Black tip fungus is a common issue in Plumeria, and to prevent it, ensure good air circulation around the plant. Avoid overhead watering and keep the foliage dry to prevent fungal growth. Water at the base of the plant to avoid wetting the leaves. Prune off any affected parts and remove fallen leaves promptly. Applying a fungicide with active ingredients like copper or neem oil can help prevent black tip fungus. Regularly inspect your Plumeria for signs of the disease and treat as needed to keep it healthy and fungus-free.