Role of the Foliar Endophyte Colletotrichum in the Resistance of Invasive Ageratina adenophora to Disease and Abiotic Stress

Plant‒fungal associations often drive plant invasion success by increasing host growth, disease resistance, and tolerance to environmental stress. In this study, to determine whether the numerically dominant foliar endophyte Colletotrichum can directly enhance the resistance of the invasive plant Ageratina adenophora to disease and environmental stressors, we planted sterile A. adenophora seedlings, inoculated them with three endophytic fungal strains of Colletotrichum with distinct genetic backgrounds and compared their role in the resistance of A. adenophora to the pathogenicity of three foliar pathogens from the Didymellaceae family, as well as to abiotic stress, including drought and nutrient stress. We observed that the foliar endophyte Colletotrichum reduced photosynthesis-related physiological indicators, increased resistance-related indicators, and decreased the biomass of A. adenophora in the absence of pathogen attack, drought and nutrient stress. However, some endophytic Colletotrichum strains exhibit positive effects on resistance to certain foliar pathogen attacks, and some endophytes increase the biomass of A. adenophora under drought or nutrient stress. Our results revealed that there are certain positive effects of foliar Colletotrichum endophytes on A. adenophora in response to biotic and abiotic stresses, which may be beneficial for its invasion.