See:
Fir, Douglas and True - Damping-off
Note: Several soilborne fungi including Fusarium, Pythium and Rhizoctonia species have been reported to cause damping-off. Seedlings are infected when favorable soil moisture and temperature conditions and seed or root exudates are present. Seedlings fail to emerge from soil or rapidly wilt shortly after emergence. Seeds may rot before they germinate (preemergence damping-off). Careful attention to planting conditions is necessary including seed depth and conditioning, media moisture, temperature and pH, and clean pots or trays. Although preplant fumigation is commonly used in conifer nurseries, cultural practices can reduce the need to fumigate. Fungicide treatment should be preceded by an accurate diagnosis since most of the chemicals are not effective against all pathogens that might cause damping-off.