Games : Does damage reduction apply fully to damage over time effects?

I just tried out my new Combusting Wounds spell, and the effect seemed to be rather underwhelming. I then noticed that my enemy has around the same damage reduction as the spell does damage over time.

Does the damage reduction apply fully to this kind of damage over time effect? If the DR is directly substracted from the damage per second value, this would make this kind of spell rather weak againt enemies with any significant DR.

How exactly do the mechanics work here?