Some of my coneflowers died during late July-August. The entire stalk is...
We can't be sure what's affecting the plants by viewing the photo. However, based upon what we can see, we suspect some kind of stem or crown rot.
The following bulletin discusses diseases that often affect coneflowers and of those listed we think Sclerotinia and Sclerotium crown rot is most likely.
Compare here and see what you think: