It is a powdery mildew and black spot fungi. Humidity levels and temperatures create the circumstances for rose rust condition to strike rose bushes. Having good airflow through and around the bushes will help counter this rose rust ailment from beginning. Also, disposing of old rose leaves will deter rose rust fungus from staying past the winter and re-infecting your roses following year. If it does strike your rose bushes, spraying them with a fungicide at interims as directed should take care of the predicament. Also, be certain to throw away any infected leaves, as they can infect the rose rust fungus to other bushes.
Now that you know how to approach it, you can help your rose’s get rid of the disease that is affecting it. Treating rust on roses is fairly straightforward and you will be repaid with rose bushes that are once again beautiful and attractive.