Introduction

Xylella fastidiosa (Xf) is a Gram-negative bacterium (Janse & Obradovic, 2010: https://www.jstor.org/stable/41998754) that affects numerous plant species, including olive trees, and causes Olive Quick Decline Syndrome (OQDS) . Xf obstructs water and nutrient transport by colonizing the xylem vessels, leading to canopy desiccation and plant death. Despite severe damage, olive trees often produce basal shoots, or suckers, which represent a survival mechanism with a peculiar behavior in Xf-positive plants (Camposeo et al., 2022: https://doi.org/10.3390/agronomy12122917; Martelli, 2016: https://doi.org/10.1007/s12600-015-0498-6).

Goals

The most common reason for the development of suckers is the abandonment of fields, which favors the natural growth processes of the olive tree. These shoots may also represent a survival mechanism as the damaged tree tries to resist by producing suckers. This study aims to determine whether olive tree suckers exhibit enhanced defense mechanisms against Xf by analyzing sucker samples of different ages from three cultivars: Cellina di Nardò, Ogliarola, and Leccino.

Methodology

Four suckers from at least five plants per cvs were collected. Samples were cut from the sucker bases using a sterilized scissors. Approximately 1 g of tissue per sample was homogenized in extraction bags (BIOREBA, Switzerland) with 5 mL of CTAB buffer using a semi-automatic homogenizer (Homex 6, BIOREBA). DNA extraction followed the EPPO Bulletin (2004) (doi:10.1111/epp.12575), employing chloroform for protein removal and ethanol for precipitation. Real-time PCR was performed following Harper et al. (2010) (https://doi.org/10.1094/PHYTO-06-10-0168), and bacterial concentration, expressed as CFU/mL, was estimated according to D’Attoma et al. (2019) (https://doi.org/10.3390/pathogens8040272).

Results

Xf-incidence in suckers of Cellina di Nardò less than 5 years old was approximately 70%, with bacterial loads varying between 10³ to 10⁶ CFU/mL. These values slightly differed from those of the Ogliarola, in which more than two-thirds of the samples were negative in less than 5-year-old-suckers, while Xf-incidence reach 90% in older ones. The bacterial load in Ogliarola is also similar to Cellina, between 10³ and 10⁷ CFU/mL. Conversely, for Leccino, the younger shoots were mainly negative, while those older than 5 years were mostly positive with a bacterial count between 10⁴ and 10⁶ CFU/mL.

Conclusion

The findings highlight age-related and cultivar-dependent variability in Xf susceptibility among olive suckers. Younger suckers generally exhibited lower infection rates, suggesting a form of temporary resistance, but infection seems to progress as expected in older ones. Further research should explore additional factors, such as endophyte presence, to elucidate their potential role in sucker resistance.