Looks like a bad case of leaf spot, pcorn, usually caused by overhead watering and humid conditions. Try to minimize the return of this fungal infection by never watering the leaves. Instead, water the soil early in the mornings, discarding plant debris, the spent blooms and leaves to the trash, replacing the mulch with new mulch and, if it fits your budget, apply fungicide as described in the link below. See the information on Cercospora Leaf Spot.

Here is a link that might be useful: ACES Diseases of Hydrangea