Answer: Sounds like either fungal or bacterial spots on the leaves of your plant. Hydrangea's are susceptible to both Cercospora and Phyllosticta, which cause reddish spots which later turn purple at the margins. Your plant will be losing its leaves soon, so rake them up and remove them from the garden. Inspect the shoots and branches for spotting, pruning them off if you find any. Pruning will remove the problem, as well as open your plant up to better air circulation. To prevent the spread, don't splash water on the leaves (supply water to the soil, not the leaves of the plant).
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