Hidden threat: reciprocal international sentinel tree plantings reveal high frequencies of cryptic foliar fungal pathogens

To improve early detection of invasive tree pathogens, foliar putative pathogenic fungal communities in reciprocal ex patria sentinel plantings across three continents were examined. Geographic location and tree species were the strongest predictors of fungal community structure, while leaf symptom status had minimal influence—thousands of potential pathogens were detected in asymptomatic plants. Some putative pathogens were even more abundant in asymptomatic than in symptomatic leaf samples (e.g., Fusarium equiseti, Verticillium dahliae, Taphrina spp.) underscoring the prevalence of latent, epiphytic, or transient colonization and the limitations of symptom-based inspections at ports of entry. Seasonality significantly structured pathogen assemblages, with distinct communities observed between early and late-season sampling. High Throughput Sequencing (HTS) greatly expanded pathogen detection compared to classical, isolation-based methods, revealing numerous unculturable fungi and previously unknown potential host-pathogen associations. Based on these findings, we recommend: (i) the involvement and active participation of National Plant Protection Organizations (NPPO) to translate sentinel outputs into harmonized phytosanitary action; (ii) securing long-term infrastructure and site commitments commensurate with tree lifespans; (iii) maximizing geographic and host diversity across networks; (iv) implementing multi-season sampling; and (v) integrating HTS with classical diagnostics on the same samples. Embedding these practices within sustained sentinel planting networks will enhance early warning capacity for tree pathogens and support global plant biosecurity.