Where is the poorest cluster of white people in the US?

The Appalachians, particularly the central and southern Appalachians. This has been the cornerstone of American poverty for the entire history of the country. In the old South, "mountain men" were at the very bottom of the social ladder, except of course for slaves. There is still a lot of dependence on coal mining, which has been getting progressively more difficult to do profitably. There is an entire federal commission, the Appalachian Regional Commission, dedicated to improving the economy of the Appalachian region.