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Simple Summary: The currently applied containment strategies against the quarantine bacterium Xylella fastidiosa, the causative agent of olive quick decline syndrome, resulted to be only partially effective in limiting the further spread towards other uninfected areas. This study reports the promising results of an endotherapeutic trial conducted using a phenolic extract from olive leaves to counteract the bacterium, in comparison with a solution based on garlic powder and potassium phosphite, in naturally infected mature olive trees in Apulia Region. The two years in planta trial demonstrated a statistically significant effectiveness of polar phenols in stimulating the vegetative growth of the treated trees, likely due to the bacteriostatic effect shown in an in vitro test. A similar effect in limiting the planktonic growth of the bacterium was also highlighted for the solution based on garlic powder, which, however, did not seem to have significant effects in stimulating the vegetative growth of plants, unlike potassium phosphite which instead confirmed its plant growth booster action. (1) Background: Since 2013, the pathogenic bacterium Xylella fastidiosa has been severely affecting olive production in Apulia, Italy, with consequences for the economy, local culture, landscape and biodiversity. The production of a phenolic extract from fresh olive leaves was employed for endotherapeutic injection into naturally infected olive trees by Xylella fastidiosa in Apulia region, Italy. (2) Methods: The effectiveness of the extract was tested in vitro and in planta in comparison with analogous treatments based on garlic powder and potassium phosphite. (3) Results: The uptake of phenolic compounds from olive leaves through a trunk injection system device resulted in a statistically significant increase in leaf area index and leaf area density, as well as in the growth of newly formed healthy shoots. Plant growth-promoting effects were also observed for potassium phosphite. Moreover, the bacteriostatic activities of the phenolic extract and of the garlic-powder-based solution have been demonstrated in in vitro tests. (4) Conclusions: The results obtained and the contained costs of extraction make the endotherapeutic treatment with phenolic compounds a promising strategy for controlling X fastidiosa to be tested on a larger scale, although the experiments conducted in this study proved not to be suitable for centenary trees. [ABSTRACT FROM AUTHOR]