Mary Perlmutter has a great solution for black spots in roses. The best part about it is that it mainly consists of the readily available ingredients of tomato leaves and onions.
10 Tomato Leaves
1 Medium Size Onion
½ Cup of Rubbing Alcohol
Stick(about the size of a chopstick)
Finely chop that tomato and onion and combine with the alcohol. Allow the mixture to sit overnight. After removing the more diseased leaves from the rose use swab tied to the stick to apply the mixture to the entire plant including the underside of the leaves.