It may be due to a root stress such as a lack of soil moisture or to too much sun. The lack of water will show up also as wilting foliage. Then there is the possibility that some sort of fungal blight (such as Botrytis blight, also known as gray mold) is affecting the blooms. This problem is usually more common in greenhouses and in cool, wet weather. Try to avoid unnecessary sprinkling of the blooms and foliage to minimize such problems. Prune out and remove affected bloom heads.
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