To prevent diseases in Plumeria, maintain good plant hygiene by removing and disposing of any fallen leaves or debris. Ensure proper air circulation around the plant to reduce humidity, which can contribute to fungal diseases. Avoid overhead watering to prevent water from sitting on leaves for extended periods. If you notice any signs of disease, such as spots, discoloration, or wilting, promptly remove and destroy the affected plant parts. Applying fungicides as a preventive measure can help protect Plumeria from fungal diseases.