Rot and decay is the general description for the disintegration of wood fiber. Decay is caused by microscopic organisms like mold and fungus that feed off the natural substances found in wood. Fungi require four conditions in order to survive: an oxygen supply, 40-100 F temperatures, a supply of moisture (most fungi need a moisture content of 30% or more to thrive), and a food source. Some woods like cedar and redwood have extracts in their heartwood that make them naturally resistant to decay. Pressure treated wood uses man made chemicals that are toxic to fungus and mold. Many stains have mildew inhibitors that should help prevent decay. The most effective method of avoiding decay is to keep your deck materials dry before and after installation.