You are right to be worried about damping-off disease. Once you've gone to the work of filling flats with potting mix and seeds, it's horrible to wake up one day and find your seedlings dying from this fungus, which causes them to collapse at soil level and fall over. Damping-off fungi live at the soil line, where air meets the soil surface. They tend to attack overwatered seedlings. There's no effective treatment for damping-off once it occurs-your best bet is to prevent it. Use sterile planting containers (use new ones, or scrub recycled containers with a solution of 1 part chlorine bleach to 9 parts water.) Fill the containers with sterile potting mix, and maintain good air circulation by installing a small fan near seedlings to keep air moving. Avoid excess water. If your plants are already infected, you may be able to save some seedlings if you allow the soil to dry out between waterings.