This could be anthracnose on your dogwood.  Check to see if the spots go through to the back side of the leaf.  It is too late this season to spray.  If the spots go through the leaves as in the below link, spray with a labeled fungicide such as Daconil this fall and again in the spring.  https://extension.umd.edu/hgic/dogwood-anthracnose-discula-trees     Sanitation is very important. Remove all fallen leaves.  http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/not_updated/hg12_IPM...   Another common problem on dogwoods is powdery mildew.  https://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs  vw

Thank you.  Could the tree die if I have to wait until the fall to spray? It is a young tree.

Of course, there are no guarantees when it comes to a young tree's ability to overcome disease.  The two most important things you can do is to rake up all fallen leaves and provide the tree with supplementary irrigation during hot, dry weather conditions.
LS