[brokenglass]

Could it be weevil damage i.e. the larvae eating the roots?     Damping off does seem likely

[Fairy Plotmother]

Just had a look at gurgle images for damping off! I think I rest my case m'lord!

[Lardman]

Just had a look at gurgle images for damping off! I think I rest my case m'lord!

Conditions really aren't right for damping off  I don't know.

I've lost another 3-4 cells of the beetroot overnight and a few more in a different tray, there's also a goodly amount of the carrots (3rd sowing) flat on their backs, they were about 2" high yesterday and sturdy plants.

I tipped some of the cells out onto white paper but couldn't see anything moving around and nothing was lurking under the cells or in the gravel trays.

Decent roots on them too, no sign of anything untoward.  Closer look at the damaged area, Doesn't it all look a little too neat to be fungal?

[sunshineband]

It actually look as though the stem has been scorched in some way. Just and idea, but is the compost too rich for the seedlings? The effect of water collecting round the stem with a high solution of "salts" from fertiliser might possibly cause this type of damage,

Clutching at straws a bit, I know, but perhaps worth a thought?

[Aunt Sally]

Put some pieces or fly paper down on the soil near sone "test" plants to see if you catch any visitors.

[azubah]

Actually, cut worm came to my mind.. but it would have to be a very small one to do that. A lot of small ones.

[Fairy Plotmother]

I've investigated several more sources. I am now convinced it's damping off. Is there air movement around your seedlings or are they contained under a propagator?

[Trikidiki]

Your second photo today looks typical of damping off. The stem has collapsed in on itself.

[JayG]

The chances that your one old, and two new composts, are all harbouring pests seem pretty remote to me, which puts me in the 'damping off' camp (unless you mixed them all together of course.  )

Are you using stored rain water or tap water to water them?

If allowed to stand for a day or so before use tap water is a safer bet for young seedlings as it's much less likely to already contain fungal spores and other potential pathogens.

[Lardman]

Are you using stored rain water or tap water to water them?

Clean fresh water, but left for a day or so, so the water is it up to the temperature in the GH. When the compost is dry I stand the cells in water and allow the compost to rehydrate then remove them. The carrot box is watered with a fine rose when it feels light enough to need it - only twice so far. 

Air circulation is good, there's not a lot in the GH (10'x8') at the moment , side & top vent have been open as has the door during the day no sign of excessive condensation and neither the cell trays or carrot box have been covered with a lid and nothing is planted too close together.

Cell trays are new, carrot box hasn't been used for veg before, the gravel trays were all rinsed, although not scrubbed in Jeys, labels vary from plastic to new lollipop sticks.

The only thing not have been cleaned is the watering can 

It actually look as though the stem has been scorched in some way. Just and idea, but is the compost too rich for the seedlings?

Might have been the case for the jacks magic but the carrots are in last years spent compost, just some BFB mixed in the lower section.

I couldn't find my yellow sticky paper yesterday but I'll give that a go - if it's damping off I'm at a complete loss as to what else I can do to prevent it. 

[azubah]

How low does the temperature go at night? It has been frosty here lately and that might contribute to the problem.

[bravemurphy]

Leather jacket.