Seedlings keel over, collapse and die. Usually there is some discolouration and rotting around the base of the seedling.
Cause
Damping off is caused by several soil and water-borne fungi that attack the stem base, which then becomes thin and brown. It is encouraged by poor light, overcrowded seedlings, cool, humid conditions and over-watering.
Treatment
Always use clean containers, fresh compost and clean water. Sow seeds thinly. After sowing water with copper fungicide, such as Murphy Copper Fungicide as a preventative or at the first sign of attack.