Infestation
Brown rot and other wood decay fungi are caused by an excess of wood moisture -- above 20% wood moisture will promote the growth of fungi in the wood. This may occur in wet crawl spaces, or any structural wood exposed to water.
Identification
The dry wood is 'checked', the wood is divided into small blocks, and these may easily fragment. This is often called dry rot, but it originates from wet conditions.
Control
Reduce wood-moisture content to below 20% by eliminating excess water in the crawl space, or fixing the leak. Use plastic to cover a portion of the soil in the crawl space.
