Mangetout seedlings: 'pinching' and dying
I' an experienced veg grower but haven't encountered this before. I planted some mangetout seeds undercover a couple of weeks ago (I don't plant veg seeds straight into the ground because the slugs would have them practically before they appear) and they quickly produced strong healthy plants. Since then almost all of them have developed a tough 'pinched' area at the point the stem emerges from the compost. They still look healthy but gradually keel over and die because clearly water isn't getting from the roots to the rest of the plant.
They have been kept well watered but have been subject so some fairly extreme diurnal temperature changes in the greenhouse. Could that be the cause? If not, why is this happening?