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Regulated deficit irrigation (RDI) is a common practice applied in irrigated vineyards to control canopy growth and improve fruit quality, but little is known of how imposed water deficits may alter root growth and colonization by beneficial arbuscular mycorrhizal fungi (AMF). Thus, root growth and mycorrhizal colonization were determined throughout the growing season for 3 years in own-rooted, field-grown, `Cabernet Sauvignon’ grapevines exposed to three RDI treatments. Vines under standard RDI were irrigated at 60 to 70% of full-vine evapotranspiration (FVET) from 2 weeks after fruit set until harvest, a standard commercial practice. Early deficit vines were exposed to a more extreme deficit (30% FVET) during the period from 2 weeks after fruit set until the commencement of ripening (veraison), and thereafter reverted to standard RDI. Late deficit vines were under standard RDI until veraison, then exposed to a more extreme deficit (30% FVET) between veraison and harvest. The production of fine roots was reduced in both the early and late deficit treatments, but the reduction was more consistent in the early deficit vines because the additional deficit was imposed when roots were more rapidly growing. The frequency of arbuscules in fine roots was greater in both of the additional deficit treatments than in the standard RDI, a response that appeared chronic, as the higher frequency of arbuscules was observed throughout the season despite the additional deficits being applied at discrete times. It appears that grapevines compensated for a lower density of fine roots by stimulating arbuscular colonization. Irrigation did not affect yield or quality of grapes, but reduced whole-vine photosynthesis during the additional deficit periods. It appears that high-quality grapes can be produced in this region with less water than that applied under the current RDI practice because the root system of the vine may be more efficient due to greater arbuscular colonization by AMF.