Your dogwood has a fungal disease known as powdery mildew. If it has not had problems with this before, it may be highly resistant to the disease and the disease pressure may be very high this year. If it has this every year, then it will be weakened and eventually need replacement. Replace with a variety that is highly resistant to powdery mildew. This page explains the situation and give varieties to use: https://extension.umd.edu/hgic/topics/flowering-dogwood-trees-selection-care-and-management-disease-problems

This webpage also lists other diseases.  We can't see any others distinctly in your photos, but look at the photos so you can recognize them should they appear. 

 Ellen