powdery mildew on Hydrangea
Hydrangeas can be susceptible to diseases such as powdery mildew or multiple fungal leaf spots later in the season, but they are generally nothing to be too concerned about and do not kill the plant. Hydrangeas in general grow best in morning sun and afternoon shade in a well drained soil with good air circulation. Pick up affected fallen leaves this fall and discard. Hopefully, given good airflow, you will not see it next season.