Damping off is a situation caused by a variety of naturally occurring fungi which attack sprouting seeds and newly emerging plants. It can also be problematic with plants set into the ground before the soil warms.
The fungi thrive in wet soil.
Sprouting seeds and slow growing or weak plants are more at risk than vigorous plants. Cool soil temperatures slow seedling growth and give the fungi an edge. Even if it does not kill the seedlings, it may stunt the plant's growth.
If an attack occurs, but the seedling can get its roots established faster than the fungus can decay them, the seedling will survive and be healthy.
To control damping off, there are things you can do!
1: Avoid soil that is too cool for germination and plant growth to happen quickly.
2: Pasteurize your soil: Place soil in a pan, 3 to 4 inches deep. Place in an oven preheated to 200 degrees. Check the soil temperature with a meat thermometer. When the soil reaches 160 degrees, turn the oven off and leave the door closes for 30 minutes. (You are not attempting to sterilize the soil here. That causes other problems.)
3: Clean tools, containers and work areas with a 10% bleach solution.
4: Put your newly planted seed trays or pots (indoors) in a warm location for sprouting (65 - 75 degrees). You can use a sprouting mat for this purpose, but I find that the top of the fridge or the room where the woodstove is located is sufficient.
4: Grow out your indoor starter plants in a warm, sunny location and/or under plant lights.
Wait for soil temperatures to warm before planting or transplanting. Ideal temperatures are 50 degrees for cool weather crops like cabbage, kale, broccoli and 65 - 70 degrees for tomatoes and peppers.