Hydrangea flowers are prone to diseases caused by leaf spots, powdery mildew, rust, drought, and heat stress. Leaf spot occurs in humid conditions due to a fungus that spreads through spores. Avoid overhead watering to control it. Powdery mildew appears on warm days and cool nights and can be prevented by reducing humidity and increasing air circulation. Keep the plant healthy by removing dead or diseased material and providing a well-draining soil, adequate watering, and nutrients.