There's no monsters on the Pumpkin Spice Trail (there used to be, but even they would get bored of the pumpkin spice scent and taste. If they have ways to fly up there or somehow fetch the berries, they don't get pumpkin spiced.

However, they should note that getting close to the pumpkins means entering the swamp that drains energy and the longer you stay on it the weaker their seasonal powers feel. The swamp covers a very large area around the pumpkins.