Cotyledons falling off tomato seedlings

[cgantt]

They did not turn yellow. I spoke with a horticulturist over the weekend and he too thought it might have been a pest but the fact that none of the other seedlings were touched boggled his mind.

[dice]

A bowl of wilted lettuce or grass clippings or some other
decaying vegetation set nearby might tell you whether you
actually have pillbugs in there. (Lettuce is good for that because
it stays moist while it rots without need of sprinkling or misting
or anything.)

[erlyberd]

You mentioned watering twice daily so that may be your problem. So, if you have to ask if your overwatering you probally are. Tomatoes like to dry a bit between watering. At least that is how I do it. Do you have good drainage? I tell people to picture a kitchen sponge. Its either rock hard (dry), wrung out (perfect), totally soaked/submerged (too wet). Also, pick up the pots, do they feel heavy? Light?

[RayR]

I'm having a similar problem with some of my seedlings. No damping off, the necrosis starts at the tip of the cotyledons and progressively works its way down, they become dry and brittle and the cotyledons finally fall off. It starts before the true leaves start to emerge. The problem is mostly happening with Brandywine and Prudens Purple, where most seedlings have been affected. I've been watching this for a week now and the few seedling of those varieties that are healthy have shown no such symptoms.
Other varieties like Eva Purple Ball, Red Brandywine, Boxcar Willy, Chocolate Cherry, Gardeners Delight, Super Sweet 100 F1, Big Rainbow, Rutgers ect.., show no symptoms at all. Only one seedling each of Black Krim, Delicious, and Big Boy F1 have shown this dying cotyledon problem. No fertilizers were applied before the problem started, just rain water. I watered with a weak fish/seaweed dilution only when the true leaves started to emerge.
I expect to lose a few weak seedlings along the way, but have never had this many die in two specific varieties. Anybody got a clue?

Here's a pic of Prudens Purple, on the bottom left and the one above it is are where you can see the cots drying up, the one on the bottom right and the seedlings on the very top have lost both cots completely. You can see the other three in the center that are perfectly fine.

[cloz]

I have had the cots dry up and fall off on Brandywine, Marianna's Peace, Wes and German Red Strawberry after up potting. In my case I suspect I was a little heavy handed with the water. Most others did not have that problem.