Regional variation strongly shapes the culturable root mycobiome of rosette-stage rapeseed (Brassica napus L.) in northwestern Spain

: Decades of agricultural intensification reliant on monocultures and synthetic agrochemicals have compromised environmental health, prompting a European policy shift toward sustainable production systems. Alternatives to agrochemicals include rotations with soil-improving crops and microorganisms that promote plant growth and/or exert biocontrol, such as endophytic fungi. This study employs culture-dependent methods to examine root-associated fungi of rapeseed ( Brassica napus ), selected for its status as an improving crop, with the primary aim of characterizing fungal diversity and determining regional variations in endophytic community composition. Twenty-four plots were sampled, distributed across six agricultural regions in northwestern Spain, characterized by distinct edaphoclimatic conditions. Five plants were collected from each plot, and fungal endophytes were isolated from their root systems. The isolates were taxonomically identified at the genus level using the internal transcribed spacer (ITS) and clustered into operational taxonomic units (OTU). After that, alpha and beta diversity analyses were performed to statistically evaluate spatial variation across the study area. Results revealed significant regional differences in fungal community composition, with distinct clusters emerging in accordance with similar environmental conditions. Furthermore, taxa belonging to genera with widely recognized agricultural potential (e.g. Trichoderma , Penicillium , or Chaetomium ) were isolated and exhibited distinct, region-specific distribution patterns. These findings support the premise that root endophytic fungal taxa vary regionally, suggesting that universal bio-inputs may not perform with equal efficacy across different environments. Thus, harnessing native strains offers promising avenues for future functional screening, paving the way toward customized bioformulations tailored to local conditions.