Arbuscular mycorrhizal fungi are soil microbes that live in vine roots. The combination of the fungus and the vine is a mycorrhiza, or a symbiotic relationship, with mutual benefits. The vine provides the fungus with its sole source of carbon and the fungus provides the vine with soil derived nutrients, namely phosphorus. Mycorrhizae play essential roles in grapevine nutrition, as evidenced by their beneficial effects on vine growth, phosphorus nutrition, and water relations. The fungal filaments hyphae that extend into the soil make up the majority of the fungal biomass. They significantly increase the absorptive surface area of the root system and enhance the ability of grapevines to utilize nutrients from the soil solution. The nutrient deficiencies grapevines develop in the absence of mycorrhizae, increase after soil fumigation.

Arbuscular mycorrhizal fungi form elaborate structures called arbuscles. The fungus actually grows within the root itself, within the space between the cell walls and cell membranes of the root cortex. These fungi are obligate colonizers, meaning they cannot grow in the absence of a root. Their fungal filaments or hyphae extend outside of the root into the soil. The fungus can absorb nutrients from the soil that can be transported to the root system via the hyphae of the fungus. The association of the fungus with the plant also permits colonization of, and access to more challenging soils. It is lower in cost to the plant, in terms of carbon consumption needed to produce new roots, versus feeding the fungal biomass. Both the plant and the fungus gain from this association. The fungus is provided with carbon by the plant. Mycorrhizal grapevines have improved shoot and root growth, higher tissue concentrations of phosphorus, and improved water relations. Mycorrhizal grapevines have been shown to resist certain fungal diseases. The external hyphae can serve as a "bridge" for nitrogen transfer between plants, and can take up nitrogen from decaying cover crops that can then be transferred to the grapevine.

The presence of mycorrhizal fungi is of greater importance in nutrient limiting soils, where their contribution to nutrient uptake may be more critical to the plant. Since the fungi receive carbon directly from the grapevine, turnover of their external hyphae provides an important source of carbon to feed other microbes in the soil, whose populations are responsible for nutrient cycling.