Answer: Hydrangeas are occasionally subject to leaf spot problems such as cercospora, which would begin as purplish spots and eventually turn grayish. It is unlikely that it would kill your shrubs, although it looks a bit rough. Remove the worst affected leaves and avoid overhead watering (avoid wetting the leaves when you water), and this fall remove all the fallen leaves to try to limit reinfection. There are fungicides that can be used if the problem is truly severe and disfiguring -- some years are worse than others, too. Your local county extension should be able to specifically identify the cause of the spots and provide the most up to date control recommendations for your state, if chemical control is warranted. They will probably need to examine a fresh leaf sample and possibly also a photo of the overall plant. Good luck with your hydrangeas.
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