If GH can stand for P as in Hiccough 
If OUGH can stand for O as in Dough 
If PHTH can stand for T as in Phthisis 
If EIGH can stand for A as in Neighbor 
If TTE can stand for T as in Gazette 
If EAU can stand for O as in Plateau

Then the right way to spell POTATO should be:
GHOUGHPHTHEIGHTTEEAU