To prevent fungal diseases in Plumeria, ensure good air circulation around the plant. Avoid overhead watering, as wet foliage can promote fungal growth. Water the plant at the base to keep leaves dry. Remove and dispose of any fallen leaves or debris to reduce potential fungal spores. Applying a fungicide as a preventive measure during periods of high humidity or when conditions are favorable for disease development can further protect your Plumeria from fungal infections. Regularly inspect the plant for signs of disease and treat any issues promptly to prevent further spread.