Tomato leaves that curl and turn yellow are usually reacting to stress, not one single problem. Common causes include irregular watering, heat or cold swings, nutrient imbalance, root damage, herbicide drift, pests like aphids or mites, and viruses such as tomato yellow leaf curl virus or mosaic virus.

Most likely causes

  • Water stress: too much or too little water can make leaves curl and yellow, especially in hot weather.
  • Nutrient issues: magnesium deficiency and other imbalances can cause yellowing, especially on older leaves.
  • Temperature stress: cool nights, hot days, or sudden changes can trigger leaf roll.
  • Herbicide exposure: drift or contaminated compost/manure can twist leaves and cause yellowing.
  • Pests: aphids, whiteflies, and mites can distort leaves and weaken the plant.
  • Disease: viruses often cause curled, pale, yellow leaves and stunted growth; infected plants usually do not recover.

How to tell which one it is

  • Older leaves only, plant otherwise healthy: often a watering or nutrient issue.
  • New growth is curled, pale, or stunted: think virus, herbicide damage, or mites.
  • Leaves are thick, leathery, and rolled upward: often physiological leaf roll from stress.
  • Twisted, distorted growth with yellowing: herbicide injury is a strong possibility.

What to do now

  1. Check soil moisture and water deeply but consistently.
  2. Inspect the undersides of leaves for aphids, whiteflies, or mites.
  3. Look for recent weedkiller use nearby or contaminated compost/manure.
  4. Remove badly damaged leaves, but do not prune heavily.
  5. If the plant is stunted and new growth keeps yellowing and curling, remove it to protect nearby tomatoes if a virus is suspected.

Quick read

If the curling is mild and the plant is still growing well, it is often stress-related and manageable. If the new leaves are yellow, twisted, and the plant is stunted, virus or herbicide damage is more concerning.

A useful next step is to describe whether the curling is on new leaves or old leaves , because that usually narrows it down fast.