Answer: Based on your description I am not certain what is happening to your dahlia flowers, however it might be due to a cultural problem such as overly dry soil. Dahlias in bloom need a generous amount of water so the soil stays evenly moist and never dries out. It might also be a pest such as broad mites, or a disease such as botrytis which causes the flower to shrivel and turn gray and fuzzy, this is most prevalent during damp or humid weather or can occur with overhead watering.
I would suggest you consult with your local county extension to get a more specific diagnosis and based on knowing that, determine how to proceed. If it is something that requires a chemical control, they will have the most up to date information on what to use and how/when is best to apply it.
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