I have 3 pagoda dogwoods in my Brampton garden. The soil is mostly clay, but has been amended over several decades with mulch and compost. The area gets sun most of the day with a few hours of shade from the house in early afternoon. 2 years ago we started to notice some dead branches with the bark turning reddish and rough looking. It has spread last year and this year in the spring. We’ve pruned those branches as they died. now I am noticing white discolourations on the trunk and branches. Otherwise the tree seems healthy with a good canopy and lots of flowers.
Thank you for contacting the Toronto Master Gardeners with your question about your pagoda dogwoods.
This sounds like the beginnings of stem canker.
https://www.mortonarb.org/trees-plants/tree-plant-descriptions/pagoda-dogwood
https://hyg.ipm.illinois.edu/pastpest/200705d.html