Q.Rose plant
How to prevent a rose plant from dying when its stem turns black?
The condition you describe can be either frost/winter kill or rose canker. In both cases, you prune the rose down as far as you have to in order to get to a good whiteish colored center pith of the canes. Seal the ends of the canes with white Elmer's glue or Tacky Glue from a craft store. They spray the canes area and soil around them with a good fungicide such as Immunox or another good fungicide of choice. Many rosebushes die back over the winter in cold climates. Many times I have had to prune my rosebushes down to within an inch of the ground. They come back nicely every year with a little fertilizer, water and the warm sunshine to call them forth! Here is a link to an article with more information to help you if it is rose canker:
https://www.gardeningknowhow.com/ornamental/flowers/roses/rose-canker-fungus.htm
