Characterization of the Mycoparasite Cladosporium oxysporum and Its Interaction with the Poplar Rust Pathogen Melampsora larici-populina

Poplar rust, caused by Melampsora larici-populina, remains a primary biotic constraint limiting poplar growth. In this study, we characterized the diversity and antimicrobial potential of fungi associated with the aeciospore and urediniospore pustules of M. larici-populina. We isolated and identified 502 fungal strains, spanning 16 families, 22 genera, and 47 species. Comparative analysis revealed that the urediniospore stage harbored a greater abundance of fungal isolates but exhibited lower overall diversity, whereas the aeciospore stage was characterized by greater species richness despite lower individual strain counts. Notably, Cladosporium oxysporum was identified as the predominant species across both life stages, maintaining a consistent isolation frequency. We demonstrated that C. oxysporum exhibits strong antifungal activity; its metabolites contain bioactive components capable of degrading the cell walls of M. larici-populina urediniospores, resulting in an inhibition rate of 78.59%. Mechanistic observations via electron microscopy revealed that C. oxysporum develops appressoria to penetrate the urediniospore cell wall and subsequently forms haustoria-like structures within the protoplast to facilitate nutrient acquisition. This hyperparasitic interaction ultimately leads to the death of the rust pathogen. Collectively, our results identify C. oxysporum as a mycoparasite of M. larici-populina and highlight its potential as a promising biological control agent for the management of poplar rust.