I have a dogwood (supposedly a cross between American and Japanese) that has not leafed out properly this year and has some sort of discoloration on the trunk and branches (see attached pictures.) It is adjacent to two others that have not been affected. One is a cornus kousa, but the other is supposed to be the same as the diseased tree. I am skeptical, though, because the diseased tree has pink flowers while the other has white. Please let me know if I can treat the tree (and how) or if I should remove it to prevent the others from becoming diseased. Thanks.
There may be several reasons for decline but we cannot say for sure. Dogwoods grow best in morning sun and afternoon shade in a well drained soil. They are understory trees. The growth on the trunk in the right photo looks like lichen and are not a reason for decline. They tend to grow on mature trees. Here is more about them. https://extension.umd.edu/hgic/topics/lichens-0
You may be dealing with drought, poor planting techniques, planting too deeply, poor drainage, root issues, excessive mulch, etc. Branch dieback can be also attributed to cankers, anthracnose, powdery mildew, and insect issues.
Here is our dogwood publication for more information http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs/...
Your dogwood does not look happy and you will have to consider replacement. If you decide to plant another dogwood, replace in the spring. See our website for the planting process and aftercare https://extension.umd.edu/hgic/plants/planting-process. Select dogwood varieties resistant to powdery mildew and spot anthracnose. See above publication. You may also want to select another type of tree suited for the location.