Oh my, now that I see a moldy stem I remember another time - only four of about a hundred were affected and they went down hard like that. I didn't know what the cause was, so I isolated them and they didn't die but at least 3 of the 4 were too stunted to bother planting out.
I think the mold is a sign of serious vascular damage in the main stem - I wouldn't waste time planting them deeper or trying to save them. And I would keep them away from others, in case it has a chance to spread.
I am remembering that I had signs of vascular trouble on other plants after plant out that year as well - swellings at the leaf nodes for example - but at the end of season when I cut them down there was no sign of a vascular disease having come from the soil - stems were solid and healthy at the base. So it was another tomato mystery unsolved - other than obvious problems with my container mix that year being the wrong stuff entirely for containers.
At the time the seedlings went down, I had also been peeling a bad batch of potatoes around the time I potted up, so I was suspecting some potato disease had gotten to them. I never found any proof of that though, and in the end I could only conclude it was physiological.
Another suspect in these potting incidents, for me, is lumps of peat in the mix that were not at all hydrated. The bulk seems all good but there are these super-dry pellets hidden away... I think it can be bad news when tomato roots get around those, and it sucks moisture out of them. But again I am speculating... When I read about edema/intumescence having multiple causes I understand how difficult it is to pinpoint one thing.
One thing for sure, those four seedlings were potted up at the same time into the same mix, as well as being the same lot of seeds (while three others were fine). So what happened to the tail end of that mix, what was different? Nothing fundamentally. Could it be unsoaked bits of peat?
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