This has killed two of my shrubs and has spread to the other four. How do I get rid of it?
What Causes a White Rectangle Spots on a Holly Bush
Certified GKH Gardening Expert
It is possible that it is scale or mealybugs. I like to treat both with neem oil. It is very effective and safe. These articles will help:
If the spots are powdery in nature, then it may be powdery mildew, which neem oil will help with as well.