I kept seeing somewhere for a while that Ohio based grape varieties are used for the root stock now used, but I can't find this anywhere now. Perhaps it isn't accurate.

Uncertain that they were from Ohio, but it was (and is) American rootstock that’s used to combat the parasite.

I had read earlier that the rootstock came from Ingleside, near Corpus Christi, TX. Wikipedia says they came from Temple, TX.

I was actually just talking to a coworker last week about this, and he told me that modern rootstock almost entirely comes from Grayson County, Texas (a.k.a. the Sherman-Denison MSA, just north of the Dallas area).

Interestingly enough, a few weeks ago I had a Riesling from Washington that had "100% vinifera rootstock" prominently printed on the label. Not every part of the world got hit by phylloxera. I understand that some parts of South America (either Argentina or Chile, I can't remember) were spared as well.

There are still pre-phylloxera vines in South Australia as well that are over 150 years old.