What are white spots on the bottom of its leaves?
These are likely mealy bugs or aphids. These articles will help:
https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm
https://www.gardeningknowhow.com/plant-problems/pests/homemade-aphid-control-a-natural-way-to-kill-aphids.htm
Planted three China hollies about five yrs ago. Have grown well (too well!) - noticed middle one had dead branches. When trimming them off, noticed white splotches or places on the foliage. What's going on and will it go into the China hollies on each side of this ailing bush?
It sounds like it could be scale. This article will help:
https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm
We have found neem oil effective on most damaging pests. Here is more information on neem oil:
https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm
Last year I planted two Chinese holly bushes. The bushes are planted within 3 feet of one another. My soil is heavy clay. When I planted them, I planted them with a mixture of soil conditioner and top soil. They both get the same amount of sun. I checked them two weeks ago and they were doing fine. They are about three feet tall. I checked them again about two days ago. One looks fine. The leaves are green and glossy. The leaves on the other bush are brown and crisp looking. What can the problem be? Can I do anything to stop the browning? I live in zone 6.
Have you had changes in weather? Increases in temperatures can cause browning of leaves.
Here is a link with more information.