About 100 forms of wood rot exist in the United States, and they do more damage annually than fires, according to the Encyclopedia Britannica. Dry rot and wet rot, the two most common forms of wood rot, do most of the damage. Dry rot comes from one specific fungus, serpula lacrymans. Wet rot comes from any one of several fungi, but principally from Coniophora Puteana, and secondarily from Poria Vaillantii. Dry rot and wet rot have similar causes, but different outcomes.
Water, particularly standing water or water trapped between layers of wood, combines with oxygen to cause every form of wood rot. When the water content in wood reaches above about 18 percent, the wood becomes highly susceptible to dry rot. Wet rot, as you might guess from its name, prefers a slightly wetter environment, around 24 percent. But once either rot gets started, it will propagate over a range of humidities.
One common form of wet rot causes wood to darken; the other causes wood to lighten. Both forms cause wood to weaken, and to both look and become spongy. The brown form of this rot attacks softwoods more often; the white form primarily attacks hardwoods, according to Dr. Michelle Seidl, an Emlab scientist writing in Emlab's Environmental Reporter. Wet rot occurs more often than dry rot but causes less damage; if you stop the source, the rot will stop spreading.
Dry rot looks somewhat like the brown form of wet rot, but it causes a lot more damage because the fungus propagates so efficiently, differing from other rots in the way it develops sturdy roots that convey the fungus from infected wood to still-intact areas. These root structures penetrate dry wood, distribute water by capillary action and then begin the rotting process anew, a process described in detail by Dr. Seidl. Dry rot fungi often form a damp white fuzz on the surface of the wood.
Several home repair websites agree that to stop wood rots you first have to eliminate their common cause: water. In basement areas, dehumidifiers will help. In older buildings particularly, but sometimes even in newer buildings, water enters because of defective or inadequate flashing (the various metal and plastic shields put around window frames, at the junction of wall and roof and at the bottoms of wood walls where they tie into foundations). If water has penetrated beneath the exterior siding and rotting has begun between the layers, remove the exterior wood and either replace it or treat it chemically. Allow the inner layers to dry out, replacing where necessary, and treating uninfected wood chemically before replacing it. In all cases, you must remove all infected wood. Thorough removal becomes particularly important when treating dry rot; any remaining dry rot will quickly spread and start a new round of infestation and destruction.
On his website Living with Bugs, Dr. Jack DeAngelis recommends boric acid or similar borates as an effective fungicide for treating dry rot. You can treat wet rot with borates as well, but once you've removed the infected wood, you can also use any number of widely available wood preservatives to coat the nearby areas and prevent reinfestation.