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Physiological or nonparasitic leaf curl occurs during wet weather or when transplants are first placed outside. Typically, the leaves roll upward,at first appearing cupped but eventually rolling continues until the leaflets overlap. Overall growth of the tomato is usually not affected. Fruit are not injured by this condition. Another cuases of leaf roll are climatic conditions Nonparasitic leaf roll is caused by an irregular supply of water or severe pruning. It is a temporary disorder in which the edges of the tomato leaves roll upward and inward, even overlapping when conditions are severe. Most leaves on the plant are affected but the condition is temporary. The plant resumes normal growth within a few days following pruning or irrigation.
Herbicide exposure on tomatoes will also cause leaf roll. 2,4-D is the most frequent cause because it is commonly used in lawns, corn, cereal crops, and in pastures.
Tomatoes are susceptible to 2,4-D damage through all stages of growth. The first symptom is the downward curling of the tips and edges of the leaves. New growth is distorted by even minute amounts of drift. Generally, if you call smell 2,4-D you will damage sensitive crops.
When exposure is more severe, stems will become stiff, thick, brittle and develop a whitish external surface. New leaves will not expand fully while the tips of the new leaves will be abnormally elongated. Leaf veins are lighter in color and with parallel veins. Fruit may be abnormally pointed or catfaced and ripe fruit is firmer and harder to remove from the plant.
Curly top virus is transmitted by the beet leafhopper. This problem is common in western Utah. Infected plants turn yellow and stop growing. Upper leaflets roll and develop a purplish color, especially along the veins. Leaves and stems become stiff; fruit ripens prematurely and it has an insipid flavor.
It is difficult to control because leafhoppers migrate from southern areas. Hot, dry springs with predominant southwest winds usually cause problems with this disease. No chemical controls are effective. Use row covers to protect tomato plants from the leafhopper or plant extra plants. Since infected plants will not recover it is best to remove them.