What is Wood Rot? And what is the cause?
It is a form of Decay. And Fungus that digests moist wood, cause wood rot. And it is considered a serious concern.

What should you do?
Recommendation:
The annual mission for the homeowner to check its sign. It is your investment, take care of it, please.
The best time, Summer or Spring.
Tools, Screwdriver with a Flashlight.

Best Solution:

Replacement, and Preventions.