Further In Vitro Assessment and Mid-Term Evaluation of Control Strategy of Xylella fastidiosa subsp. pauca in Olive Groves of Salento (Apulia, Italy)

Abstract

During recent years; Xylella fastidiosa subsp. pauca (Xfp) has spread in Salento causing relevant damage to the olive groves. Measures to contain the spreading of the pathogen include the monitoring of the areas bordering the so-called “infected” zone and the tree eradication in case of positive detection. In order to provide a control strategy aimed to maintain the tree productivity in the infected areas, we further evaluated the in vitro and in planta mid-term effectiveness of a zinc-copper-citric acid biocomplex. The compound showed an in vitro bactericidal activity and inhibited the biofilm formation in representative strains of X. fastidiosa subspecies, including Xfp isolated in Apulia from olive trees. The field mid-term evaluation of the control strategy assessed by quantitative real-time PCR in 41 trees of two olive groves of the “infected” area revealed a low concentration of Xfp over the seasons upon the regular spraying of the biocomplex over 3 or 4 consecutive years. In particular, the bacterial concentration lowered in July and October with respect to March, after six consecutive treatments. The trend was not affected by the cultivar and it was similar either in the Xfp-sensitive cultivars Ogliarola salentina and Cellina di Nardò or in the Xfp-resistant Leccino. Moreover, the scoring of the number of wilted twigs over the seasons confirmed the trend. The efficacy of the treatment in the management of olive groves subjected to a high pathogen pressure is highlighted by the yielded a good oil production

Progress towards Sustainable Control of Xylella fastidiosa subsp. pauca in Olive Groves of Salento (Apulia, Italy)

“…Comparison of the three cultivars located at Galatone indicated that Leccino trees had a lower bacterial concentration (mean of 9.0 × 10 2 CFU g −1 ), in comparison to Cellina di Nardò (1.7 × 10 4 CFU g −1 ) and Ogliarola salentina (8.7 × 10 3 CFU g −1 ) (Figure 7). The comparatively healthy status of the Dentamet ® -treated trees was confirmed by their yield, with a production estimated at about 18 to 23 Kg of olives per tree recorded in autumn 2019 in the farms of Galatone and Cannole, whereas the untreated trees were completely wilted [34]. These results further supported the conclusions from the first study and clearly showed that X. f. subsp.…”

“…Dentamet ® biofertilizer showed bactericidal activity towards all the X. f. subsp. fastidiosa, multiplex and pauca strains tested, including the one isolated from olive trees showing OQDS symptoms in Apulia [33,34]. The antibacterial activity was ascertained both in broth tubes and on a bacterial culture substrate.…”

“…Aliquots of 10 µL and 10-fold dilutions from 10 7 to 10 3 CFU mL −1 of each X. fastidiosa subspecies were spotted on PD2 medium agar plates supplemented or not with 1:10, 1:50 and 1:100 Dentamet ® dilutions and incubated at 28 • C. Ctr (control): X. fastidiosa cultures grown on PD2 medium Dentamet ® -free. Reproduced from [34].…”

“…A statistically significant difference was obtained between Xfp, Xff and Xfm PD2 cultures (untreated control) and the same subspecies added with Dentamet ® dilutions (normalized with absorbance obtained by tubes without bacteria), according to one-way ANOVA, Dunnett's test (**** p ≤ 0.0001 vs. Control). Reproduced from [34].…”

“…pauca is possible in the so-called "infected area" of Salento (i.e., the province of Lecce, and part of the provinces of Brindisi and Taranto). The tree crown nebulization of Dentamet ® (a biocomplex containing zinc, copper, and citric acid) during spring, summer, and early autumn significantly reduced both the symptoms (i.e., twig and branch wilting) and the bacterial cell density within the leaves of the sensitive olive cultivars Ogliarola salentina and Cellina di Nardò as measured by quantitative real-time PCR [33,34]. Dentamet ® treatments resulted in a rapid reprogramming of some indicative metabolic pathways, as observed by metabolomics, which re-established a more normal tree physiology [39,40,46].…”