Some species of wood are more naturally durable because they contain substances toxic to fungi, and the heartwood of these will only be attacked by certain fungi. It can be generally assumed, however, that no wood is entirely immune to attack if placed in conditions favourable to fungal growth, allowing for some variation in the susceptibility to decay of different kinds of wood. It is possible to eliminate the food supply by treating it with certain substances toxic to fungi but harmless to men and animals. This is the basis of wood preservative treatment. If there should be insufficient moisture, after growth has started, the fungi do not necessarily die, but will probably become merely dormant.
Active growth can start again, sometimes years later, when sufficient moisture returns.
It is necessary for the substratum to be moist and the humidity of the surrounding atmosphere to be high for the germination of fungal spores. About 35 to 50 per cent moisture is required for wood rotting fungi to flourish, the actual moisture content depending on the species of fungi and the kind of wood. Fungal spores do not germinate readily on wood that has a moisture content below the fibre saturation point, commonly reached at around 25 to 30 per cent. When wood is put into service at such low moisture contents, subsequent conditions can cause high local moisture contents, which may be conducive to fungi growth. Once started fungi can produce a certain amount of moisture by the chemical decomposition of the wood, and can thus increase the moisture content of the wood if evaporation loss is low.