I have black spots on the stems of my knockout roses. I read your atmrticke on black spot and am still not sure if that’s what I have. Can you look at these pictures and confirm?
Black Spots on Rose Stems
It looks like black spot or winter die back. At any rate, prune out the large, blackened stem all the way to where shoots are coming out at the base of the rose. The inside of the cane should be white with a thin green line under the bark. Any cane that is not completely white in the center should be removed. Cut back to 1/4 inch above an outward-facing bud. The rose should recover nicely.