It is a powdery mildew infestation. Common in hydrangeas. I have even seen some at plant nurseries when they pack the pots closely to take less time when watering.

To get rid of the fungus, make life miserable to the fungal spores by following some clean housekeeping guidelines: do not overhead water the shrub; instead, water the soil early in the mornings; throw away the spent flowers and leaves into the trash; discard plant debris under the shrub; replace the mulch with new mulch if the infestation is heavy; do not overwater as this increases the humidity in the environment and the spores will love you for that; separate plants to improve the air flow between them; consider fungicides if this problems occurs early (as it is doing now) and has spread a lot.

First fungicide to try is milk: spray weekly during the day using 1 part milk to 9 parts water or slightly more concentrated. Because it works more to prevent PM, I sometimes use 4 teaspoons of baking soda, 2 tablespoons of horticultural oil and 1 gallon of water instead.

If you decide to apply chemical fungicides, be aware that some can cost $$. Apply them on the top/bottom of the leaves per label directions.

Consider reapplying next year before the time that this issue started (early May?). Fungicides Cleared for the Control of Powdery Mildew on Hydrangea: azoxystrobin (Heritage; has the smallest application rate and largest repeat interval); fenarimol (Rubigan); fenarimol (Cleary's 3336). You can also use Sunspray Ultrafine Oil.

Luis

Thank you very much! I will try the milk first.