First, take a close look at the tomato plants:
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?
Observe the plants over a span of a few days to see if there is any progression. This overview helps you identify the reason why your tomato plants are having trouble, and tells you what you can do about it.
Environmental Causes
Planting Tomatoes Too Early
Tomatoes that have not been properly hardened off, or planted too early when the weather was still too cool, can suffer from leaf curl. It usually starts with the lower leaves curling upwards, then rolling inwards lengthwise. The leaves also become thick and leathery but usually remain green. Make sure to plant tomatoes only when daytime temperatures consistently reach between 70 and 79 degrees Fahrenheit and nighttime temperatures are no lower than 61 to 65 degrees Fahrenheit.
Heat, Drought, and Wind
In extremely hot, dry, or windy weather, tomato leaves twist and curl upwards to protect themselves from further water loss. 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. Mulch around their base so the soil stays evenly moist.
Chemical Causes
Nutrient Imbalance
Tomatoes that have been overfertilized with nitrogen may show upward curled leaves that are thicker and greener than usual, and also lots of foliage instead of setting fruit. Insufficient phosphorus might also lead to leaf curl. The only reliable way to determine the nutrient content in the soil and find out how to remedy the imbalance is a soil test done in a specialized laboratory (your local Extension Office sells soil test kits).
Herbicide Drift
Tomato plants are highly sensitive to herbicide damage. If the tomato leaves are curling downwards and growing in a twisted way, they might have been exposed to drifting herbicides—not necessarily from your own use but it could be from neighbors treating their lawn, or from farm fields in the area. When glyphosate or 2,4-D, dicamba, and other hormone-type herbicides are being sprayed, even wind speed of five miles per hour can lead to herbicide drift. 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. If the new growth continues to show herbicide damage, which in addition to curling leaves might include white and splitting stems and deformed fruit, you will unfortunately have to pull the plants.
Herbicide Residue
If you used manure 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. These chemicals remain on treated hay and hay products, grass clippings, manure, and compost for many months. Cows and horses feeding on that hay then pass it on via the manure. To prevent this, make sure to get manure or compost only from reliable sources that don’t use these herbicides, or use chicken manure, which does not have the problem.
Biological Causes
Broad Mites
When broad mites feed on young tomatoes leaves and flowers, they inject toxins into the plant that cause the leaves to severely twist and become distorted. The mites are so tiny that they cannot be detected with the naked eye and 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. Broad mites may come from infested greenhouse plants so make sure to buy only plants from reliable suppliers. To combat a moderate broad mite infestation, you can use a sulfur-based miticide. Read the label instructions carefully, as some tomato cultivars cannot tolerate treatment with sulfur. Organic alternatives to miticides are horticultural oils and insecticidal soaps. If the damage is severe, pull up the plants and dispose of them.
Tomato Viruses
If curling tomatoes leaves are caused by a virus, pinpointing it is rather difficult as there are numerous viruses that can cause tomato leaf curling and stunted growth. The additional symptoms vary in each virus and the transmission method is also different. Two common viruses that cause tomato leaf curl are tomato yellow leaf curl virus and tomato mosaic virus. Tomato yellow leaf curl virus is transmitted by whiteflies. New leaves become cupped and turn pale green in color. The leaf edges turn yellow, and the undersides of the leaves may turn purplish. The growth entire plant is stunted and fruit production is poor. One way to reduce the spread of this virus is to control whiteflies with horticultural oils and insecticidal soaps. Tomato mosaic virus is transmitted by aphids. In addition to curling, the leaves become mottled, and new leaflets are small. Infected fruit is brown inside. What all the viruses have in common is that there is no treatment once a plant is infected and you need to pull the plants and dispose of them in the trash. An important 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.
