It is called damp-off, a fungus disease that eventually destroys the stem at the soil line and is caused by keeping them too wet and with inadequate air circulation. That allows the fungus to proliferate.

If you search 'damp-off' you'll find both FAQ's and many previous discussions on it all over the forums.

If you catch it early enough it may be possible to save a few of the plants by transplanting them deeply into fresh mix or by cloning the growth tips in fresh mix. But be advised that the plants may already be infected with it once it gets into the plant system.

Dave

Here is a link that might be useful: Damping off discussions

yep

Thanks Dave, I thought damping-off only happened to tiny seedlings. Now I know WHY it happened this year--I used potting mix that had a moisture-retention ingredient inside! I bought it by mistake and didn't realize it was in there, and I probably watered too much. They will all be either planted tomorrow or repotted in new "normal" potting mix.

Suzie

Thanks Dave, I thought damping-off only happened to tiny seedlings

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Most folks would agree with you Suzie but the fact is that seedlings can go down with damping off even at the 4-6 inch size as a picture in one of my tomato pathology books shows.

There's pre-emergent damping off where the seeds are destroyed so no germination, and then post-emergent damping off which can affect newly germinated seedlings up to those several inches tall.

Actually there's three different genera and species of fungi involved and the folks who seem to have the most trouble are those who don't use an artificial mix for seed germination b'c it has the lowest number of those fungi.

Carolyn

I had it happen to many of mine, but potted up put them deeper. Seemed to be alright after that.

I concur with rj. Happened to a few of mine at the 4" stage, and they're OK now.

There is a fairly easy way round all of these problems,even if you discover you have used the wrong mix already , I have done it myself this year too and used a MG potting mix with a wetting agent in it, same results as you ??, however this can be cured and perhaps save any remaining plants that have not shown any signs of the disease as yet -if it is not too late already.
All you have to do ia obtian a block of Coir fibre, or even loose finely ground Coir fibre and sprinkle it on the existing suraface of the container the young plants are in, to about a depth of half an inch, this then wicks away any surplus moisture and prevents any form of damping off disease, naturally it is of course always best to buy compost with the coir fibre already incorperated in the mix all the way through-you get no problems with damping off after that, Coir fibre is ground up Coconut fibre that surrounds the nut, the best source of coir fibre bricks or blocks is a company called (Fertile Fibre)-it's on the internet.

After treatment you just water the trays as normal, as the coir fibre will take care of the rest.you may notice one or two little plants still keel over with it later on-these are the plants that were already just infected with the disease before you treated them,but not yet showing.

Here is one method of killing damp-off in the soil and your plants published by Oregon State University Extension. Damp-off can certainly kill off a stand of seedlings because it shows up only when the seedlings rather suddenly keel over. I've never had it but I know someone who lost all her tomato seedlings because of it. I'm sure her soil was too cold (and wet) since she didn't use heat under them.

Here is a link that might be useful: Damping-off in Vegetable Seedlings

Mickeyfinn, I happen to have two blocks of coir so I will try that on the plants I will not put out in the beds yet. You say to sprinkle the coir on top, I am assuming you mean after the block has been moistened with water, right?

Another thing wrong I did this year, that I have never ever done, is put several pots close together in a shallow container so that they could all be moved from one location to the next while I was hardening them off. They are now all far apart and kept dry. Thanks everyone!

Suzie