Phenolic Extract from Olive Leaves as a Promising Endotherapeutic Treatment against Xylella fastidiosa in Naturally Infected Olea europaea (var. europaea) Trees

Abstract

(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.

Keywords: Olea europaea L.; Xylella fastidiosa; endotherapy; polar phenols.

Figure 1

Positioning of the Arbocap^® syringes for endotherapeutic treatment in a secular plant (a) and in each of the 4 principal branches of an adult olive tree (b).

Figure 2

Percentage of planktonic growth (a) after 3 and 6 dpi (days post inoculation) and biofilm formation (b) after 6 dpi of Xylella fastidiosa subsp. fastidiosa (Xff) strain Temecula1 (NCPPB 4605) grown in PD2 broth added with four formulations (G, PE1, PE2, PE3) at different concentrations (1:10, 1:100) and PD2 without substances (Ctr). Values are means ± SD of three independent biological replicates, normalized with absorbance obtained by blank values (non-inoculation), of the most representative experiment of two independent experiments. (statistically significant difference obtained between control and the concentrations of the different formulations, according to one-way ANOVA analysis, Dunnett’s multiple comparisons test: **** p < 0.0001, ** p < 0.01).

Figure 3

Effect of treatments on canopy density expressed as increments of leaf area index (LAI) and leaf area density (LAD). Different uppercase and lowercase letters indicate statistical significances at 99% and 95% levels, respectively, through post hoc multiple-mean comparison by Tukey’s honestly significant difference (HSD) test.

Figure 4

Pictures of three plants subjected to the three different treatments at the beginning (a,c,e) and at the end (b,d,f) of the experimental test. a and b: G; b and c: KP; E and f: EP3.

Figure 5

Box plots showing the elongation rate of shoots for the four treatments (G, PE3, KP and T). Different uppercase letters indicate statistical significances at 99% level.