Most people equate a healthy plant with vibrant green coloration, even if they don't know why healthy plants stay green. The green coloration of your tomato plants means that its cells are hard at work at photosynthesis, producing energy to power new growth, flowering and the development of juicy tomatoes. When leaves begin to turn yellow, it could be the first sign of a problem.
The leaves of a green plant are like little power plants, pulling carbon dioxide from the air, absorbing sunlight and producing energy. However, in order for all of this to go on, leaves must open tiny pores called stomata, through which they also lose water. When a plant is stressed, yellowing leaves are often the first sign. Yellow leaves often mean that the plant isn't getting enough water or nutrients or that it is losing leaves to protect itself from water loss.
There are many reasons why tomato leaves turn yellow. The Colorado State University Extension lists yellow leaves as a symptom of nearly every disease and pest that afflicts tomatoes, including curly top virus, psyllids, early blight, whiteflies, aphids, cucumber mosaic virus, Fusarium wilt, flea beetle, tomato hornworm, and septoria leaf spot. Deficiencies in iron or nitrogen or excesses of other minerals may cause leaf yellowing. To further complicate matters, yellowing is a normal part of the aging process for mature leaves.
Plants get their green coloration from chlorophyll, a chemical in their leaves that allows them to convert sunlight to food. When leaves turn yellow, it means that chlorophyll is no longer present, so that leaf is no longer producing energy. Even if the plant survives whatever is causing the yellow leaves, decreases in energy mean lower tomato yields.
With so many possible causes, identifying the exact cause of yellow leaves is important in order to find a solution. The University of Missouri Extension recommends first determining common problems for tomato plants in your area. Contact your local extension office to find out what pests and diseases are commonly reported and consider those first when making your diagnosis. Inspect the plant closely for additional signs. For example, you may be able to observe damage caused by insects or the insects themselves. Early blight causes leaves to yellow but also produces spots. If you need help, your local extension office can provide advice and services to help you diagnose and treat common tomato problems.
Just like healthy people, healthy tomato plants are better able to resist disease and insect infestation. Have your soil tested for deficiencies and stick to the fertilization program recommended by your local extension office. Tomatoes are heavy feeders and need a nutrient-rich soil to thrive. Keep your plants well-watered and watch for early signs of problems so that you can treat them before it's too late.