The English walnut (Juglans regia L.) is the main species cultivated for the production of edible nuts. Due to heterozygosis, the characteristics of agronomical interest of walnut cultivars are not inherited via seed propagation. In vitro plant tissue culture thus plays a key role in mass propagation of high-quality walnut rootstocks. Micropropagation of walnut explants needs an ex vitro acclimatization phase to repair the in vitro induced abnormalities, and further requires a post-acclimatization growth in greenhouse conditions when plantlets become photoautotrophic. However, poor survival and slow growth rates are common difficulties encountered in nurseries when establishing micropropagated walnut saplings. As many other fruit and nut bearing trees, walnut exhibits a high dependency on symbiotic soil-borne arbuscular mycorrhizal (AM) fungi for development due to a coarse root architecture that limits soil inorganic phosphate (Pi) uptake. In the context of rootstock production, we investigated at different development stages the establishment of seven walnut rootstocks of economic interest, upon inoculation or not with an AM fungus and two contrasting Pi fertilization regimes. We demonstrated that biotization with the AM fungus Rhizophagus irregularis decrease micropropagated walnut rootstock dependency on Pi fertilization both at the acclimatization and post-acclimatization stages, together with improving adequate sapling development and quality, especially in the hybrid rootstocks VLACH, VX211, and WIP3. We also showed that these benefits are rootstock-dependent, indicating that walnut mycorrhizal dependency for Pi nutrition varies between cultivars, and that the symbiotic program may be differentially recruited depending on the chosen rootstock.