Answer: What you describe sounds like a fungal disease called Botrytis. It is a gray fungus that generally lives on dying tissue. With the right conditions, any dead plant tissue can release thousands of Botrytis spores. Botrytis infection occurs when water remains on leaves or buds.
To control, cut off all infected blossoms as soon as they droop or die. This will prevent large numbers of fungal spores from being released and infecting the rest of the plant. Fungicide application may be necessary. A preventive spray program can include chemicals such as Funginex or Orthenex.
Best wishes with your roses.
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