Are you getting the leaves wet? Overhead watering causes all kinds of fungal diseases so it is best to water the soil and never the leaves. Water using soaker hoses or drip irrigation. If you use a regular sprinkler, set the station that waters the hydrangea to activate around 6-8am so the leaves do not stay wet for long periods of time.

Dispose of the affected leaves when they brown out or when they fall. Do the same with all plant debris (blooms, dead stems, etc.). Throw them in the trash since they will have fungal spores. Replace the old mulch with new mulch too.

Since we are near the end of the growing season, I would not worry too much about the leaves since they will fall shortly. But take steps to minimize conditions that might help fungal spores thrive. You should get new foliage in Spring. Continue taking the steps mentioned every year and the leaves will improve with time.

Is it in sun or shade? Probably wants afternoon shade in your location.