Q.what causes a white rectangle spots on a holly bush
This has killed two of my shrubs and has spread to the other four. How do I get rid of it?

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:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm
https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm
If the spots are powdery in nature, then it may be powdery mildew, which neem oil will help with as well.