Holly Bushes Covered With White Scale
I have a Holly Bush hedge which has developed white spots mostly on the underside of the leaves. The spots are approximately 1/4 inch long, and 1/8 inch wide, powdery in appearance. Some leaves have 10-15 spots on each leaf.
Certified GKH Gardening Expert
It is possible that it is scale or mealybugs affecting your holly. I like to treat both with neem oil. It is very effective and safe. These articles will help:
However, the white spots may also be powdery mildew, as you stated they were powdery in nature. Again, neem oil would be effective in treating this as well.