Answer: Based on your description I suspect they are suffering from a fungal infection. I would suggest you consult with your local county extension for a specific diagnosis and based on knowing that, determine how to proceed. If chemical control is needed, they will have the most up to date information on what to use and how/when to apply it for best results. In the meantime, clean up any fallen foliage and dispose of it in the trash to limit reinfection. Also do a thorough clean up of the stems and foliage this fall for the same reason.
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