Peter Rohland ought to be mentioned. Sadly, he died young, but not before he'd given the German folk revival of the Sixties and Seventies a big push.

Also, some of those who made folk a force in the then GDR; Folklaender, Wacholder, Duo Sonnenschirm, Piatkowski & Rieck, Bettina Wegner (who is in a league of her own) - to mention but a few.

And I forgot Helmut Debus, writer of Low German songs from East Frisia.