Q.Treating variegated red twig dogwood
Leaves are turning red on established variegated red twig dogwood bushes. What does this mean? Lost one bush already. What can I do to save the others?
Dogwood is effected by a few different diseases.
From your description, I would say it is likely Septora Leaf Spot.
I have listed some links for you that include images to help you pinpoint the cause and determine if treatment is necessary.
http://plantpath.caes.uga.edu/extension/plants/woodyornamentals/SeptoriaLeafSpot.html
https://extension.psu.edu/pests/plant-diseases/all-fact-sheets/dogwood-diseases
http://www.uaex.edu/publications/pdf/fsa-7564.pdf