Decay occurs when organisms such as fungi use wood as a food source. Fungi require several conditions to survive. They must have water, the right temperature, oxygen, and food. In these situations, the best way avoid fungi infestation is to poison the food source. This can be done using treated wood, or wood that has extractives in the heartwood, which are toxic to fungi.
Pressure treated lumber is the best choice to use when wood is to be exposed to weather in use.
When selecting pressure treated material it is important to use the proper treatment for the job. The amount of preservative in the wood after treatment is called the retention level. The preservative retention levels are printed on the quality mark tag that is stapled to the end of the board. Often trees grown around the farm can be used and although not treated, many of these species will provide several years of good service. There are still several species commonly available that provide excellent resistance to decay and should last several years.