When you meet a guy who's been turned into a tree, you know that something is up. And sure enough, Fraudubio, the bleeding tree, has a sad story to tell about being tricked by the evil Duessa and being turned into a tree along with his lady, Fralissa.
Redcrosse accidentally comes upon Fraudubio—he actually tears a branch off of him that starts to bleed, yikes—in the midst of being tricked by Duessa himself, so you'd think that Fraudubio's story might make Redcrosse examine his own recent choices. Sadly, however, Redcrosse fails to learn from his encounter with Fraudubio and follows Duessa into further unhappiness.
Fraudubio, whose name suggest doubt, represents the danger of doubting too frequently those who you ought to trust (like Una).