Dogwood tree diseased
Your dogwood is very mature and has lived a long life. There may be several reasons for branch dieback and decline including fungal diseases (powdery mildew, spot anthracnose) insect issues (borers), trunk damage, cankers, drought, poor site conditions, environmental factors, etc.
Dogwoods are understory trees which grow best in morning sun and afternoon shade. They require more moisture in full sun in dry periods.
The last several years have been tough on woody plants due to excessive rainfall and moisture which reduces oxygen in the soil and the roots decline. We also had a drought which affects the root system too.
All you can do is keep the tree happy. No chemical controls are recommended. Prune dead wood and make sure mulch is no thicker than several inches and away from the trunk. Water during dry periods.
The tree will limp along and you will have to consider removal in the future.
If you decide to replant a dogwood, look for resistant varieties. See our web pages on care and management of dogwoods, diseases, and resistant varieties
and abiotic factors (Not insects and diseases ) https://extension.umd.edu/hgic/topics/tree-and-shrub-problems-not-caused-disease-or-insects-abiotic