*4.8. Histological Examination of Least Pathogenic Fungal Endophyte Samples*

Infected *D. nobile* and *D. o*ffi*cinale* samples were used for visual histopathological assessment of endophytic fungi by staining the infected tissues using a method modified from Ding et al. [36]. Infected seedling segments were stained using lactophenol cotton blue to identify the tissue infected with endophytic fungi. The distribution of fungal endophytes and their localization was studied using a microscope (Nikon Eclipse E200, Model Eclipse E200MV R, Nikon Corporation, Tokyo, Japan) with DS-1600 Panasonic CMOS sensor. The endophytic fungal colonies were observed as blue after staining with lactophenol cotton blue in vascular bundles and cortex region of *D. nobile* and *D. o*ffi*cinale* leaves and stems. Photographs were captured under different magnifications (10× and 40×).

#### *4.9. Statistical Analysis*

Statistical calculations were completed using SPSS11 software (USA) and the Fisher's least significant difference procedure. Correlations of disease index caused by *C. tropicicola* (ZF01), *F. keratoplasticum* (ZF02), *F. oxysporum* (ZF03), *F. solani* (ZF04), and *T. longibrachiatum* (ZF05) were calculated using SAS CORR procedure.

#### **5. Conclusions**

In this study, we evaluated fungal pathogenicity and colonization inside plant tissues under in vitro conditions. Firstly, *T. longibrachiatum* (ZF05) was found to be the least pathogenic or a conditional pathogen that supports the development of the *D. nobile* protocorms and *D. o*ffi*cinale* seedlings without the presence of any nutrients in the media. *C. tropicicola* (ZF01) is a highly pathogenic strain, responsible for the host death. We concluded that endophytic fungi were cross-transmitted from host plant inoculated to uninoculated cells, which was confirmed by histopathological examination and re-isolation of the same endophytic fungi from uninoculated plant tissues. Future investigations should determine what role, if any, the plant host specificity plays in the interior plant passage and differential tissues establishment by test fungal endophytes. How fungi are able to precisely move

within tissue (host plant) should also be examined. The symbiotic seedling and protocorm growth are advantageous and expedient method to improve orchid growth under the experimental conditions and could help with the reintroduction of *Dendrobium* orchids to the natural environment.
