Red Twig Dogwood
There are few different diseases that cause "leaf spots" on dogwoods so I'm unable to provide a specific recommendation. However, I am including a link to a few resources that may help narrow down the symptoms you may be seeing:
Please feel free to send us photos as soon as you start seeing symptoms and we'll try to narrow down which problem it may be. Ultimately, it may be best to contact your state university diagnostic clinic for positive identification by a plant pathologist.
Thank you for your question.