The spotting is due to cercospora, a common but minor fungal ailment of bigleaf hydrangeas, typically encouraged by high humidity or overhead watering.

The plant is also suffering from a nutrient deficiency that is making it chlorotic. Have you had a soil test done recently? Do you know what your soil pH is? If the pH is too high, it could very well be iron chlorosis. And applying chelated iron will only be a stopgap measure at best until you can bring the pH in line.

It is also possible that this is due to a magnesium deficiency, which will result in similar symptoms. That's why it would be good to have a soil test to find out exactly what may be amiss.

I agree. The yellow to white ish leaves indicate iron chlorosis. If you live in the Piney Woods Region, it could be magnesium deficiency. If you are located in the Hill Country or the DFW Metroplex, your soil is likely too alkaline and it is needing regular amendments like aluminum sulfate (keep away from azaleas and rhododendrons) or garden sulfur. Iron chelated liquids can also be applied but you have to reapply them more often. See the package for directions on how much and how often to apply. You can get those products are plant nurseries.

The granular products have to be reapplied less often than the liquids but the liquids work faster than the granular products. "Faster" is relative term as they all are slooow to fix the leaves... Whatever you use, apply the product and forget about it until it is time to reapply. If you keep looking for the leaves to turn dark green overnight, you will definitely fall asleep! Do not forget to reapply or else the soil will return to its original soil pH value.

The leaf spots are cercospora, usually caused by overhead watering at the plant nursery or in the garden. If you use a sprinkler, set it to water the hydrangea at sunrise so the leaves are not kept wet for a long time. Other things you can are pick up plant debris, do not overwater to minimize high humidity and see if there is a way to improve air flow.

https://ugaurbanag.com/cercospora-leaf-spot-on-hydrangea/

Down here, the leaf lesions become visible usually about a month after the Summer Solstice.

I replaced my sprinkler with a drip irrigation. You can help minimize the leaf spot greatly by not getting the leaves wet using drip irrigation or soaker hoses. Discard the worst looking leaves. When the plant goes dormant in December-ish, discard dried out leaves and bloom in the trash.

The are some cheap fungicides available but all they do is control cercospora, not cure it so I rarely buy them ($8). If you decide to apply it, start around the end of June per label directions.

Chelated Iron is some funky stuff. One year, I was mixing some into a watering can. I had some natural stone samples (tile samples) sitting on the same table. Must have dripped some of the already diluted mix on it by mistake. It ~etched~ the samples in a heartbeat

Ugh, thanks for letting us know!