In last week's column for the S.F. Chronicle (September 27, 2006), I wrote about the fact that roses are prone to occasional dying canes and that this can be caused by many different stresses. Cold winters can cause canes to die, but so can botrytis (gray mold), powdery mildew, black spot disease, soil that is too wet, too much fertilizer. Less commonly, there are insects borers or canker diseases that can kill canes. Many cane deaths start with an injury that isn't lethal in itself, but allows a fungus in that kills the cane. And these fungi aren't ones that cause a specific disease, just opportunistic fungi that attack weakened plant tissue. And the injury could be just normal deadheading or pruning.
With all of these causes, one begins to feel it must be the exception that rose canes survive, and yet, survive they mostly do. The bottom line is that a few may die, you cut them out and try to figure out if you can do anything else to prevent it from happening again.
I think that learning to grow plants is often a process of learning when to worry and how much to worry. New gardeners often worry when a single leaf turns yellow, a single twig dies. It is right to worry when rose canes die, but, like an occasional yellow leaf, it might not have an underlying cause you could have anticipated, or, it might.
Read my columns on www.sfgate.com. You can find them by searching for my name (Pam Peirce) or for a specific topic.