Answer: I'm so sorry your plants aren't doing well! Overwatering can cause plants to wilt, because saturated roots eventually die, and can't supply the plant with moisture.
There are a number of diseases that can lead to tomato wilt.
Fusarium and verticillium wilt symptoms include wilting, and a yellowing of leaves from the
bottom of the plant upwards. If you cut through a thick
basal stem, you'll see dark streaks in the center. These are soil-borne fungal diseases that must be prevented since they cannot be cured. The best way to avoid the problem is to relocate
your tomato patch to a new spot every year, and to keep rotate the
planting on a 3-5 year schedule.
If you check each tomato variety listed in the catalog, you'll
notice that some say "disease resistant, resists verticillium
and fusarium wilts, etc." It's very important to choose varieties that specify that
they are resistant to fusarium.
Note, however, that even resistant plants can contract
fusarium, especially if you have nematodes in your soil that
create wounds in the roots. It is very important to destroy
your infected plants. Do NOT put them in your compost.
Bacterial canker, which shows up first as wilting, and develops into small
white spots with dark centers on leaves, and brown,
cracking stems. There's no cure for it -- you need to get rid
of the plants. The disease can be spread from infected seeds
or plant debris. Bacterial wilt is sneakier. Plants wilt in the
afternoon, and by morning recover, and this cycle continues
until an entire side or stalk of the plant has wilted. Yellowing
seldom occurs -- leaves and stalks just wilt and curl up. If
you open a stem, it'll be filled with oozing slime (yuk!).
Prevention is the only "cure" for these bacterial wilts. Use
resistant and disease-free varities, and clean up all plant
debris at the end of a season.
I wish I could be more hopeful for you, but at this point, I would suggest pulling the wilted plants and destroying them. Perhaps the disease will not spread to the rest of the plants. Good luck!
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