It seems as though this damage may be Canker caused by a fungal organism, Coriothyrium sp.
One authority says that after long cold wet winters roses can be expected to have dead and cankered stems. The symptoms are dark red stains and cracking of bark; the stems go yellow with red spots and then go brown or black. Black erumpent spots in the discoloured tissue are the fuiting structures of the fungus containing spores. I found a picture of a birch which had a canker exactly like the one on the bottom of the cane. The pathogenic fungi may cause considerable damage especially on old established roses that have lost vigour. The cankers can encircle the cane and cut off the flow of nutrients to growth further up.
In early Spring, pruned stems provide wound sites colonised by the canker causing fungi, and the fungi can be transfered from one rose to another by contaminated pruners. Pruners must be cleaned in disinfectant between each bush. The discolouration and damage to the bark is not caused by weather damage as I supposed.
Controlling the canker includes the promotion of vigorous growth, the removal of dead canes and stubs to reduce the primary source of inoculum, and to use a fungicide. The fungicide which controls blackspot is effective against canker.
'Alchemyst' rose Under Attack