Diseases can do everything from cause your tomato plant to shrivel and die, to cause a decreased fruit yield. The two most common diseases that diseased tomato plants are often lumped together and called "tomato blight." However, they are actually separate afflictions called Late Blight Disease and Early Blight Disease--different types of fungus that cause spores and infection. You can tell the plants are infected if they have fruit rot, as well as leaf and stem lesions.
Early and Late Tomato Blight
Get rid of last year's tomato debris. This will prevent the disease from carrying over to the new crop.
Water tomato plants at their base only. Do not let the water get on the leaves or fruit because the blight will spread. It's best to water them in the morning so they have time to dry during the day.
Remove all diseased parts of the plant as soon as you notice them. Cut them off with pruning shears. Seal them in a plastic bag or bury them. Do not add them to the compost pile because they can live there.
Apply a fungicide such a copper spray to cure diseased tomato plants. Spray it once a week when tomatoes are susceptible to late blight disease.
Move diseased tomato plants to a warmer location. They need a sunny, dry location. If it's not possible to move them, add a new layer of soil to the top of the planting bed to limit the chances of the tomato blight flaring back up.