Kerria looks very sick

I recently cut back my rather overgrown kerris bush, and found as I went along that many of the stems were black or brown and blotchy. Added to this, those that weren't in this state had very weedy growth of leaves. I fear I have hastened its end by cutting back, but I don't think this would have caused the blotching. What are the known problems with kerria, and can I recover this one? Should I cut all the blackened stems out to ground level? In its heyday it was a lovely floriferous bush, but it had outgrown its patch, which is why I set to work on it. Please advise.