Damping off can be caused by species of Pythium, Phytophthora, or Rhizoctonia. A laboratory diagnosis is required to determine the exact cause of damping off of seedlings and cuttings.
Phytophthora, like Pythium, is a a lower fungus favored by excess moisture and excess nitrogen fertility. Unlike Pythium, species of Phytophthora are more aggressive, more likely to be host specific, and less frequently found in greenhouses. The most likely source of origin is plant material.
Start with soilless growing media and avoid contaminating media with soiled hands, tools, or containers. Promptly remove diseased plants, avoid splashing water when irrigating and keep hose ends off the floor. The best means for controlling Phytophthora is with drenches of systemic fungicides which will move up into the crown area such as mefenoxam (Subdue Maxx), foestyl-Al (Aliette) azoxystrobin (Heritage), or phosphonate-Al (ProPhyt). Check label for crop appropriateness. Rotate fungicides to prevent resistance development.