There are several diseases which can affect roses. The most common is black spot. It can be treated with a labeled fungicide applied periodically depending on the weather conditions. The wetter the weather the more often you will need to apply the fungicide. I suggest you visit with a garden center manager in your local home depot and ask them to direct you to a product with triforine or some other fungicide labeled for black spot control on roses. These products also control other rose diseases quite well should the spots be caused by another fungus.
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