In the past two years I've had a problem with my tomatoes. The plants start out fine but eventually the leaves start getting brown spots, dry up and fall off.
Several fungal diseases such as early and late blight, septoria leaf spot and anthracnose, can cause the symptoms you describe. Minimize the problem by rotating your plantings whenever possible. Move your tomatoes to an area of the garden where unrelated crops, such as beans, lettuce or onions, were growing last season. Always clean up and dispose of tomato and other disease infected plant material in the fall. Mulch the soil to help keep soil borne fungal spores off the plant. Properly space and stake or tower the plants for better air circulation and fewer diseases. This is often enough to keep these diseases under control. As a last resort you may choose to use a fungicide. Select one labeled for food crops and apply at the first sign of the disease. Repeat applications are needed in wet seasons. Be sure to read and follow all label directions carefully.
Photo: Harry F. Schwartz, Colorado State University