Bacterial cane blight. Notice the purplish area and then the black? This darn bacteria is ruining roses all over the valley, and this infection was found on my Princess Diana rose bush. When the infection is localized, cut below it several inches and be careful that the cane doesn't touch any other canes. Try not to touch the infected area. Put the cane in the trash. Sterilize clippers with Lysol after every cut. If there is a lot of infection, pull up the whole plant and discard. Do not compost the clippings. I have found this on every rose plant in my yard except for the miniatures.