Genetic diversity and divergence of global and French populations of Septoria steviae , the causative agent of Septoria leaf spot in Stevia rebaudiana

This study investigates the genetic diversity of worldwide Septoria steviae, the causal agent of Septoria leaf spot on Stevia rebaudiana, focusing on French and Argentine populations. Multi-Locus Sequence Typing (MLST) of five loci (ITS, LSU, CAL, EF1 and ACT) was used to compare the diversity of 14 French and 14 Argentinean new isolates with previously published data from other regions. Phylogenetic analyses based on ITS and LSU loci from 61 isolates from 6 countries confirmed that Paraguayan isolates form a genetically distinct group compared to other populations, highlighting the importance of geographic isolation. A more detailed analysis using five loci in 59 isolates from five countries (France, Argentina, the USA, Paraguay, and Japan) showed significant genetic differentiation between Japanese isolates and those from other countries, with French and Argentine isolates being closely related, suggesting common ancestry or gene flow. Local genetic variation among French isolates was also examined, showing minimal differentiation between nearby sites, but with slight evolutionary divergence observed over time. This suggests a slow rate of genetic evolution in French populations, likely due to shared environmental factors and agricultural practices. The results highlight the role of geographic factors in shaping the genetic structure of Septoria steviae populations, and the importance of continued monitoring of genetic diversity to inform management strategies and breeding programs for disease resistance in Stevia rebaudiana.