Most likely there are a couple of diseases on your rose bush. Please keep in mind that it is impossible to diagnose diseases with certainty without proper lab testing. That being said; your rose has a case of powdery mildew and possibly either downy mildew or leaf spot. It also looks as if your bush is in the shade. The optimum environment for roses is: full sun; rich,well-draining soil; fertilizer formulated for roses; and enough water to sustain the plant. When a plant is not given its optimum growing environment, it is not able to fend off disease that it otherwise might be able to. Some preventative steps to take would be to: avoid overhead watering; remove and properly (do not compost) dispose of all affected plant parts and litter; and provide the plant with good air flow.
Here are a few links that might be helpful:
http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/rose.html
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7493.html
http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/DISEASES/downymildew.html
http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/DISEASES/leafspotdis.html
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