Foliar application of a Rugulopteryx okamurae extract enhances soil resilience in greenhouse grapevines

Mildew disease poses major challenges in viticulture, with copper-based compounds remaining the primary treatment, despite their associated environmental drawbacks, including effects on soil microbiota due to prolonged applications. Seaweed based extracts are emerging as sustainable alternatives, but their impact on non-target microbiota remains poorly understood. This study evaluated the belowground effects of an aqueous Rugulopteryx okamurae foliar extract (RO) on two Tempranillo clones (RJ43 and VN40) under greenhouse conditions, assessing soil response compare to copper (BB) and water (W) treated plants. 16S rRNA and ITS high-throughput sequencing was used to profile bacterial and fungal soil communities, alongside belowground physicochemical analyses (soil pH, NPK, and fresh-to-dry weight ratio). Microbial communities differed significantly between clones prior to the treatments, highlighting the scion's influence on soil microbiota. Copper increased the proliferation of wood-decaying or potentially pathogenic taxa, elevating the risk for grapevine trunk diseases ( Phaeoacremonium sp.). In contrast, the seaweed extract promoted beneficial fungal taxa, including Hypholoma fasciculare (RJ43), Trichoderma hamatum (VN40) and Candida subhashii (in both clones). Notably, RO-treated soils of RJ43 showed a 75 % reduction in pathotrophic fungal groups. RO also enhanced microbial connectivity, suggesting a more stable and resilient soil fungal community. Results therefore suggest that R. okamurae extract may improve soil status by modulating the belowground microbiome. These responses were clone-specific, highlighting scion-driven plant–microbe interactions in biostimulants performance. Overall, the enhancement of the soil–vine alliances observed supports the potential of R. okamurae as a sustainable strategy for improving soil microbial balance and resilience in viticulture systems.