Your dogwood tree may be affected by powdery mildew which can be controlled with the proper application of a fungicide. Typically, spray applications are made on 7, 14 or 21 – day intervals depending on the level of disease pressure. Pathogen resistance to fungicides is well known and the performance of many fungicides has been affected to some degree by pathogens developing resistance. Using fungicides with different modes of action in a rotation program will lessen the risk for resistance development. In future, preventative fungicide applications will prevent further infection. I have attached links below to a couple of factsheets with additional fungicide recommendations for powdery mildew on dogwoods.

https://www.tnstate.edu/extension/documents/Dogwood%20Powdery%20Mildew%20Factsheet.pdf

https://www.tnstate.edu/extension/documents/Dogwood%20Powdery%20Mildew%20Factsheet.pdf

https://plantpathology.ca.uky.edu/files/ppfs-or-w-13.pdf