Mildew killing plants
This is powdery mildew. Take a look at our web page about powdery mildew, which offers recommendations for managing this disease. https://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs
You can use a horticultural oil product labeled for powdery mildew control. Follow the product label instructions.
Clean up fallen infected leaves as best as you can and put them in the trash. This sanitation step will help reduce locations for overwintering fungal spores. If you have had persistent problems with powdery mildew on certain plants, it might be because they are not in an ideal location (e.g., not enough air circulation). Check plants regularly for early symptoms of mildew and apply horticultural oil as soon as you see the first symptoms.