7 Causes of Tomato Leaves Curling: From Heat to Virus

Problem: 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 typically remain green.

How to Fix It:

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. 

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.

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 1 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. 

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.

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.

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 kits).

Problem: Herbicide Drift

Tomato plants are susceptible to herbicide damage. If the tomato leaves are curling downwards and growing twistedly, they might have been exposed to drifting herbicides—not necessarily from your use, but it could be from neighbors treating their lawns or from farm fields in the area. 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.

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. 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 and re-plant.

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. 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 use chicken manure, which does not have the problem. 

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. 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. 

Both viruses are major diseases causing havoc worldwide, including in the U.S. The mosaic virus is exceptionally hardy, surviving in plant debris for over 100 years.

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 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 if you are a smoker.

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.

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. 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.

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. Organic alternatives to miticides are horticultural oils and insecticidal soaps.

If the damage is severe, pull up the plants and dispose of them.