Hi, PollinaterPlanter! It sounds to me as if they got some kind of wilt. There are several kinds of wilt, bacterial and viral. Unfortunately they aren't something that can be treated. The virus or bacteria can remain in the soil and contaminate the next tomatoes planted there, so it's best to dump everything and start over.
There are many tomatoes now that are resistant to wilt; when buying tomato plants ( hybrids) look at the letters after the name.
Here's the list of the resistance letters.
AB Alternaria. ( early blight)
ASC Alternaria stem canker
BER blossom end rot
BW bacterial wilt
CF Cat facing
CLM Cladisporum leaf mold
CR crack resistance
CS crease stem
FW1 fusarium wilt race 1
FW 2 fusarium wilt race 2
GW grey wall
GLS grey leaf spot
LB late blight
NHR nail head rust
RKN root node nematode
SLS septoria leaf spot
ST stemphylium spot
SUN sunscald
TMV tobacco mosaic virus
VW vercillium wilt
Now obviously it would be really had to find a tomato with all these attributes! I choose tomatoes resistant to problems in my area, and some of these qualities are pretty standard for any tomatoes that have been out for a few years.
The ones to look for are the wilts and the spots. So here's an example.
Mortgage lifter VFN. Resistant to vercillium wilt, fusarium 1, and root node nematode. The single N indicates root node. A tomato that was listed with 2 Fs would be resistance to both fusarium races.
Most well known hybrids are resistant to vercillium, fusarium and nematodes, these were the first resistances developed.
I plant in very large containers; I can't say the gallon size, but I do know it takes nearly 2 40# bags of product to fill. I use the best quality top soil I can find, and either compost or compost/manure mix in a ratio of 50/50.
In the planting hole I put bone and blood meal, a handful of very finely crushed eggshell, some Epsom salts, two regular aspirins, crushed, and a low number slow release granulated fertilizer, I think it's 3-6-8. Then when the plants start to bloom I side dress with the tomato fertilizer. And that's it.
Have to water often even in large pots. Not too wet and not too dry. Tomatoes like even moisture. Put cages or stakes in place when you plant. Mulch really is needed, keeps the soil surface cooler, holds in moisture, keeps weeds out, but most importantly, keeps water from splashing from the dirt to the leaves; a main way the tomatoes can get infected, specially by fungal diseases. I spray with copper from 10 days from planting throughout the life of the plant. I have to, or I lose my plants to fungal and leaf spot diseases.
