dry rot, fungus disease that attacks both softwood and hardwood timber. Destruction of the cellulose causes discoloration and eventual crumbling of the wood. This frequently results in the collapse of wooden structures such as house flooring, mine shafts, and ship hulls. Because the fungi require moisture for growth, dry rot occurs most often in places where the ventilation is poor or humidity is high or when the wood has been improperly seasoned. In the United States it is most frequently caused by a pore fungus ( Poria incrassata ) and by the dry-rot, or house, fungus ( Merulis lacrymans ). It may be prevented by application of creosote or other preservatives. Dry rot sometimes attacks standing conifers. The name is also used for other fungus diseases that attack the roots or stems of plants (see diseases of plants).