There's probably no warranty coverage under the basic factory 3/36 bumper-to-bumper warranty, unless it's rusted through from inside the hatch. If so, you might be able to convince the dealer to fix it under the rust perforation provision (60 months/unlimited miles). Also, if you have an extended warranty, check the coverage provisions in it to see if rust is covered.
I used the "perforation" warranty to have the entire roof cut off and replaced on a prior vehicle. They did such a sloppy job, however, that I soon traded it for my current PF.