The Enemy of My Enemy? Emergence of the rust pathogen Austropuccinia psidii is associated with the rapid decline of the invasive tree Syzygium jambos in the Galápagos Islands

Abstract The introduction of invasive plant species has become a major threat to the unique biodiversity and ecological integrity of the Galápagos Islands. One of the most aggressive invaders is Syzygium jambos (Myrtaceae), which forms dense tree stands that suppress native vegetation and alter ecosystem functioning. Remarkably, populations of this invasive rose apple tree have undergone a pronounced decline on San Cristóbal Island in recent years. In this study, we combined remote sensing with pathogen identification to investigate this phenomenon. Spectral and spatial analyses revealed an approximate 73% reduction in S. jambos tree cover between 2018 and 2024. Concurrently, symptomatic leaves were examined using morphological characterization and molecular diagnostics. Nested PCR amplification followed by internal transcribed spacer (ITS) region sequencing confirmed the presence of Austropuccinia psidii , a myrtle rust fungus known to infect members of the Myrtaceae family, in symptomatic S. jambos leaves. This study combines morphological and molecular identification of A. psidii infecting S. jambos in the Galápagos Islands with a landscape-scale assessment of the invasive host’s decline. Our findings document an ecological paradox in which an invasive fungal pathogen infects an invasive tree, potentially contributing to its decline while posing a risk to native and endemic Myrtaceae. This interaction highlights how associations among non-native species may influence invasion dynamics, with potentially contrasting consequences for biodiversity conservation in vulnerable island ecosystems.