Virus or Herbicide
- Cucumber mosaic virus causes yellowing in tomato plants. The plants will eventually produce bushy and stunted growth after contracting the virus. This virus actually starts in the seeds and exhibits itself as the plant matures. Unfortunately, this virus displays the same symptoms as a plant that has been injured by herbicide. One of the few ways to tell if it is viral or the result of a herbicide application is to have a sample tested at a laboratory. If no evidence of herbicide comes up in the results, you must remove the plant from the garden.
- Several diseases can infect tomato plants and cause yellowing, including early blight, alternaria canker and powdery mildew. Most bacterial and fungal diseases spread through the wind or pruning tools with residue from an infected plant. Hot and humid growing conditions cause these diseases to spread. Generally, pruning diseased areas of the plant and applying a fungicide will help control the disease in your yard. To prevent future diseases from popping up in the yard, it is important to water your tomato plants early in the day so that the water evaporates. Wet leaves exacerbate problems.
- Spider mites are a common foe of tomato plants. Gardeners have problems seeing these tannish mites without using a magnifying glass, but you will notice small yellow spots on your plant caused by the insect. In addition, you may also see webs created in the foliage or on the stems of the plant. It is important to remove these mites with hard sprays from the gardening hose. If water doesn't wash off the spider mites, you can use an insecticidal soap to kill these insects.
- Sunscald or sun-burnt fruit occurs when the plant is exposed to too much sun. Sunscald will often occur after a gardener has heavily pruned off the leaves of the plant. If your plant has had to be heavily pruned, it is important to dig up the plant and transplant it to a shadier location. You can also place the plant in a pot filled with potting soil and keep it under a tree with dappled shade.