I had the same thing happen to a couple of my rose bushes last summer. Still have no idea what caused it, but the seemingly perfectly healthy bushes died, one branch at a time until the entire plant was dead. I too assumed it must be some kind of rot or infection, but never found anything abnormal in the wood--other then the browness spreading down the cane.

I replaced the bushes this spring. The new bushes are thriving and healthy.

I speculated that after a bad drought one summer, I got in the habit of over-watering the following summer in compensation and that maybe the over-watering caused the problem--but I don't really know if any of that is relevant.

Best I can advise for now is to cut those canes turning brown down all the way to the base, even if the bottom half of the cane is still green. And take care not to overwater.

But I have no idea if that will help or not except to say that I tried that late last summer on a David Austin rose that had been growing beautifully for 5-6 years in my garden when it suddenly started showing the symptoms you mention. This year that DA rose is still alive and is bursting out in buds right now--although it is not the most robust looking rose I've ever seen--but we had that brutal winter, so I'm not sure how much of its less than optimum vigor is due to a hard winter and how much is left-over damage from last summer. But it is alive and looking healthy and getting ready to put out a good set of blooms--so let's all cross our fingers and think good wishes for my DA rose. : ) And for yours also.

Kate

Possible causes are verticillium wilt (a soil-borne disease that clogs the plumbing) or something wrong with the cane stub that underlies the dying cane (canker or insect damage). In eastern zone 7 and colder there was a lot of winter damage this year, but unlikely in zone 8.

V-wilt sounds like a real possibility. I have had it in two roses, which developed resistance (immune reaction) and survived. Canes die back separately to the crown or graft. You can look up images.

I have also thought of stem canker but I don't see it on the affected stems.