Answer: It is difficult to tell exactly what is affecting your plants. However, there are a couple of things you can try. Always plant varieties labeled resistant. You will see the initials on the seed packet or plant label: V for verticillium, F for fusarium, T for tobacco mosaic. If it is fusarium, buying seed for a resistant variety should stop the problem.
Next, be fastidious and clean up all tomato plant residue during the growing season and in the fall. Remove and destroy it because some of the diseases live in the soil, some on the plant tissues including debris, and some in the seeds themselves. Remove any self sown plants in or near the garden too. If you are growing your own seedlings, disinfest all of your tools, pots, benches, and so on.
Next, be sure to rotate your plantings. Since you already have a problem in your garden, you might use a four year schedule for tomatoes and their relatives.
You can spread some diseases by handling a diseased plant and then touching a clean one, so you might keep that in mind as you work in the garden. I suppose deer could do that too, so you might consider fencing them out.
Finally, healthy plants will be least vulnerable, so make sure your growing conditions are the best possible; you might begin by running some basic soil tests and checking for pH and any particular amendments you need to add. Your County Extension can help you with this. Their phone numbner is 652-3370.
I'm sorry about your plants -- good luck with getting the problem under control!
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