One of my customers recently had two 8' holly trees planted by professional tree people. Some of the leaves (more lower than higher) have developed gray splotches, which look like mildew or mold or something fungal. We live in southern NH and have had very unusal weather this summer. . . lots of rain, cool nights, now very hot and dry. She is 91 and is having a fit about the trees since it has taken her 20 yrs to finally do something to remove the old arborvitaes and replace them. As her regular gardener for many years, I am seeking your opinion because I surely don't want to make a mistake! Any guidance would be appreciated.
Prince Holly Trees
Certified GKH Gardening Expert
This sounds like powdery mildew. Here is more information: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm
Neem oil is also effective for this problem. This article will help: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm