I planted my 30 cm high, healthy, happy tomato plants into the polytunnel, but a couple of days later they all (bar 1) had gone rotten at the base of the stem. No brown marks, the leaves seem quite normal. I can't find anything that seems similar except dampening off, I didn't know it could affect plants as large as these.

They were transplanted from 7cm pots to 30cm pots with a very rich mix of organic compost and soil. These were then sunk into the border of the polytunnel. The tomatoes themselves were planted very deeply so they could root all the way up the stem. Could the mix I planted them into have been too rich?

The polytunnel was particularly hot and moist, even with the door and the back window open there was no wind.

Any ideas please, I'm about to plant up some of the remaining plants. What precautions should I take?