Certified GKH Gardening Expert
Bordeaux mixture is an old school copper fungicide that is not appropriate for all disease issues. It is a preventive dormant spray for use when the plant has dropped all foliage for winter dormancy, killing fungal sprores and suppressing new disease incidence for spring. It is not a cure for an active infection.
General advice is to improve soil fertility as the first line of defense, by enhancing nutrient density and plant resistance to pests and diseases. Water management is vitally important, water deficit/drought or other physiological stress makes plants more susceptible to disorders, diseases, and pests. Use horticultural oil or neem oil, and/or biological fungicides and botanical pest repellents early in the season as pro-active measures. Don't wait for advanced problems before reacting.
Here is an article on how to make bordeaux mix and one on dogwood leaf diseases. https://www.gardeningknowhow.com/garden-how-to/info/diy-bordeaux-fungicide.htm