Sorry for the delay in responding! Hard to say definitively without more info, but looks like at least part of the problem is Powdery Mildew (the white powdery mildew spots). The green mottling (among the yellow tissue) could be the symptoms of another fungal disease called Rust, so we suggest you look at the bottom of the leaves (it is usually orange/rust in color). The spotting of the upper surface of the leaf could be a result of that, or it could be a combo of things. See the links included for more info on Powdery Mildew and Rust. We also recommend that you provide air circulation in/around the plant as much as possible, and keep water of off leaves as many fungal diseases spread with water. We also recommend proper water and fertilizer applications. Excesses of either one (or insufficiencies of either one) can reduce plant health and cause them to be more susceptible to pests and disease. The links we’ve provided will also give you info about those cultural practices. You should also remove any infected leaves (without defoliating the plant) and in the case of Rust, consider asking your local garden center for a fungicidal recommendation.
http://www.ipm.ucdavis.edu/QT/powderymildewcard.html
http://www.usna.usda.gov/Gardens/faqs/hydrangeafaq2.html (look for Rust at this link)
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