Answer: First of all, I'll assume that you have given the plants adequate water--keeping them moist but not soggy, and that they get good sun.
You did not say what variety of tomatoes you planted. Some varieties are more resistant to wilt diseases and viruses than are others. In the future, buy plants that are labelled as resistant to these problems.
The letters V, F, N which often follows a variety name and refer, respectively, to that variety's resistance to verticillum wilt, fusarium wilt and nematodes. When two of the same letters occur together, such as FF, it denotes resistance to 2 'races' of the disease. (T) stands for tobacco mosaic and (A) means resistance to alternaria leaf blight.
Once tomato plants have started wilting from a disease problem, there is nothing that can be done for that plant. You should pull it up and get rid of the plant. Do not add it to the compost pile.
The varieties I have had the best luck with are Celebrity and Better Boy. Both have resistance to all the wilts listed above. These are large tomatoes. The Better Boy needs staking (indeterminate). The Celebrity is determinate and needs support, but staking is not required.
Best wishes with your garden!
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