In autumn 2013, the presence of Xylella fastidiosa, a xylem-limited Gram-negative bacterium, was detected in olive stands of an area of the Ionian coast of the Salento peninsula (Apulia, southern Italy), that were severely affected by a disease denoted olive quick decline syndrome (OQDS). Studies were carried out for determining the involvement of this bacterium in the genesis of OQDS and of the leaf scorching shown by a number of naturally infected plants other than olive. Isolation in axenic culture was attempted and assays were carried out for determining its pathogenicity to olive, oleander and myrtle-leaf milkwort. The bacterium was readily detected by quantitative polymerase chain reaction (qPCR) in all diseased olive trees sampled in different and geographically separated infection foci, and culturing of 51 isolates, each from a distinct OQDS focus, was accomplished. Needle-inoculation experiments under different environmental conditions proved that the Salentinian isolate De Donno belonging to the subspecies pauca is able to multiply and systemically invade artificially inoculated hosts, reproducing symptoms observed in the field. Bacterial colonization occurred in prick-inoculated olives of all tested cultivars. However, the severity of and timing of symptoms appearance differed with the cultivar, confirming their differential reaction.

After the first confirmed outbreak of Xylella fastidiosa in the European

No abstract

The dynamics of Xylella fastidiosa infections in the context of the endophytic microbiome was studied in field-grown plants of the susceptible and resistant olive cultivars Kalamata and FS17. Whole metagenome shotgun sequencing (WMSS) coupled with 16S/ITS rRNA gene sequencing was carried out on the same trees at two different stages of the infections: In Spring 2017 when plants were almost symptomless and in Autumn 2018 when the trees of the susceptible cultivar clearly showed desiccations. The progression of the infections detected in both cultivars clearly unraveled that Xylella tends to occupy the whole ecological niche and suppresses the diversity of the endophytic microbiome. However, this trend was mitigated in the resistant cultivar FS17, harboring lower population sizes and therefore lower Xylella average abundance ratio over total bacteria, and a higher α-diversity. Host cultivar had a negligible effect on the community composition and no clear associations of a single taxon or microbial consortia with the resistance cultivar were found with both sequencing approaches, suggesting that the mechanisms of resistance likely reside on factors that are independent of the microbiome structure. Overall, Proteobacteria, Actinobacteria, Firmicutes, and Bacteriodetes dominated the bacterial microbiome while Ascomycota and Basidiomycota those of Fungi.

Olive quick decline syndrome (OQDS) is a devastating disease of olive trees in the Salento region, Italy. This disease is caused by the bacterium Xylella fastidiosa, which is widespread in the outbreak area; however, the “Leccino” variety of olives has proven to be resistant with fewer symptoms and lower bacterial populations than the “Ogliarola salentina” variety. We completed an empirical study to determine the mineral and trace element contents (viz; ionome) of leaves from infected trees comparing the two varieties, to develop hypotheses related to the resistance of Leccino trees to X. fastidiosa infection. All samples from both cultivars tested were infected by X. fastidiosa, even if leaves were asymptomatic at the time of collection, due to the high disease pressure in the outbreak area and the long incubation period of this disease. Leaves were binned for the analysis by variety, field location, and infected symptomatic and infected asymptomatic status by visual inspection. The ionome of leaf samples was determined using inductively coupled plasma optical emission spectroscopy (ICP-OES) and compared with each other. These analyses showed that Leccino variety consistently contained higher manganese (Mn) levels compared with Ogliarola salentina, and these levels were higher in both infected asymptomatic and infected symptomatic leaves. Infected asymptomatic and infected symptomatic leaves within a host genotype also showed differences in the ionome, particularly a higher concentration of calcium (Ca) and Mn levels in the Leccino cultivar, and sodium (Na) in both varieties. We hypothesize that the ionome differences in the two varieties contribute to protection against disease caused by X. fastidiosa infection.

The vector-borne bacterial pathogen Xylella fastidiosa is widely distributed in the Americas; in the last decade it has emerged as a serious threat for agricultural crops, natural environment and landscape in Europe. Following the first EU outbreak in 2013 in southern Italy, associated with a severe disease in olive trees, annual mandatory surveys are now in place in the Member States, leading to the discovery of bacterial outbreaks in different countries. Among the latest findings, an outbreak has been reported in the Italian region of Tuscany, with infections identified in seven different plant species. In this work, we report the isolation and the genetic characterization of isolates associated with this newly discovered outbreak. Multilocus sequence typing approach revealed the occurrence of isolates harbouring a new sequence type, denoted ST87, genetically related to strains of subsp. multiplex, but different from the genotypes of this subspecies previously characterized in Europe. Five cultured strains were successfully recovered from four of the seven host plants, an important achievement for advancing the studies on genomics and pathogenicity of these isolates and thus assess their potential threat for European agriculture.

Pathogens ultra-sensitive detection is vital for early diagnosis and provision of restraining actions and/or treatments. Among plant pathogens, Xylella fastidiosa is among the most threatening as it can infect hundreds of plant species worldwide with consequences on agriculture and the environment. An electrolyte-gated transistor is here demonstrated to detect X. fastidiosa at a limit-of-quantification (LOQ) of 2 ± 1 bacteria in 0.1 mL (20 colony-forming-unit per mL). The assay is carried out with a millimeter-wide gate functionalized with Xylella-capturing antibodies directly in saps recovered from naturally infected plants. The proposed platform is benchmarked against the quantitave polymerase chain reaction (qPCR) gold standard, whose LOQ turns out to be at least one order of magnitude higher. Furthermore, the assay selectivity is proven against the Paraburkholderia phytofirmans bacterium (negative-control experiment). The proposed label-free, fast (30 min), and precise (false-negatives, false-positives below 1%) electronic assay, lays the ground for an ultra-high performing immunometric point-of-care platform potentially enabling large-scale screening of asymptomatic plants.

Xylella fastidiosa subsp. pauca strain De Donno has been recently identified as the causal agent of a severe disease affecting olive trees in a wide area of the Apulia Region (Italy). While insights on the genetics and epidemiology of this virulent strain have been gained, its phenotypic and biological traits remained to be explored. We investigated in vitro behavior of the strain and compare its relevant biological features (growth rate, biofilm formation, cell–cell aggregation, and twitching motility) with those of the type strain Temecula1. The experiments clearly showed that the strain De Donno did not show fringe on the agar plates, produced larger amounts of biofilm and had a more aggregative behavior than the strain Temecula1. Repeated attempts to transform, by natural competence, the strain De Donno failed to produce a GFP-expressing and a knockout mutant for the rpfF gene. Computational prediction allowed us to identify potentially deleterious sequence variations most likely affecting the natural competence and the lack of fringe formation. GFP and rpfF- mutants were successfully obtained by co-electroporation in the presence of an inhibitor of the type I restriction–modification system. The availability of De Donno mutant strains will open for new explorations of its interactions with hosts and insect vectors.