Drought influences the fungal community structure, diversity, and functionality inhabiting the grapevine xylem and enhances the abundance of Phaeomoniella chlamydospora

The productivity of grapevines in Mediterranean regions faces significant threats from global warming, which may intensify competition for water resources. Recent research highlights the impact of water deficit on the root-associated microbiota of grapevines, particularly organisms capable of mitigating abiotic and biotic stressors. This study explores the influence of drought on the structure, diversity, and functionality of xylem- inhabiting fungal communities of grapevine, with a focus on the fungal pathogen Phaeomoniella chlamydospora associated with esca and Petri diseases. One-year-old grapevine rootlings grown under greenhouse conditions were subjected to three water regimes: severe water deficit (SWD) at 25% of field capacity, moderate water deficit (MWD) at 50% of field capacity, and no water deficit (AWD) at 100% of field capacity. Wood samples were non-destructively collected before planting (t0) and one (t1) and two (t2) growing seasons after planting from the bottom, medium, and apical parts of the rootstock. Fungal composition and P. chlamydospora abundance were assessed using ITS high-throughput amplicon sequencing (HTAS) and droplet-digital PCR (ddPCR), respectively. The induced water stress not only altered the diversity and composition of the fungal microbiome in the xylem vessels but also affected co- occurrence networks, resulting in less complex networks with fewer correlations between taxa, potentially increasing grapevine vulnerability to various biotic and abiotic stresses. SWD significantly reduced microbial diversity, leading to a shift in the abundance of pathotrophs such as P. chlamydospora in the xylem. This underscores the interconnectedness between water stress, microbiome dynamics, and plant health. The combination of compromised plant defenses, altered physiological conditions, and shifts in the surrounding microbial community may create conditions conducive to increased P. chlamydospora abundance in the xylem vessels of young vines following water stress.