Pocket BotanistTomato leaves curl for a lot of reasons, and the first thing to know is that the most common one is harmless. Every summer, gardeners strip healthy plants or reach for a spray because the lower leaves have rolled up like cigars. Most of the time the plant is simply managing water. The trick is knowing which curl you are looking at, because a few of the others are worth catching early.
1. Physiological leaf roll (the usual one)
The pattern: older, lower leaves roll upward and inward along their length, feel thick and leathery, but stay green. The plant keeps flowering and setting fruit. It shows up after a heat wave, a spell of heavy rain following drought, hard pruning of suckers, or on plants that were transplanted late. Indeterminate (vining) varieties do it more than bush types.
It is a stress response, not a disease: the leaf reduces its surface to lose less water. What to do: nothing to the leaves. Water deeply and evenly, mulch to keep the soil moisture steady, stop removing so many suckers at once, and shade the plants in the worst heat if you can. The rolled leaves stay rolled, but the new growth comes in flat and the harvest is unaffected.
2. Herbicide damage
The pattern: new growth twists and cups downward, stems bend, and the leaves look narrow and fern-like, sometimes with thickened veins. It appears within days across the whole plant, and often on neighboring plants too. Tomatoes are extraordinarily sensitive to the growth-regulator herbicides used on lawns, so a neighbor spraying dandelions on a breezy day is enough. Contaminated compost or manure from treated pasture does the same, and lasts longer.
What to do: if the exposure was a single drift, the plant usually grows out of it in a few weeks; keep it watered and wait. If it came from compost or manure, the affected plants rarely recover and the soil should not be used for tomatoes, beans or potatoes until it has broken down, which can take a season or more. Do not eat fruit from plants showing active herbicide symptoms.
3. Aphids and other sap suckers
The pattern: young leaves curl and pucker, there is a sticky film on the leaves below, sometimes with black sooty mold growing on it, and the undersides carry clusters of small green, black or pink insects. Whitefly and thrips cause similar distortion with their own signatures: clouds of tiny white insects when you brush the plant, or silvery streaks on the leaves.
What to do: a firm spray of water knocks aphids off; repeat every few days. Insecticidal soap or neem for heavier infestations, in the evening so bees are not caught. Encourage ladybirds and hoverflies. Curled leaves do not uncurl, but the plant recovers once the pests are gone.
4. Viruses
The pattern: leaves curl upward and the margins turn yellow, the leaves are small, the plant is stunted and sets little fruit, and it started on one plant and is spreading. Tomato yellow leaf curl virus is carried by whitefly and is the serious one in warm regions; other mosaic viruses add mottling and a fern-leaf look. There is no cure for any of them.
What to do: pull and bin (not compost) infected plants promptly to protect the rest, control whitefly, wash hands and tools between plants, and next season choose varieties labeled resistant. Buying plants from a reputable source matters more than any spray.
5. Wind, cold nights and transplant shock
A freshly planted tomato exposed to wind, a cold night below about 10 °C (50 °F), or a sudden move from a greenhouse to full sun will curl and go a little purple for a week or two. It is the plant adjusting. Protect young plants with a cloche or fleece for the first fortnight, harden them off gradually, and they straighten up on their own.
Telling them apart in thirty seconds
Where is it? Lower, older leaves only: leaf roll. New growth: herbicide, pests or virus. Is it sticky or crawling? Pests. Is it twisted and fern-like across every plant at once, including ones in other pots? Herbicide. Is it yellow-edged, stunted and spreading plant to plant? Virus. Is the plant otherwise thriving? Leaf roll, and you can stop worrying.
Photograph the leaf and check
If you are not sure, open Pocket Botanist and photograph the curled leaves. The diagnosis feature returns the likely cause and the first thing to try, and the follow-up chat can ask about your watering and the weather. Free on iPhone and Android.
Sources
- University of California Statewide IPM Program, "Tomato: physiological leaf roll." ipm.ucanr.edu
- Royal Horticultural Society, "Tomatoes: grow your own." rhs.org.uk