dry rot

Short definition from Wiktionary

Noun

1. 1
   The crumbly, decayed portions of wooden members of buildings, especially at or below grade, usually caused by a fungal infection.
2. 2
   Any progression of decay, corruption, or obsolescence.
3. 3
   A fungal infection which affects plants, in particular potatoes.
4. 4
   Alternative form of dry rot.