Facultative parasite

A facultative parasite is an organism that may resort to parasitic activity, but does not absolutely rely on any host for completion of its life cycle.

Examples of facultative parasitism occur among many species of fungi, such as members of the genus Armillaria. Armillaria species do parasitise living trees, but if the tree dies, whether as a consequence of the fungal infection or not, the fungus continues to digest the wood without further need for parasitic activity; some species even can infest dead wood without any parasitic activity at all. As such, although they also are important ecological agents in the process of nutrient recycling by microbial decomposition, the fungi become pests in their role as destructive agents of wood rot.