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. Unless you used garden soil in your planters, I doubt that fusarium or verticillium wilt are the culprits. 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. Use fresh soil-less medium in your planters. Sorry I can't be more helpful!
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