Japanese Hollies have a white powdery substance
My 10 yr old Japanese hollies have a white powdery substance on the stems and branches. The leaves are unaffected. One of the hollies is especially bad looking where the leaves and branches are brown and dead. I did not notice the fungus type stuff as you have to look directly into the bush and spread the branches to see it. I never would have noticed anything until the one plant began to turn brown in sections.
Powdery mildew is caused by fungus that attack Holly leaves, sometimes covering them entirely with a thin white powdery coating. For more information, here is an article that you may find helpful: https://www.gardeningknowhow.com/nonorganic/get-the-cure-for-powdery-mildew.htm