dry rot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fungal disease that causes timber to become brittle and crumble into powder.
• noun A plant disease in which the plant tissue remains relatively dry while fungi invade and ultimately decay bulbs, fruits, or woody tissues.

from The Century Dictionary.

• To affect with dry-rot.
• noun A decay affecting timber, occasioned by various species of fun-gi, the mycelium of which penetrates the timber, destroying it.
• noun Figuratively, a concealed or unsuspected inward decay or degeneration, as of public morals or public spirit.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The crumbly, friable decayed portions of wooden members of buildings, especially at or below grade, usually caused by a fungal infection.
• noun Metaphorically, a progressive malaise of decay, corruption, or datedness.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a crumbling and drying of timber or bulbs or potatoes or fruit caused by a fungus
• noun a fungus causing dry rot
• verb affect or be affected with dry rot