Thank you for your question about your dogwood tree.  Your tree appears to have dogwood anthracnose, which is common in the Pacific Northwest.  It will not kill the tree, but it obviously unsightly.

This is a fungus, as explained in this article, that lives from year to year, overwintering in foliage that has fallen from the tree in the fall.  To prevent it recurring next year, be sure to rake up and destroy (do not compost) leaves, twigs, etc. throughout the year.  As the article indicates, there are 8 chemical fungicides available to home gardeners (signified by the "H" in a box after the name) that can be applied in the spring.  Be sure to read and follow all label directions and, to prevent the fungus becoming resistant to one chemical, apply several ones alternately.

Hope this is helpful.  Good luck!