mildew problem on this bush
The leaves have probably been affected by fungal disease called powdery mildew. Go here to learn about it:
Most shrubs grow normally despite occasional insect and disease leaf damage. This is especially true at this time of year when the leaves have mostly served their purpose and begin to decline. Consequently no control is needed at this time.
On the other hand, frequent defoliation is harmful and requires intervention to protect plant health.