Reduced foliar disease promotes mycorrhizal mutualism in invasive plants and their native counterparts: The role of root exudate chemistry

Abstract Interactions among foliar pathogens, host plants and soil mutualists, regulated by plant chemicals, can influence plant performance. While invasive plants in their introduced ranges are often attacked by fewer foliar pathogens and form stronger arbuscular mycorrhizal fungal (AMF) associations compared to native plants, it remains unclear whether lower foliar pathogens are related to higher AMF associations. In particular, it is uncertain what role root exudate chemistry plays in linking foliar pathogen release and AMF promotion. In field surveys, we examined the relationships between foliar fungal disease and AMF colonization in 15 invasive and 13 native Asteraceae species in central China. In complementary experiments, we manipulated foliar fungal diseases using fungicide treatments and sooty mould inoculations to assess their effects on AMF colonization and investigated whether the flavonoid quercetin (a key signalling compound in AMF symbiosis) in root exudates mediates their interactions. In field surveys, invasives had lower leaf area diseased and higher AMF colonization than natives with a negative relationship between leaf area diseased and AMF colonization. These patterns were consistent in the experiments. Foliar fungicide application increased AMF colonization while sooty mould inoculation reduced it. Across treatments, AMF colonization was negatively related to leaf area diseased. Moreover, quercetin exudate concentrations increased with fungicide application, decreased with sooty mould inoculation, were negatively related to leaf area diseased and were positively related to AMF colonization. Synthesis . Our findings suggest that decreased foliar pathogens can strengthen AMF mutualism in both invasive plants and their native counterparts by altering root exudate chemistry. This above‐ and below‐ground linkage underscores the importance of enemy release in shaping plant–microbe interactions in plant invasions and the role of chemical signals in modulating such interactions.