Damping-off disease (early seedling death), DOD.
This is one of the commonest forms of seedling loss, yet it is very hard to find a useful googleable access. If you find this article helpful please comment to keep it " in the headlines".
Damping-off disease is a general term to refer to a number of conditions that result in early seedling death. In a typical case seedlings will germinate then fail to thrive, eventually collapsing about a week after germination, though DOD can cause death as much as a month later. There are a variety of causes, both bacterial and fungal, which it is usually impossible to identify specifically. The only effective treatment is prevention as, once a seedling is already showing symptoms of DOD, it is usually too late.
The most effective treatment is complete sterilization of the growing compost before sowing. This can be effected in a number of ways but principally the temperature of the compost must be raised to close to 100C, 212F (boiling) for at least five minutes. The compost should be dampened before treatment. Small quantities of compost can be sterilized very effectively in a microwave oven. Chemical sterilization of compost is not very effective.It is not wise either to rely on commercially purchased compost being sterile.
After sowing keep the sowing area very well ventilated, if necessary use a fan. This will reduce the number of fungal spores ( which are present in any normal atmosphere) that settle on the compost. Water with a dilute solution of Bordeaux mixture (Copper Sulphate) for at least a month. Other commercial mixtures are available (though expensive) but do not rely on them without pre-sterilization. Small seeds are the most vulnerable.
Good luck - Ian.