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Cause of Tomato Leaf Curl

Symptoms / Characteristics: The rolling or curling of tomato leaves can be a symptom of environmental stress, herbicide damage or viral infection.

Physiological leaf roll may be associated with environmental stresses such as excess moisture, excess nitrogen, and transplant shock. Leaf roll may also be related to moisture conservation during periods of extreme heat and drought. Improper cultural practices such as severe pruning and root damage during cultivation can also cause leaf roll symptoms. Physiological leaf roll involves an initial upward cupping of the leaves, followed by an inward roll. In severe cases, the leaves roll up until the leaflets overlap. Symptomatic leaves become thickened and leathery. Symptoms typically appear first in lower leaves but may spread to the entire plant, depending on the severity of the condition. Leaf roll is more commonly associated with staking varieties of tomato, such as "Early Girl" and "Big Boy", rather than bush types. In most cases, the condition is temporary and will have little or no affect on plant growth or fruit production.

 

Tomato leaf roll symptoms may also be a direct result of herbicide injury. 2,4-D is a hormonal herbicide commonly used on lawns to control broadleaf weeds such as dandelion and chickweed. Tomato plants that have been exposed to 2,4-D drift exhibit downward curling of the leaves and overall deformed, twisted growth. Leaf veins are light coloured and very prominent. The vein pattern may be more parallel in symptomatic leaves. Affected stems turn whitish, thicken and often split. Tomatoes may be deformed or "catfaced". Plant recovery depends on the severity of the exposure.

Tomato leaf roll may be associated with viral infection. Tomato yellow leaf curl virus is transmitted by sap-sucking insect vectors and causes leaf roll symptoms in infected tomato plants. Purplish veins on the leaf underside will help to distinguish this virus from physiological leaf roll and herbicide injury. In addition, new leaves appear chlorotic (pale green) and cupped, and overall plant growth becomes stunted. Early infection often inhibits fruit production. Tomato mosaic virus has also been known to cause leaf roll symptoms. Other symptoms of this virus include mottled leaves and smaller than normal leaflets. Infected fruit appears brown on the inside and blotchy on the outside

Control / Preventions: First of all, it is extremely important to examine the symptomatic plants and evaluate the environmental conditions to determine the true cause of the disorder. Once a cause has been determined, an appropriate control measure can be administered. If an entire mass of plants is exhibiting leaf roll symptoms, then the cause is more likely to be environmental stress or herbicide injury. If symptomatic plants are scattered throughout a planting, the disorder is more likely to be of viral origin. A sudden appearance of symptoms is usually indicative of herbicide injury or perhaps environmental stress, whereas a progression of symptoms over time usually indicates viral infection. Spreading of symptoms throughout the planting will also indicate virus transmission by insect vectors. Disorders related to environmental stress and herbicide injury cannot be transmitted from a symptomatic plant to a healthy one.

Preventing physiological leaf roll requires effective water management and a balanced fertilizer program. Avoid overwatering and irrigate during periods of dry weather. Make sure that plants are properly hardened off before transplanting into the garden. Avoid severe pruning of indeterminate (staking) varieties and minimize root damage if cultivating around the roots. Plant growth and fruit production will not be affected if the problem is recognized early and the contributing factor is amended.

Once herbicide injury has occurred, the damage is irreversible. However, plants that have experienced limited exposure will likely recover and new growth will be normal. Never apply 2,4-D (or any herbicide) in windy or dead calm conditions. Contrary to popular belief, dead calm conditions are often associated with a phenomenon known as temperature inversions. Spraying under such conditions can actually increase drift distance. Always follow manufacturer's directions and recommendations regarding application rates, timing, equipment and safety.

Plants infected with tomato yellow leaf curl virus cannot be cured. For most viruses, e.g. the tomato mosaic virus, removing and destroying symptomatic plants effectively prevents further transfer to healthy plants. However, transmission of the tomato yellow leaf curl virus from an infected tomato to a healthy one is uncommon. Source plants are often wild. Keeping growing areas weed-free effectively reduces virus transmission. Removal of symptomatic leaves, stems and fruits will not eliminate the virus from otherwise healthy looking plants and may only lead to further spread via infested tools, etc. Thoroughly disinfect tools and equipment that may have come in contact with the diseased plants. Sap on infested tools and plant parts can be an ideal way to transmit the virus from one plant to another. Controlling large populations of aphids or other sap-sucking insects may help to reduce virus transmission to healthy tomato plants. Avoid unnecessary injury during pruning and cultivating. Keeping the tomato plants as healthy as possible will build their immunity and make them less susceptible to insect attack.

Causes of Tomato Leaf Curl

Tomato leaf curl can be caused by a variety of things of which bugs (usually aphids) are only one. Look at the underside of the leaves that are curling. If you don't see aphids, it will either be normal for the leaves to curl or you have a fungus/viral problem. Tomato leaves curl naturally if it's too dry, too hot, too humid or too windy. In other words, the leaves will tend to curl if growing conditions are not optimum for the plant. If it's too hot and dry, water more deeply. If it's too wet, water less frequently (make sure you water deeply though). You can tell if it's a viral or fungal problem by looking at the entire plant, especially the leaves. Tomato leaves differ in color dependent upon the plant. There are some varieties with deep green leaves and others have lighter green leaves. If the leaves are lighter in color than they should be for that variety of tomato, you probably have a viral or fungal infection. The same holds true if you have spotting on the leaves. Cut off a small branch and take it to your local nursury to see what they recommend for your area of the country.

The oxygen in the air mixed with the carbon dioxide causes the tomato leaf to curl.