Prevent damping-off in Plumeria seedlings by using sterile, well-draining potting mix to avoid soil-borne pathogens. Water the seedlings from the bottom to prevent excess moisture on the leaves and stems. Maintain good air circulation around the seedlings and avoid overcrowding to reduce humidity. Use a fungicide labeled for damping-off prevention if necessary. Regularly inspect the seedlings for signs of disease and address any issues promptly.