I planted this ninebark (Coppertina) about four or five years ago. It grew to be a nice full shrub, about 6 feet tall. Last summer, I noticed powdery mildew on it for the first time. I also noticed an aphid problem for the first time last summer. This was the only plant in the garden affected by these problems. Several rounds of treatment with neem oil seemed to take care of the aphids, but the powdery mildew never completely went away.

When the winter came, I noticed there were some leaves that never fell from the plant. I had intended to knock them all off and collect all the dropped leaves to help with the mildew problem but I was lazy and never did it. Now it's spring, and it's starting to grow again, so I figured I'd go out and prune out these dead pieces. When I got out there, I noticed there was a lot of this dead stuff. It's very stiff and black and mostly concentrated on the lower half of the inside of the plant. The outer leaves appear normal, although the powdery mildew (or at least what I assume is powdery mildew) is visible again.

A few high-resolution pictures are available here: https://imgur.com/a/X6wU455 . Any ideas what is affecting this plant? Is it treatable, and if so, how should I treat it? Thank you!