Yellowing leaves is often a sign of too much water. 40 degrees is a little chilly for the plants, too.

I think daytime temperature is getting higher than you think. Try more ventilation and diffusing sunlight during peak of day. Some heat ballast would also help if temp inside house is falling to 40f.

40F won't kill tomatoes. But it will slow their growth a lot and maybe stunt them. High temps? Tomatoes love high temperatures. True about yellow leaves suggesting overwatering, but it looks to me that these plants have vastly more problems than yellow leaves. Might want to read up on early blight, but if the soil was packaged potting soil, it was probably sterile, so not clear where it could have come from.

Looks like blight or fungus to me. I agree with Dan. Tom's can take a LOT before they die from no water. I had a plant down on the floor and was 100% sure it was dead (missed watering it before vacation). Wish I had a pic. It 80% recovered. I mean a 1 foot tall plant on the ground (in a pot).

Point is vary water as a test could also be to little water.

Thanks to all. I have kept them just to see what happens. I will work with the water levels.

I suspect the problem is due to the heating mat. Once seeds germinate, the heating mat should be removed. The plants are suffering the temp swings, what looks to me like overwatering, and then are also basically having their roots cooked.

I will try that. Thanks Peggy

That's a fair point about heating mats. In principle, those should only be used for seedlings, which is why they're called seedling mats. In the summer, the soil temperature is always cooler than the daytime air temperature. Though I wouldn't expect such serious damage from a little warmth down below. Different mats have different temperatures. For mature plants, the soil temperature probably shouldn't be more than 10F over the air temperature. You might put a pot without a plant on the mat, and measure the temperature at the soil base.

Yes, I have been using the mat, because we have had a cold snap and the nights have been cold, but I can see that it might be too much for the seedlings during the day. And they need to learn to manage... The sickest of the tomatoes are perking up just a bit today. I will not toss them yet, and let you know how they do. It has been a cold and cloudy several days until today....

Well, your pictures aren't of seedlings. Seedlings are with the cotyledons and maybe the first set of true leaves. Your plants appear to be long past that. Heating the soil past seedling stage can also tend to make plants leggy, but yours don't really seem to suffer that. Note that peppers, which thrive in heat, can do well with heat mats under more mature plants if the ambient air is cold.

I agree that excessive soil temperature swings will kill the roots and seriously stress the plants. they also look as if they're over watered but both issues will kill off roots so both issues would look the same. Under watering would look different with branches and leaves doing some serious wilting before turning yellow.

Have the plants always been pale green/yellow, or at one time a nice green and then turned sickly? Do you fertilize your plants because as Dan said they're well past the seedling stage and should be fertilized. In cold soil a lot of plants including tomatoes have a hard time taking up nutrients even if the roots didn't die back and will also make the plants look like yours with plenty of fertilizer in the soil.

Cut off those lower branches because they aren't going to survive anyway to remove some demand on the roots. Stick a thermostat in the plant and see what the temperature is, and try to keep it at between 65 and 85 and fertilize if you haven't already. They still can be saved.

I second the thought that it might be blight. Note the spots on the lower leaf in the 3rd pic and the yellowing and drying-up leaves.

IDK if blight could have survived on the tray and you brought in with the tray; if the tray wasn't thoroughly washed and there was some blight-y soil on it, maybe? If it were me, I'd toss them just to be on the safe side. If it is blight it's going to spread to other plants and isn't worth the risk. It's nasty business.

Agree with others, probably a blight of some type. A few of those pics remind me of when a friend of mind accidently watered his plants with a liquid fertilizer concentrate rather than picking up the diluted bottle.

They all look pretty sickly. Have you been feeding them at all, or are they just in a soilless mix w/o any fertilizer?

I start feeding seaweed extract, compost tea and diluted liquid plant food once true leaves are established. I usually add a little granular fertilizer, not touching the stems - too.

When you don't feed a plant, they don't end up looking like they're on death's door. Their growth just gets stunted.

Is it possible it might be either - too little or too much? I've had plants do that from fertilizer 'burn', but there's no mention of feeding in the OP...

Well, yes, fertilizer burn could have that result.

Hello again everyone, and thanks for your interest. My two worst tomato plants have pretty much died, and all the rest look great. I am going to toss the sick ones very soon. I did put some powdered fertilized in with the potting mix - it is possible these two got an overdose. All the other plants seem to be doing fine, so I am not sure. I guess it might just remain a mystery - as I tell my husband all of the time -- not all of the plants make it. thanks again for your thoughts...

You put powdered fertilizer into the potting mix? Was that according to directions? Most powdered fertilizers are intended to be mixed with water at a strong dilution. MiracleGro, for example, is about a tablespoon or less per gallon. When you water with that solution, most of that fertilizer drains away. This sounds potentially like a killer proposition to me.