Diseases and Fungi

• Diseases like Early blight can strip tomato leaves of their color and cause yellowing. The symptoms of this disease become especially noticeable during the end of spring and the summer season, when the weather is warm. In addition to yellow leaves, Early Blight also causes dark brown or black spots on leaves, and can spread to the stems and fruit if not treated. To get rid of the disease, it's best to clip the affected leaves, remove them from the planting site, and refrain from planting tomatoes there the following year. A fungal infestation like Fusarium Wilt could also be the reason for yellow tomato leaves. This condition also results in browning of the inside of the stems, and according to the Growing Tomatoes website, there isn't a fungicide strong enough to get rid of Fusarium Wilt. Tomatoes must be removed from the site and not planted there again next season.
  
  

Lack of Sunlight or Soil Nutrients

• When the leaves at the bottom of a tomato plant begin to yellow, this is simply a sign that the leaves aren't getting enough sunlight or nutrients from the soil. In most cases, this is nothing to worry about, since yellow base leaves often occur after the fruit has matured and is ready to be picked. Sometimes tomato leaf plants will begin to yellow when there isn't enough nitrogen in the soil. This can easily be remedied once you check the soil's nitrogen levels. If the levels are deficient, add organic matter or nutrient-rich soil enhancements from your local gardening store, and your leaves should return to their lush green color shortly.
  
  

Insects

• Aphids or whiteflies like to feed on tomato plants. As they suck the vitamins and nutrients from the stems and leaves, this changes the plants color from a healthy green to a pale yellow. These insects also leave a residue on tomato plants known as "honeydew", which makes the leaves sticky and shiny, but the actual tomato fruit usually remains intact. Psyllids, on the other hand, can stunt the growth of the tomato plant after sucking the juices from the leaves and stems. The leaves become yellow and start to curl, and it is common for a plant affects by psyllids not to produce any fruit at all.
  
  

Prevention and Treatment

• To keep your tomato leaves green, water the plant often. This will also reduce the risk of the tomatoes, leaves and stems becoming pest-infested. However, don't over-water the plant either, as this can cause root rot, which leads to yellow leaves. Treating the soil or leaves with iron chelate will also prevent or treat yellowing if the discoloration is an indicator of a poorly nourished plant. Allowing ladybugs to roam free in your tomato garden will help to cut down on the pests that feed on tomatoes, like aphids, as ladybugs love to eat them.