Answer: Unfortunately based on your description I am not able to diagnose what is happening to your plant. Hydrangeas are potentially subject to a number of foliage problems. I would strongly suggest you consult with your local county extension to obtain a specific diagnosis of the problem(s). They may appreciate a photo of the overall plant and a close up of the affected leaves as well as a leaf sample enclosed in a clear plastic bag and kept cool so it stays fresh.
Based on knowing the exact cause of the spots and discoloration, you can decide how to proceed. If a chemical control is needed, they will have the most up to date information on what to use and how/when is best to apply it for maximum results.
In the meantime, water as needed to keep the soil evenly damp like a wrung out sponge, not sopping wet and not dried out. Also avoid wetting the foliage when you water. Also be sure to clean up and remove all of the hydrangea foliage this fall after frost to limit sources of reinfection for next year.
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