Hi,

There is a collection of spots on the trunk/ some branches of my small dogwood bush. These spots feel like a thin marshmallow blob and they have a brown top on them. I don't see any insect material. Some of the leaves look like they are drying up (curling and brown). Last summer/fall there were a lot of what looked like house flies sitting in the foliage (not sure if there is a connection).

It sounds like scale but it's very hard to determine from description. If you can send a photo, that would help get a positive identification and then we can recommend control from there.

This is cottony maple scale. In its overwintering stage, it is brown and flat, but in spring the white cottony egg sac expands looking like tufts of cotton. June through July over 1000 eggs are released from those sacs and are refrred to as crawlers. Treatment for this insect is during that time. The timing is when Queen Anne's lace is blooming. Various insecticides are recommended - Insecticidal soap, malathion, imidachloprid - to name a few. However, since you are in MN, you may need to find what materials are registered for use in your state.