Answer: There are a number of possible diseases that cause the symptoms you describe. First of all, I would recommend planting disease-resistant varieties. Secondly, always rotate your garden plants. Don't plant crops from the same family in the same spot more than once every three years, to help break disease and insect life cycles. (Eggplant, peppers, potatoes and tomatoes are in the same family.)
Possible disease include early blight, late blight, and fusarium wilt. My first guess would be late blight. This disease is caused by the fungus Phytophthora infestans and, unfortunately, there are no tomato varieties resistant to this disease. Symptoms include brown to black spots, starting on lower leaves, followed by spots on stems and leaves. To help control the disease, immediately pick off and dispose of infected leaves; mulch plants with straw or hay, to keep water from splashing up--the disease needs a wet leaf surface to spread. Spray plants with a solution made from 1 teaspoon baking soda per quart of water with a few drops of mild liquid soap added. When spraying, cover both upper and lower leaf surfaces. If your plants are very full and lush, consider pruning them a bit to help improve air circulation, speeding leaf drying.
Unfortunately, because this disease is windborne, there is no way to treat the soil to help check its spread.
Q&A Library Searching Tips