It is a disease caused by fungus. There is more than one kind of Canker. From MSU website:

"Brand canker develops while plants are under winter
protection. Infected roses have a black sooty appearance
when uncovered in the spring. This disease is sporadic
and does not directly affect the blossoms or leaves.
Destroy infected canes.

Brown canker causes cankers with light chestnut brown
centers and deep purple margins. Small flecks and purple
spots form on the leaves. This disease attacks canes
while the roses are under winter protection. Brown canker
may be worse on bushes hilled with soil.

Common stem canker gains entrance through a wound.
Young cankers are pale yellowish or reddish, becoming
brown, sunken and cracked. Destroy cankered canes. Black
spot sprays usually prevent new infections.

Dieback, cane blight canker causes leaves on the
cane above the canker to wilt, turn brown and die. Destroy the
cankered canes. Sprays for black spot may prevent new
infections."

Here is a link that might be useful: from Michigan State University website

That's a good link, Hoovb! (Of course, I'm an MSU fan anyway). Here in the north, the whole issue of winterizing and canker is such a conundrum. If we winterize, our roses are more likely to develop canker, and if we don't, our roses are more likely to die or suffer severe winter kill. I mound my roses with the best draining mulch I can find, which helps. I spray them well with dormant oil before mounding them. But canker still rears its ugly head in spring. Fortunately, it doesn't seem to do too much long-term damage, as long as I prune to below the canker (sometimes leaving only a few inches of cane).

Often the fungus enters through a wound in the bark. It requires dampness and cool to moderate temperatures. Lesions sometimes grow aggressively when the plant is dormant or semi-dormant, because active plants seem to have the ability to isolate and stop it growing. You see this on pruning stubs where the stub cankers but it doesn't spread much into the live cane.

Botrytis canker or cane blight could be added to the list. It is black with a purple leading edge.

I always soak my roses and beds with Manzate before I cover for the winter.