll have many bright green leaves and fewer tomatoes.3
The only reliable way to determine the nutrient content in the soil and find out how to remedy the imbalance is a soil test.
Ask Your Extension Office
Need a soil kit to determine your nutrient content? Call your local Extension Office, which advises on agriculture, pest control, and gardening; they should sell soil kits.
04
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Problem: Herbicide Drift
Tomato plants are susceptible to herbicide damage. If the tomato leaves are curling downwards and growing twisted, they might have been exposed to drifting herbicides—not necessarily from your use, but it could also be from neighbors. When spraying glyphosate or 2,4-D, dicamba, and other hormone-type herbicides, even a wind speed of five miles per hour can lead to herbicide drift.2
How to Fix It:
Once the herbicide drift has occurred, there is nothing to be done. Depending on the severity of the herbicide damage, the plant may recover, and the new growth may look normal.
Or, the new growth may continue to show herbicide damage which, in addition to curling leaves, might include white and splitting stems and deformed fruit. If this is the case, you will, unfortunately, have to pull the plants and re-plant.4
05
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Problem: Herbicide Residue in Mulch and Compost
If you use mulch or compost from a third-party source and the leaves of your tomato plants become cupped or in any other way distorted, the reason might be residue from herbicides such as aminopyralid or clopyralid, the active ingredients in weed killers used by farmers.5 These chemicals remain on treated hay and hay products, grass clippings, manure, and compost for many months. Cows and horses feed on that hay and then pass it on via the manure.
How to Fix It:
To prevent this, make sure to get manure or compost only from reliable sources that don’t use these herbicides. Or, get manure from sources that use chicken manure, which does not have this problem.
06
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Problem: Tomato Viruses
If a virus causes curling tomato leaves, pinpointing it is rather tricky as numerous viruses can cause tomato leaf curling and stunted growth. The additional symptoms vary in each virus, and the transmission method differs.
Two common viruses that cause tomato leaf curl are tomato yellow leaf curl virus and tomato mosaic virus.6 Whiteflies transmit tomato yellow leaf curl virus. New leaves become cupped and turn pale green, the leaf edges turn yellow, and the undersides of the leaves may turn purplish. The plant's entire growth is stunted, and fruit production is poor.
Tomato mosaic virus is transmitted from seed, grafting, human handling, and tobacco products. In addition to curling, the leaves become mottled, new leaflets are small and infected fruit is brown inside. The virus is a concern throughout the growing season.
How to Fix It:
To reduce the whitefly feeding that causes tomato yellow leaf curl virus, use reflective mulches (aluminum or silver-colored) between rows. To repel whiteflies and reduce feeding, mix a solution of 0.25 to 0.5% oil spray (2 to 4 teaspoons of horticultural or canola oil with a few drops of dish soap per gallon of water) and apply weekly.
Remove plants and bag them for disposal at the end of the growing season. Consider rotating crops the next growing season.
Because tomatoes are in the same plant family as tobacco (nightshades), tobacco users and smoke can transmit a mosaic virus to their tomato plants by touching them. Don’t allow smoking near your garden, and wash your hands or glove them before tending tomatoes.
A vital step to prevent your plants from getting infected is to keep your garden weed-free because weeds are often host plants for insects that then transmit a virus to your tomato plants.7
07
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Problem: Broad Mites
When broad mites feed on young tomato leaves and flowers, they inject toxins into the plant that causes the leaves to twist and become distorted severely.8 The mites are so tiny that they cannot be detected with the naked eye or even a magnifying glass; only the damage will indicate their presence.
If the infestation is severe, the underside of leaves and fruit might also turn bronze or russet in color.
How to Fix It:
Broad mites may come from infested greenhouse plants, so buy only plants from reliable suppliers.
You can use a sulfur-based miticide to combat a moderate broad mite infestation. Read the label instructions carefully, as some tomato cultivars cannot tolerate treatment with sulfur.9 Organic alternatives to miticides are horticultural oils and insecticidal soaps.
If the damage is severe, pull up the plants and dispose of them.
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