Evaluation of foliar fungus‐mediated interactions with below and aboveground enemies of the invasive plant Ageratina adenophora

Plant–fungal associations are frequently key drivers of plant invasion success. Foliar fungi can benefit their invasive hosts by enhancing growth promotion, disease resistance and environmental stress tolerance. However, the roles of foliar fungi may vary when a given invasive plant faces different stresses. In this study, we designed three independent experiments to evaluate the effects of a foliar fungus, Colletotrichum sp., on the growth performance of the invasive plant Ageratina adenophora under different soil conditions, as well as the responses of A. adenophora to the foliar fungal pathogen Diaporthe helianthi and to herbivory. We found that the soil type was the most influential factor for the growth of A. adenophora. The role of the foliar fungus Colletotrichum sp. varied in the different soil types but generally adversely affected leaf development in A. adenophora. Colletotrichum sp. may be a weak latent foliar pathogen that can enhance the pathogenicity of D. helianthi on leaves of A. adenophora and marginally reduce signs of herbivory by natural insects in the wild on A. adenophora seedlings. In general, the benefits of the foliar fungus Colletotrichum to the fitness of A. adenophora are not significant in the context of this experimental design. However, our data highlight the need to consider both aboveground and belowground biota in different soil habitats when evaluating the effects of foliar fungi.