Tomato leaf curling is a common symptom that can point to a number of different underlying issues. This distortion is not a disease itself; rather, it is a protective reaction or a sign of damage, and the specific pattern of the curl helps diagnose the root cause. Distinguishing between a benign environmental stressor and a serious, systemic infection is the first step toward effective treatment and plant recovery.
Environmental Stress and Physiological Leaf Roll
The most frequent cause of tomato leaf curling is Physiological Leaf Roll (PLR), a non-pathogenic condition and the plant’s natural defense mechanism. When a tomato plant experiences stress, it rolls its leaves upward and inward along the mid-vein to reduce the exposed leaf surface area. This action helps the plant conserve moisture by limiting transpiration.
PLR is often triggered by inconsistent moisture levels, such as a cycle of drought followed by heavy watering, or from excessive heat, especially temperatures exceeding 95°F. The condition typically begins on the older, lower leaves of the plant, causing them to become thickened and leathery, but they maintain a healthy green color. Indeterminate (vine-type) tomato varieties are more susceptible to PLR than determinate (bush-type) counterparts.
Excessive pruning can also induce PLR by creating an imbalance between the root system and foliage, leading to stressed growth. Unlike diseases or pest damage, this environmental curling does not usually affect fruit production or overall plant health, though it can persist throughout the growing season. The upward curl on the oldest leaves, without yellowing or stunted growth, helps differentiate this harmless condition from more serious problems.
Sucking Pests and Insect Damage
Curling can result from feeding by small insects that extract sap from plant tissues. This damage is typically localized and concentrated on the newest growth, leading to distorted, twisted, or stunted leaves. Sticky residue, known as honeydew, often indicates an infestation of Aphids, which are found near growing tips and the undersides of leaves.
A subtle but devastating culprit is the Broad Mite, which is too small to see without magnification. As these mites feed on tender new leaves and flowers, they inject a toxin that causes a severe downward curling, twisting, and bronzing of the foliage. The damage appears concentrated at the growing point and can be mistaken for a viral infection due to the extreme distortion of the new leaves.
Systemic Viral and Bacterial Infections
The most concerning cause of leaf curling is a systemic infection, meaning the pathogen has spread throughout the plant and is often irreversible. Tomato Yellow Leaf Curl Virus (TYLCV) is a destructive example, characterized by a distinct upward cupping and rolling of the new leaves. Plants infected with TYLCV become severely stunted, with new leaves being significantly reduced in size and often exhibiting yellowing along the margins or between the veins.
This begomovirus is transmitted by the silverleaf whitefly (Bemisia tabaci), which acquires the virus from an infected host and spreads it as it feeds on healthy plants. Once a plant is infected, the virus prevents normal growth, leading to a drastically reduced fruit set, especially if infection occurs before the flowering stage. The plant’s vascular system is compromised; since there is no chemical cure, the only management strategy is immediate removal to prevent further spread by whiteflies.
Bacterial infections can also cause leaf distortion, such as the wilting and collapse caused by Ralstonia solanacearum, the bacterium responsible for bacterial wilt. While the primary symptom is wilting, which can mimic curling, the key diagnostic is the sudden, rapid nature of the wilt, where the plant collapses while the leaves are still green. This soil-borne pathogen clogs the plant’s xylem, blocking water flow and leading to the plant’s rapid demise, particularly in hot, moist conditions.
Accidental Herbicide Exposure
A distinct cause of curled foliage is exposure to broadleaf weed killers, even in minute amounts. Tomatoes are extremely sensitive to synthetic auxin herbicides, such as 2,4-D and dicamba, which mimic natural plant growth hormones. The resulting damage is unique: new growth exhibits severe, downward cupping, twisting, and strap-like distortion, a process known as epinasty.
This damage often occurs from herbicide drift, where the chemical is sprayed on a nearby lawn or field and carried to the plants by wind or volatilization. Symptoms can appear within hours or days of exposure, dramatically affecting the newest, most actively growing tissues. While low exposure may allow the plant to grow out of the damage, the initial injury is a clear sign of external chemical contamination.