Rhododendron kanehiraeWilson is an endemic species of Taiwan that is listed as extinct in the wild by the Red List of Vascular Plants of Taiwan. R. kanehirae seedlings can form mycorrhizae with the ericoid mycobiont Oidiodendron maius. A completely randomized experimental design was conducted to investigate the effects of three different O. maius isolates (strains CBS110450, RhYM3, and RooDK1) as well as nitrate and ammonium (inorganic nitrogen) on the growth of R. kanehirae by evaluating height and shoot, root, and total fresh weight. Three isolates of O. maius showed different colony morphology and growth rate, which was further verified by molecular analysis. The results showed that application of ammonium fertilizer improved the height and shoot, root, and total fresh weight of all of the R. kanehirae seedlings. Among them, seedlings inoculated with RhYM3 had the highest fresh weights and heights. By applying an appropriate amount of nitrogen combined with RhYM3 inoculation, the rhizome of R. kanehirae grew well. The information provided here is crucial for the conservation of R. kanehirae, as this species is on the verge of extinction.