Given Barth's well-known polemical style, this quotation is perhaps not that surprising: 

Mariology is an excrescence, i.e. a diseased construct of theological thought. Excrescences must be exised.

Barth, Church Dogmatics I.2, 139

So there you have it! And then, just a few pages later, he comes up with a cracker: 

What in fact makes revelation revelation and miracle miracle is that the Word of God did actually become a real man and that therefore the life of this real man was the object and theatre of the acts of God, the light of revelation entering the world.

Barth, Church Dogmatics I.2, 147