FYI: Landreth name is variant of a German name.
Based on Virginia land patent research in the Grayson (now Carroll) Co, VA area, Landreth is a Anglicized variant spelling of Landwrath. Landwrath is also seen as Lundwrath.
Perhaps a check of the Landwrath/Lundwrath name in Pennsylvania German immigrant sources could provide a link to the Landreth name seen in early 19th-century land records in southwestern Virginia.