Tomatoes are generally easy to grow, and a healthy plant will provide you and your family with many sweet, juicy tomatoes from midsummer until the first fall frost. But tomatoes need good soil, plenty of sun and nutrients in order to thrive. They sometimes fall victim to insects, which can cause leaves to become curled. The plants' health and productivity can also suffer if they contract a disease that causes the leaves to curl.
When tomato plants are small, they benefit from one to two applications of a high nitrogen plant food. As soon as your plant starts to form blossoms, stop giving it nitrogen because it causes the foliage to grow at the expense of the flowers, which will soon turn into fruit. Too much nitrogen will cause leaves and stems to become very dark green and thick. While this might look like a good sign, curled leaves are another effect of excessive amounts of nitrogen. Choose a plant food specially designed for tomatoes, or spread compost around their root zone instead of using fertilizer.
Limit Excessive Water
Tomato leaves can develop a curled appearance when the plant is in soil that stays wet longer than it should. Summer rains can cause leaf curl, according to Texas A&M University. If you rely on irrigation to keep your tomato plants healthy, always allow the soil to dry out between waterings. Plant your tomato in a basin, which you can flood to water your plant deeply once every week or 10 days if needed
The small sucking insect called a psyllid attacks tomato plants, but does not necessary present a problem every year, according to the University of Colorado. Because the psyllid sucks out the plant's juices, it loses the ability to maintain turgidity in its leaves---they become yellow and curl under at the edges. When this occurs, your tomato plant can be stunted and may produce little or no fruit. Aphids are a common insect pest in vegetable gardens and also can cause tomato leaves to curl. If you think your plants may be infested with psyllids or aphids, spray with insecticidal soap. Follow label instructions and repeat every few days until you see signs of healthy new leaves emerging.
The tomato leaf curl virus (ToLCV) can attack tomato plants. The plant becomes stunted, and leaflets curl and grow increasingly smaller, exhibiting a puckered appearance. Whiteflies cause this disease and help it to spread from plant to plant. Hang yellow sticky traps among your tomato plants to keep whiteflies away and prevent the virus from being introduced. Also, choose a hybrid variety of tomato that is bred for its resistance to this virus.