**1. Introduction**

The Orchidaceae is the chief family of plants, with over 25,000 plant species worldwide. They are also some of the most vulnerable flowering plants, as numerous genera are endangered and nearly all genera are at risk of habitat harm and over-assortment [1]. China is a rich source of orchid plants, with over 1447 species, mostly located in the subtropical and tropical provinces in the southwest and south [1,2]. Many *Dendrobium* orchids are horticultural plants and have been used for profitable trade due to their flowering profusion, extensive variety of flower colors, shapes, sizes, and year-round producibility, along with long lifespan [3]. Most species are in danger of extinction. Orchids extend their lives as herbaceous plants using two evolutionary methods: sympodial growth and monopodial development, which are influenced by the many endophytic fungal species as main pathogens of

orchids [4,5]. *Dendrobium o*ffi*cinale* has been extensively used in traditional medicines for over 2000 years to decrease fever, inhibit tumors, increase antioxidant activity, treat hypoglycemia, recover from loss of eyesight, and control the immune system, according to the best of China Pharmacopeia [6,7].

*Dendrobium* plants parts, for example, roots, stems, buds, and leaves of tropical orchid, harbor diverse fungal taxa, including mutualistic mycorrhiza, and endophytic fungi and considerably diverse nonmycorrhizal fungal associates. The role of the root-allied fungi is not well understood. They typically originate in the velamen, without causing any disease symptoms [8]. They may encourage the growth of *Dendrobium* by activating soil chemicals in the rhizosphere. The impact of quantities or variations of secondary metabolite have been investigated. At large, they act as a supply for bioactive molecules that defend the host from rhizospheric pathogens [8]. Endophytic fungal groups commonly establish a sole host specificity at the species level, which can be additionally encouraged by microclimatic conditions and microhabitat [9]. The relationship is chiefly stimulated by the endophyte fungi, yielding an overabundance of natural compounds as soon as endophytic fungi are cultured in the external environment of their ordinary hosts or environmental niches under in vitro test conditions [10].

Protocorm and seedling expansion is an important step in commercial orchid production, and its conservation is crucial tool for maintaining the genetic diversity of the orchid plant [11]. Orchid (*Dendrobium nobile* and *D. o*ffi*cinale*) protocorms initiating from the orchid seeds are typically very small, similar to dust, and deficient an endosperm. Subsequently, orchid seed incubation and seedling expansion require well-matched endophytic fungi to deliver the carbon, nutrients, and water to the seeds under usual plant conditions [3]. Consequently, an in vitro proliferation method could be a useful for the mass measure proliferation of these orchids for their marketization. The protocorms and seedlings rarely survive after relocation into nature from well mature sterile culture. The orchid protocorm may be reliant on appropriate endophytic fungi for seedling existence [12].

Most examinations of *Dendrobium* myco-endophytes (DMEs) focused on symbiotic in vitro practices using fungal endophyte strains obtained from fully-grown dendrobium roots, buds, stems, and leaves [3]. Understanding whether these endophytic fungi are pathogenic, conditional pathogenic, or non-pathogenic for the host plant is important. DMEs can be transmitted horizontal or vertically. Vertical transmission occurs when the seeds are contacted through the fungal endophytes and are transmitted to the host plant. Horizontal transmission involves the formation of exterior spores and their airborne dispersal infects many other hosts [13].

A steady state between the fungal pathogen and its host plant is achieved when the pathogen resides in equilibrium with the surrounding host tissues and causes little damage, which is also called least pathogenic. The virulence expression of the pathogen is dependent on the particular host environment. When the pathogen is isolated from an asymptomatic host and introduced into a new host, strong pathogenic reactions may be observed [14]. Commonly, fungal endophytes have functions ranging from latent pathogens to mutualistic symbionts. Reliant on the host genome type, some endophytic fungi may be pathogenic in stressed hosts, whereas they can be helpful in other conditions due to conditional pathogenic properties [15]. In the environment, orchids are chiefly dependent on these fungal endophytes for their nourishment and propagation along with the succeeding seedling (protocorm) phases. However, only imperfect quantitative approaches for assessing the *Dendrobium*–fungus connections at the protocorm and seedling phase are available at present, which places major constraints on understanding the host–endophyte relationships [16].

In this study, we focused on species-specific endophytic fungi pre-isolated from *D. nobile* plant parts, which were inoculated onto well-developed protocorms and seedlings of *D. nobile* and *D. o*ffi*cinale* for the evaluation of pathogenicity using in vitro inoculation and histopathological examination of infected tissues. The test endophytic fungus was re-isolated from the leaves tissues for examining Koch's postulates.
