Found something interesting yesterday. An experienced grower in South Carolina claims that although the parent cultivar 'Zéphirine Drouhin' was always badly disease-prone for him, 'Kathleen Harrop' is almost always healthy without being sprayed.

See: http://paulzimmermanroses.blogspot.com.au/2010/02/rose-to-know-kathleen-harrop.html

That page doesn't mention the no-spray aspect, but his commercial website (linked from that page) makes it clear that he doesn't spray in his own garden.

It's odd that the sport would have markedly different resistance to disease, but it appears to be the case, at least in some locations.