Answer: Tomato plants can have a variety of problems with similar symptoms, so I would need a bit more detail to give you a definitive answer. I'm going to assume that the problem isn't cultural (e.g., how you water, fertilize, soil fertility, etc.) and give you a brief description of two diseases that attack tomatoes. If leaves die from the ground up, and the plant dies prematurely while there is fruit on the plant, it might be Early Blight. It's nearly impossible to get rid of it.
But you can manage to reduce the damage done by knowing how the blight fungus works. It lives from year to year on plant material, so be sure to remove all vines and fruit from the garden at the end of the season and dispose of it. Look for varieties listed as "resistant to early blight" or "alternaria resistant." (Burpee's Northern Exposure, Big Beef and Celebrity are resistant varieties). Pay close attention, and as soon as you see spots forming on the lower leaves, remove them. Also, separate plantings of tomatoes from potatoes. Potatoes can pass the disease on to tomatoes. Follow the same practices for getting rid of potato vines as you do with tomatoes.
Another possibility is Fusarium or Verticillium wilt. If the wilts strike, the leaves curl up turn yellow and drop off. Fusarium wilt will stay indefinitely on plant debris or in the soil. It enters the plants through the roots and can be spread by seeds, tools, soil and plants. There is no control available. You need to destroy the infected plants. The best solution is to rotate the crops (don't plant the same crop in the same place each year) and use plants that are resistant to wilts. This will be indicated on the plant descriptions and seed packs with a V for resistance to Verticillium or an F for resistance to Fusarium wilts.
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