Answer: What you describe sounds like botrytis, a fungal disease. Since it is such a prevalent fungus, prevention is the best approach ? plant roses that are not susceptible to botrytis blight; reduce the humidity around plants by providing good air circulation, modifying irrigation and reducing ground cover; deadhead any infected flowers immediately and dispose of fallen leaves and petals; prune out infected canes, buds, and flowers and generally maintain good garden sanitation.
The disease can be controlled by spraying with fungicides like Daconil 2787 or Fungi-gard.
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