As you can see from the description of rust above, it can spread quickly. Because rust likes warm, moist conditons, it is a common problem on the west coat. Also as some cultivars are very susceptable to this disease, it is a good idea to check the next year in case it has rust again. After taking off the infected leaves and cleaning up around the rose bush, spray with any rose fungicide. Also spray on the ground around the rose and make sure the fungicide gets up under the leaves as that is where the rust will start.