A mycorrhiza is a symbiotic association which exists between a fungus and the roots of a vascular host plant. The term mycorrhiza refers to the role a fungi has within a plant’s rhizosphere (its root system). Mycorrhizae play extremely important roles in soil biology and soil chemistry.

In a mycorrhizal association, the fungus colonizes the host plant’s root tissues, either intracellularly as in arbuscular mycorrhizal fungi (AMF or AM), or extracellularly as in ectomycorrhizal fungi. The association usually has mutual benefits to both the fungus and the plant, although in particular species and/or circumstances, mycorrhizae may actually be pathogenic within host plants.