That's illogical, Toni. You say it was discovered "in fields of multiflora roses being grown for use as rootstocks." If they were being grown for use as rootstock, they must have been own-root roses themselves.
Own-root roses are just as susceptible, if not more so in view of the fact that R. multiflora, which is always own-root, and the Knock Out roses, most of which are own-root roses, are the ones that are being affected most.
I wonder why they were growing multiflora for rootstock in Texas. As far as I know, only the Canadian nurseries (Hortico, Pickering, and Palatine) use R. multiflora as rootstock. The rootstock of choice in Texas is Dr. Huey, and possibly R. fortuniana.