See:
Petunia (Petunia spp.) - Damping-off
Notes Damping-off refers to a disease that develops on the seed, radicle, hypocotyl, epicotyl, or stem of an emerged seedling. Damping-off is caused by many organisms including Pythium spp., Rhizoctonia spp., and Fusarium spp. It is difficult to determine which organism is involved by casual observation so send into a Plant Clinic for an accurate diagnosis. Poor stand development or seedling collapse are indicative of damping-off. Careful attention to planting conditions is necessary including seed depth and age, media moisture, temperature and pH, and clean pots or trays. Chemical treatment should be preceded by an accurate diagnosis since most of the chemicals are not effective against all pathogens that might cause damping-off.