Hi and thanks for reaching us.

A standard Cornus florida aka flowering dogwood has diseases affecting its leaves.  Some hybrid or varieties are more resistant to diseases.

I see on one of the picture with powdery mildew on the leaves. It's the white coating on the leaves.  It's a fungus.

The holes in the other pictures can be caused by spot anthacnose.  It's also a fungus.

All diseases are due to a wet and cold spring. It's possible to treat the tree with fungicide for both diseases but it's not easy.  I'm talking by experience. My dogwood is 35 feet tall.

One of the best thing you can do for your tree is to keep it watered especially if it's in full sun all day. Clean up and dispose of al infected leaves on the ground.  Do not compost or pass the mower on them.

More info about the diseases : https://hgic.clemson.edu/factsheet/dogwood-diseases-insect-pests/

Good luck.