Answer: It is cercospora and very common on hydrangeas, especially when grouped tightly together either in the garden or at the nursery. Not harmful, just unattractive. Good air circulation and an occassional spray of Immunnox minimizes the condition. Bayer also makes a systemic product. New leaves will come out clean so prevention is key. If there aren't too many, I pick off the spotted leaves. If the whole plant is covered, just wait until next season and try to head it off at the pass.
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