I am a fence contractor with over 40 employees, and have been for 30 years. The fact is we all know where fence posts rot. Not underground but at grade. Fungus needs oxygen to live, fuel- the damp posts.
Watch this video. We use these on all our jobs now. You can make your own I suppose, but not like these.
Fence posts rot because of wood to soil contact. If you have been around a while we have seen the posts snap off at ground level or grade. We have been using post colars for a few years now at our fence company. I have been a fence installer for about 20 years. Either metal posts or wood posts with posts collars. They are galvanized metal with the inside that touches the post made of almsot a Gracie's material. It has a peel off plastic film that keeps allows the sticky stuff inside to be able to bind to the post, creating a water tight seal. Its easier to look for yourself. It works or metal posts.
The Post Collar