From DIYinfo.org
If exterior doors have not been adequately protected with paint or varnish, rot may set in, especially near the bottom and at joints.
If the rot is not extensive, it can be repaired with a wood preservative made of a wood hardener, a wood filler and preservative pallets.
1 On a warm, dry day, dig out the rotten wood and if possible allow the remaining wood to dry thoroughly. Warming with a hot air gun can help.
2 Treat the area with a wood hardener, and when this has dried fill the cavity with a two part epoxy filler. Build it up slightly above the surface, and sand it flat when it has hardened; hardening will take about 30 minutes.
3 Drill holes in the timber surrounding the affected area at intervals of about 150mm.
4 Insert wood preservative pellets in the holes and then fill the holes with more filler.
5 Repaint the door to disguise the repair.