Olive Quick Decline Disease (OQDS) is a devastating olive disease emerged few years ago in the region of Apulia (southern Italy) as a result of the bacterial infections caused by Xylella fastidiosa subsp. pauca. Bacterial infections were consistently associated to severe desiccations on the local cvs Cellina diNardò and Ogliarola salentina,whereas traits of resistance, i.e. mild symptoms of OQDS were found in the cvs Leccino and FS17. Investigations on the olive microbiome of OQDS-resistant and susceptible cultivars were undertaken to identify potential protecting endophytes for a strategy of biocontrol. The xylem bacterial and fungal communities of infected trees of the cvs FS17 and Kalamata, respectively symptomless and highly symptomatic, were analysed by barcode (16S rRNA V4 and ITS1-spanning amplicons) and whole shotgun shotgun (WSS) sequencing. The core microbiome was dominated by fungi, accounting 99.8% and 88.4% of the total reads by barcode and WSS sequencing, respectively, while Proteobacteria andAscomycota are the most represented phyla with both techniques. This fungi/bacteria ratio was maintained in all FS17 olives while it was inverted in the susceptible cv. Kalamata, where Xylella colonized the majority of the ecological niche in the heavily affected plants. Moreover, bacterial communities of trees of the cvs FS17 and Leccino were purified and co-inoculated with X. fastidiosa into potted olives of the cv. Cellina di Nardò for evaluating potential effects on the progression of Xylella-infections in this susceptible cultivar. Data on the progress of Xylella infections and/or symptoms appearance in these microbiome-Xf co-inoculated plants will be presented.