First time growing tomato plants from seed this year, but the seedlings have brown marks on the cotyledons. I've put the affected seedlings aside through fear of blight, but honestly have very little idea what it could be.

From what I've read, I've been keeping the seedlings in a small (think 1.5m high, 1m across) plastic greenhouse to keep the temperature up. I keep the door open during the day for a breeze, but closed at night. It can get very humid in there during the day. I water the ground around the plant, but have noticed water droplets on the leaves from time to time, perhaps condensation...

Any ideas?

[![Tomato Seedling with Brown Marks](https://i.stack.imgur.com/BfRPB.jpg)](https://i.stack.imgur.com/BfRPB.jpg)