Phenolic extract from olive leaves as a promising endotherapeutic treatment against Xylella fastidiosa in naturally infected Olea europaea (var. europaea) trees

(1) Background: Since 2013, the pathogenic bacterium Xylella fastidiosa has been severelyaffecting olive production in Apulia, Italy, with consequences for the economy, local culture, landscapeand biodiversity. The production of a phenolic extract from fresh olive leaves was employed forendotherapeutic 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 withanalogous treatments based on garlic powder and potassium phosphite. (3) Results: The uptakeof phenolic compounds from olive leaves through a trunk injection system device resulted in astatistically significant increase in leaf area index and leaf area density, as well as in the growth ofnewly formed healthy shoots. Plant growth-promoting effects were also observed for potassiumphosphite. 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 andthe contained costs of extraction make the endotherapeutic treatment with phenolic compounds apromising strategy for controlling X fastidiosa to be tested on a larger scale, although the experimentsconducted in this study proved not to be suitable for centenary trees.