I've had a number of Cosmos Rubenza and Antiquity plugs, bought online, developing in small pots for a couple of weeks now and gradually hardening them off. For the last couple of nights I've left them out as it was very mild, but this morning I noticed two had suddenly keeled over with what appears to be stem rot half way up. I'm certain it isn't slug damage.
The plants themselves are about 2-3 inches high and growing well, but with two gone I've only eight left.
What might be the cause of this, because I don't fancy losing anymore. Thank you.