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). Some gardeners have good luck protecting their crops by spraying or dousing the leaves with compost tea. There are beneficial fungi in the tea that compete with the blight and keep it from developing. Gardens Alive offers a product called Soap Sheild that can be applied as a preventive measure (contact them at email@example.com or call 812/537-8650).
Another possibility is Fusarium Wilt. When the lower leaves get brown and die, followed by the upper shoots. Eventually, the whole plant goes. Generally it occurs on one side, then the other. By slicing the stem lengthwise near the soil line you can see some dark brown tissue about an 1/8th of an inch under the surface.
There are many strains of the Fusarium fungus, and each strain is specific to a different type of plant. 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. It spreads via the water-conducting vessels in the plant. When these vessels get clogged it prevents nutrients from getting to the leaves and fruit.
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 fusarium wilt. This will be indicated on the plant descriptions and seed packs. If neither of these fit all of your symptoms, please write again with more detail.
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