I'm not very comfortable giving this kind of advice, but I have an opinion, for what it's worth. Have your soil tested before abandoning it or adding things to it without knowing it's condition. It may be fine, but it would be easy enough to have it tested to see if it might be the problem, I would think.
And then, to kind of contradict the advice of finding out what it is before acting, if you were inclined to try a spray, you might try a 3 in 1 rose spray. It seems to cover the more than one possibility. I hate spraying, but I had an issue this spring and I tried it, and it cleared up my problem, whatever it was.
