Hello Frank,

Thank you for contacting us with your question.

The foamy substance on the stems of your red twig dogwood is from a spittlebug.  If you pinch off the liquid in your fingers you will find the small nymph insect inside this liquid bubble.  The good news is that although they do suck sap from the stems, they do not cause any noticeable or lasting damage to your plants.  If you'd like to remove them for aesthetics, a strong stream from a garden hose should wash them off.  

You can read more about them by clicking this link from Iowa State University below.

https://yardandgarden.extension.iastate.edu/article/1995/6-30-1995/dogwood.html

--Sandra, CCE Broome master gardener volunteer