Q. Why do my dogwoods (Cornus florida) keep dying?
If you are having problems with Cornus Florida it may be the growing conditions causing Powdery mildew, which is most common in dense, shady areas where the air circulation is poor. Warm, dry days and cool, damp nights favor disease development.more here:
Have you grown:
Cornus kousa; common name Kousa Dogwood (family Cornaceae)
A small, vase-shaped tree with horizontal branching
Mottled exfoliating bark
Pointed bract tips in comparison to the rounded bract tips of C. florida
Blooms about 2 or 3 weeks after C. florida
Red, raspberry-like fruits
Flower buds pointed and shaped like and onion
How to plant a tree: http://libguides.nybg.org/selectingsitingandplanting
Hope this helps.