The Sclerotinia fungus that cause damping off is present in most soils. It may be transported from place to another by garden tools. Its primary targets are seeds and young seedlings, as the plant ages, it becomes more resistant.
When the weather is warm and humid and the soil is very wet, the conditions are perfect for the fungus. There is no warning that a plant is infected with damping off disease. One day the seedlings are fine and the next day they are bent over. Once damping off has started, nothing can be done to stop it. Prevention is the key.
If you live in cooler climates and start your seeds indoors, use fresh sterile potting soil that drains well. A thin layer of sand or sphagnum moss on the surface of the soil will help to keep the seedling's stem dry and reduce the chance of fungal growth. Damping off fungi thrive in nitrogen-rich soil. Use a mist of chamomile tea to acidify the surface of the soil or dust with fine peat moss.
Specimen plants which need a warmer climate zone than you have do well in sheltered, south-facing walls. The wall acts as a solar collector, absorbing heat during the day and releasing it at night, creating a small zone that is warmer than the rest of the garden.