Answer: Let's start first with tomato culture. Plants need full sunshine all summer and a reasonable amount of water to remain healthy. It's natural for some of the lower (older leaves) to begin yellowing late in the season. If your plants are wilting suddenly, or if the leaves turn brown, then you might be dealing with a disease. It's difficult to tell from your description.
Tomato plants are either determinate (set blossoms all at once), or indeterminate (continue to set blossoms all season). On the indeterminate varieties it's natural for blossoms to grow but not set fruit late in the season if nighttime temperatures dip below 55F degrees, or if there has been no insect activity because of cool weather.
Next year plant tomatoes in a different garden spot (just in case this year's plants were diseased), and make sure they get all day sunshine and plenty of water. Try growing some early summer flowering plants near your tomatoes to encourage insects to visit your garden.
If you're following these guidelines already, your tomatoes are probably suffering from a fungal wilt disease, such as Fusarium or Verticillium Wilt. These fungi are present in the soil, so the best way to avoid the problem is to relocate your tomato patch to a new spot, and to keep rotate the planting on a 3-5 year schedule. There are a number of tomato varieties are resistant to both diseases, though resistant means they are less vulnerable than other varieties.
Best wishes with this year's tomatoes!
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