Question
I have been enjoying my knockout roses for about 4 years.  Now, this summer I am experiencing my first problem with them.  One bush is the the picture of health and about 6 feet tall.  My other bush-planted at the same time- and in the same bed has developed some kind of "die back" About a third of the plant suddenly turned brown and "died" The leaves looked almost as if some had hit them with a blow torch. They were completely dead.  We had excessive rain in May for a two week period. However, the other 2 roses are fine. One the age of the problem one and the other one planted 1 year ago. I have pruned back the effected canes. I am concerned that this condition might spread to the other bushes.

Answer
You  usually see fire blight on fruit trees which of course are the same family as the rose but you seldom see it on roses but they do get fire blight and that is what your roses has got. It is a bacterial infection and should be pruned out. Make sure you cut way past the fire blight to to at least 6 inches. In the late fall or early spring use a copper spray on the rose and those near it to make sure you get the infection if it is in them. Sterilize your pruning tools in diluted bleach 1/2 cup of bleach to 4 cups of water or use rubbing alcohol before you use them on any other plants.
Don't use fertilizers with high nitrogen in them  or do excessive or hard pruning, both of which promote succulent growth which will encourage fire blight.