**3. Discussion**

*Dendrobium* is largest genera of Orchidaceae, with more than 1000 species globally [3]. *D. nobile* and *D. o*ffi*cinale* are a wild epiphytic orchid found in the tropical rain forests in China, especially in Guizhou province, China. The solitary and attractive inflorescence of these orchids is slightly exclusive and valued amongst orchid cultivators. Due to their high commercial demand, they are being removed from their usual habitat, placing them at high risk of becoming extinct. To preserve and reinstate threatened and rare orchid plants, these plants must be reintroduced with fungal co-culturing. Hence, some pure endophytic fungal strains that stimulate or decrease *Dendrobium* protocorm and seedling growth must be determined in vitro [17]. For this purpose, co-culturing will be essential for *D. nobile* and *D. o*ffi*cinale* success. Biosynthesis of many active compounds will be necessary, predominantly where specific fungal strains are vital for actively providing molecules to the protocorm with or without producing symptoms.

This study provides valuable information about the orchid bionetwork with endophytic fungi associated with *D. nobile* and *D. o*ffi*cinale* under laboratory environments. The in vitro pathogenicity evaluation process was used to effectively assess well-suited and species-specific endophytic fungi in terms of their symptoms and pathogenicity index for *D. nobile* and *D. o*ffi*cinale*. For the first

time, we found that *T. longibrachiatum* (ZF05) produced the least pathogenic effects on *D. nobile* and *D. o*ffi*cinale*, which provided the plants with nutrition and helped them build an active defense mechanism to survive without the presence of any nutrients in the media. This phenomenon was observed as asymptomatic colonization due to the balanced antagonism between the host and the endophyte [13]. Endophytes also possess various virulence factors that are contradicted by host plant defense mechanisms. If endophyte virulence and host *Dendrobium* defense are well balanced, the relationship is avirulent and asymptomatic. This stage is only a transitory time where environmental influences play a key role in destabilizing the delicate equilibrium of antagonisms. [18]. Endophytic fungi can deliver appropriate carbon sources, amino acids, vitamins, and hormones that are important for seedling and protocorm development [19]. Khamchatra et al. [3] also stated that *Beauvaria* and *Fusarium* species are endophytic fungi, which we also recovered, that may play a role in the growth and survival of the plants like *D. friedericksianum*. Though grown in orchid stems and roots, the fungi may be non-casual and non-mycorrhizal endophytes. Many of these endophytes were reported in many white rot fungi and are incapable of phytopathogenicity [3]. In another study, Meng et al. [20] found that several fungal species recovered from some species of *Dendrobium* protocorms and seedlings have the ability to cause disease symptoms. Zi et al. [21] described *Epulohiza*, an anamorphic phase of *Tulasnella*, in the *Dendrobium* protocorms. Athipunyakom et al. [22] reported that *Trichosporiella multisporum* is present in *Paphiopedilum* roots. In an ecosystem, orchids depend on orchid fungal endophytes to provide needed nutrients for growth, a process termed an asymptomatic relationship. The symbiotic plant relationship, specifically in vitro approaches, is adopted because it enables higher growth rates and/or symbiotic seedlings and protocorms progress faster than without this relationship [11]. Although extensive information about the co-culturing expansion is not available, the process was endorsed as an effective process for improving the growth of many orchids [19].

From this study, we found that *C. tropicicola* (ZF01) is a highly pathogenic strain for the protocorms and seedlings of both *D. nobile* and *D. o*ffi*cinale*. *Dendrobium* endophyte contact might, in addition to balancing between defense and virulence, might more precisely control this complex contact [10]. Plant–pathogen association may be responsible for the growth conditions for plant disease. Because several fungal endophytes may be latent plant pathogens, certain inherent or environmental influences may prompt pathogenic effects [18]. Many fungal endophytes are silent/conditional pathogens, only resulting in disease as the plant ages or under stress conditions. Fungal endophytes accomplish asymptomatic colonization through a stable neutral antagonism between the fungal virulence and its response to the plant defense mechanism [23]. We hypothesized from our research that *T. longibrachiatum* (ZF05) mycelium may migrate from inoculated to uninoculated tissues of host plants. The histological image of *D. nobile* and *D. o*ffi*cinale* seedlings clarified that the fungus infects tissues. This paper is the first report on the cross-transmission of *T. longibrachiatum* (ZF05) from inoculated tissues to uninoculated tissues confirmed by re-isolated from host plant segments that fulfilled the Koch postulates. *Colletotrichum, Fusarium*, and *Trichoderma* include morphologically similar taxa that are commonly found as endophytes, saprobes, and plant pathogens [13–15].

As endophytes exist in within plant tissues and endlessly network with their host tissues, fungal endophytes may be linked intracellularly, which is responsible for the cross-transmission of the fungus into the new cells. Along this line, we strongly suggest that endophytic fungus is re-isolated from the uninoculated infected segments of the *D. nobile* and *D. o*ffi*cinale* seedlings. This means that these endophytic fungi are cross-transmitted intracellularly from one cell to another healthy cell, creating comprehensive endophytic molecular connections, cross-species appearance, and on/off switching of the compulsory gene cascades that constantly yield a chosen molecule [10].
