It's difficult to identify "genius" among folksingers, but I think it's possible to find a handful of traditional instrumentalists who deserve that description.

Just as the great jazz musicians took an established form and carried it to a new level -- I would add Bix Beiderbecke and John Coltrane to that list along with Louis Armstrong and Charlie Parker -- there are some "folk" musicians who have done the same thing. Certainly Bill Monroe, and arguably Muddy Waters, Jimmie Rodgers and some great fiddlers like Benny Thomasson. Each of them changed their type of music, leaving it different from what they started with and creating a new model for those who followed them. So did J.S. Bach and Phil Spector.

And using that defintion, Bob Dylan certainly qualifies. Certainly he was influenced by Guthrie, Ramblin' Jack, and others (including Van Ronk himself), but he carried the singer-songwriter genre to a new and different level. After the sixties, most new singer-songwriters were held up to the "Dylan standard."

I don't include the best of the 20th century bluesmen like Robert Johnson, Son House and John Hurt as geniuses, mainly because we don't know what their influences sounded like; we really can't tell whether Johnson, for example, was directly imitating somebody who came before who was never recorded. It's unlikely, but it is possible.