Can a Great Unclean One ruin regions immediately on summoning?
In the FAQ it says:


  Errata
  Rulebook (pages 19–20)
  If a region is ruined, each player who placed at least
  one corruption token in that region during this round
  immediately scores the number of victory points
  indicated for ruiners on that ruination card


So that means the Nurgle player would count as a ruiner if that region was ruined that turn. However, ruination is only checked in the ruination phase, and so the region would not be ruined immediately.