Mechanisms of pathogenic microorganisms, root antioxidant systems, and secondary metabolites of Codonopsis pilosula (Franch.) Nannf. in response to continuous cropping obstacles

Understanding the mechanisms that give rise to obstacles in the continuous cultivation of C. pilosula is essential for addressing or mitigating these challenges. The findings of this study suggest that repeated cultivation significantly reduced the content of polysaccharide in roots, and significantly increased the dead seedling rate in the field. The vascular bundles of the affected plant were extensively colonized by fungi. Furthermore, the root vascular bundles exhibit significant woodiness and corkiness, accompanied by cellular fractures and structural collapse. It was determined that the pathogenic endophyte is Fusarium oxysporum , and the exacerbated disease manifestation corresponds to an acute wilting type. Additionally, the root-zone soil microorganisms Cladosporium austroafricanum , Fusarium foetens , Fusarium petersiae , and Acaulium retardatum may significantly contribute to the yield-reducing phenomenon associated with continuous cropping. The proliferation of pathogenic bacteria during continuous cultivation initiates a complex interaction mechanism between the host plant and these pathogens. This process is characterized by a rapid increase in calcium ion (Ca²⁺) concentration, which subsequently leads to an upsurge in reactive oxygen species (ROS), particularly manifested as elevated levels of hydrogen peroxide (H₂O₂). Additionally, this response triggers thickening of cell walls and other immune mechanisms aimed at inhibiting the invasion of pathogenic bacteria. At the same time, to prevent ROS from inducing oxidative damage and triggering oxidative stress, there is a notable increase in both antioxidant enzyme activity and antioxidant substances content. • This study found that continuous cropping significantly reduced the polysaccharide content of the pharmacologically active ingredient in C. pilosula , while also markedly increasing the dead seedling rate in the field. • A substantial number of fungi were observed adhering to the cell walls of vascular bundles in diseased plants, and there was severe lignification of the root system's vascular bundles, accompanied by cell rupture and collapse. Fusarium oxysporum was identified as the pathogenic endophyte, the disease type was acute blight type. • Furthermore, rhizosphere microorganisms such as Cladosporium austroafricanum , Fusarium foetens , Fusarium petersiae , and Acaulium retardatum may serve as primary rhizosphere pathogens associated with disorders stemming from continuous cropping. • Continuous cropping boosts pathogenic fungi, triggering an interaction between C. pilosula and them. It features a quick Ca²⁺ level rise, leading to ROS outbreak (H₂O₂ increase), which causes cell wall thickening and immune responses to block pathogen invasion. • Concurrently, to mitigate oxidative damage induced by ROS, antioxidant stress is activated; this includes significant increases in both antioxidant activity and antioxidant content.