Answer: With tomato plants, the initials VFN and often also T are used to indicate disease resistance to Verticillium, Fusarium, Nematodes and Tobacco Mosaic. These are all problems that widely affect tomatoes. If you see the initials on the plant tag or seed packet, that is a good thing.
If these problems are prominent in your area or you have had them in your garden in the past, you should really plant only resistant varieties. You should also rotate your plantings so that tomatoes (also the closely related plants -- peppers and potatoes and eggplants) are not planted in that spot again until several years have elapsed. Another step is to always make a thorough clean up of all plant debris at the end of the season and dispose of it in the trash. This will help to reduce the chances of carryover of pests and diseases from one year to the next.
If you do have a problem with your tomatoes, I would strongly recommend you contact your local county extension to obtain a specific diagnosis and then based on knowing that, determine how to proceed. If it is something that can be treated, they will have the most up to date recommendations on what to use and how/when is best to apply it.
Good luck with your tomatoes this year!
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