Orchids are among the most prized ornamental plants in many societies throughout the world. As a result, consumer demands have created a significant pressure on wild populations of many species, including Cymbidium goeringii Rchb. f., a rare terrestrial orchid endemic in China, Korea, and Japan. To help conserve natural populations of C. goeringii, we recently started investigating methods to cultivate these orchids. Here we fulfilled Koch’s postulates and demonstrated that fungal strains isolated from the roots of natural Cymbidium plants increased fresh mass, plant height, number of leaves, and root length of C. goeringii, and that the two fungal strains originally isolated from C. goeringii showed overall greater effects on growth than two other strains from other Cymbidium species. Internal transcribed spacer sequence analyses revealed that the four fungal strains likely represented at least two new taxonomic groups, both belonging to the family Ceratobasidiaceae of the Rhizoctonia fungi. Our study demonstrated that these fungal strains could potentially help the commercial cultivation of the increasingly rare and endangered orchid C. goeringii.