Oh how sad to be losing part of an old rose bush.  It is hard to tell from your picture exactly what the problem is.  I would first trim out the worst looking parts of the rose bush.....sterilize your trimmers before and between each cut with 10% bleach water so you do not spread what you have.  You may also have a rose virus.  This often happens when roses are grafted in the nursery.  Most viruses are fairly harmless unless there is extensive yellowing or browning of the leaves.  No cure is available for virus infected plants.  Rose viruses rarely spread from plant to plant; therefore, only the infected part needs to be pruned out.  You may also wish to take a sample branch with the infected leaves to your local Extension Office for further detection of what ails your rose bush.  They will send the sample to a plant pathologist who will look under a microscope at the leaves to determine the problem.   

Thank you for your information.  I have already removed the diseased portion of the plant and am hoping that it will not be transmitted to the rest of this plant or my other roses.  We shall see.

I believe that the question has been addressed by Emily and received by the guest.  It seems that no further response in needed.

Tony

The anonymous guest above is the original questioner - me.  Thanks for responding.

Good luck with your rose garden. Lots of established plants suffered from the unusually harsh winter which can ofter lead to other ailments as well.  I am happy that half of the plant survived for youir enjoyment.

Tony
.