Answer: Dogwoods do not like hot, dry weather, so I'd bet the heat is weakening the plants, if not actually causing the leaf yellowing. There are a number of possible diseases, including an anthracnose disease that is becoming more and more of a problem. Symptoms include small, purple-rimmed leaf spots or large tan blotches--these spots may enlarge to kill the entire leaf.
The trees like cool, moist soil, so I would be sure to water the plants during hot spells, and mulch with a thick layer of organic mulch (keeping the mulch a few inches away from the trunk.) On the other hand, the tree does not like heavy, poorly drained soils and will decline over the years in such conditions.
I doubt the problem is due to lack of fertilization, and if the plant is truly declining, I would hold off on fertilizing--it's best to fertilize an actively growing plant. I would inspect the plant carefully for signs of disease such as those described above, as well as insect pests. If you find any specific signs, you might bring them in a sealed bag to a local nursery or arborist for help in identifying the culprit.
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