Answer: Black spot is really difficult to control and some roses are more susceptible than others so you may have a real battle on your hands. The product I use is called Rose Defense and it controls both powdery mildew and black spot on my roses. Give it a try. You may have better results if you first prune off as much of the diseased plant material as possible (within reason!) and then spray. The new growth will be protected as it sprouts and grows. Be sure to apply according to label directions.
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