Old growth on holly bushes have a black substance on top and white powdery nodules on underside. The leaves are falling off but new growth is coming out on top. What is disease? What do I do with bushes?
What Is Disease on Holly Bushes
Certified GKH Gardening Expert
This sounds like sooty mold. Sooty mold is normally a result of a pest infestation, and the most likely in your case. Powdery mildew is caused by fungus that attack holly leaves, sometimes covering them entirely with a thin white powdery coating. These articles will help: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
Neem oil can be used to treat both the pests as well as the fungus. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm