A pioneering attempt to simultaneously introduce an ericoid mycorrhizal (ErM) fungus, Oidiodendron maius Barron, and two strains of a dark septate endophyte (DSE) Phialocephala fortinii Wang & Wilcox (strain P. fortinii F and P. fortinii H) into root systems of individual Rhododendron cv. Azurro plants was conducted in split root systems. The inoculation had no effect on the total biomass of inoculated rhododendrons. However, plants accumulated more root biomass into compartments inoculated with P. fortinii H than to those non-inoculated or inoculated with P. fortinii F. Plants with the highest foliar P concentrations had been inoculated with O. maius, co-inoculated with O. maius and P. fortinii H and inoculated with P. fortinii H. Inoculation with O. maius and co-inoculation with O. maius and P. fortinii H also altered N uptake. Inoculation with O. maius and its co-inoculation with both P. fortinii strains decreased the foliar C:N ratios. All fungi colonized host roots at low levels, P. fortinii F being the most successful colonizer. Contrary to the other fungi, O. maius also colonized Rhododendron microcuttings at low levels in vitro, and the colonization pattern was distinct from Hymenoscyphus ericae. Both P. fortinii strains exhibited a typical DSE colonization pattern in vitro. Our study indicates that O. maius and P. fortinii positively affect host plant nutrition and demonstrates interactions between separately developing ErM and DSE fungi, which significantly affect plant physiology.