In situ and in vitro specificity between Rhizoctonia spp. and Spiranthes sinensis (Persoon) Ames, var. amoena (M. Bieberstein) Hara (Orchidaceae).

Abstract

The relationships between the orchid Spiranthes sinensis (Persoon) Ames. var. amoena (M. Bieberstein) Hara and Rhizoctonia spp. were investigated in situ at germination and in adult plants, Seeds of the orchid placed in cotton gauze were buried at 210 sampling points in turf grassland, the orchid habitat (in situ germination). Eight weeks later, protocorm development of the orchid was confirmed at 67 of the 210 sampling points. Isolation of fungi from protocorms showed that in situ germination was induced mainly by Rhizoctonia rapens Bernard. Similarly, R. repens was the dominant mycorrhizal fungus isolated from roots of adult plants. The number of adult plants within a radius of either 30 or 50 cm of burial points did not influence seed germination. The distribution of Rhizoctonia spp. other than R. repens in the sample site was examined with a baiting method using buckwheat stems. Thirty-two isolates consisting of binucleate Rhizoctonia anastomosis group (AG)-A, AG-B, AG-G, and AG-1, R. solani Kuhn AG-4, Waitea circinata Warcup & Talbot, which anastomozed with WAG-O and WAG-Z, and a multinucteate Rhizoctonia sp. were isolated. Three AG-G isolates were obtained from the points at which protocorm development was induced by R. repens. Seeds of S. sinensis var. amoena were inoculated in vitro with these isolates to test for symbiotic germination. Most Rhizoctonia spp, not associated with the germination in situ induced seed germination in vitro. Seedlings which developed with these isolates in vitro were transferred to ex vitro conditions. New leaves developed and elongated as seedlings continued to grow for 3 months, The seed burial method enabled the clarification of the differences in orchid-fungal specificity in situ and in vitro. We concluded that the specificity between S. sinensis var. amoena and fungi in situ conditions was different to that in vitro.