Xylella Research Articles

An Individual-Based Spatial Epidemiological Model for the Spread of Plant Diseases

AbstractIn the study of plant disease epidemics, the state of each individual in the population and their spatial location should be considered when modeling disease spread. We present a model to describe the spread of plant diseases, where the infection of a susceptible individual depends on the transmission rate of infected individuals and the spatial correlation. This latter is introduced through the Matérn correlation function, accounting for spatial dependence based on distance. Almond leaf scorch disease, caused by the bacterium Xylella fastidiosa, was used as a case study to test the behavior of the model parameters and the variability due to the characteristics and location of initial disease introduction using a proposed simulation algorithm. The greatest variability in the results depended on the range parameter of the Matérn correlation, i.e., the distance at which two observations can be considered spatially uncorrelated, and the initial introduction. The spatial distribution of individuals also had a strong influence on disease spread, highlighting that areas without trees acted as barriers when their extent was greater than the range parameter. It should be stressed that this individual-based model can be applied to other plant diseases, adapting the parameter values to their particular epidemiological characteristics.

A Physiological and Molecular Focus on the Resistance of "Filippo Ceo" Almond Tree to Xylella fastidiosa.

The impact of Xylella fastidiosa (Xf) subsp. pauca on the environment and economy of Southern Italy has been devastating. To restore the landscape and support the local economy, introducing new crops is crucial for restoring destroyed olive groves, and the almond tree (Prunus dulcis Mill. D. A. Webb) could be a promising candidate. This work focused on the resistance of the cultivar "Filippo Ceo" to Xf and evaluated its physiological and molecular responses to individual stresses (drought or pathogen stress) and combined stress factors under field conditions over three seasons. Filippo Ceo showed a low pathogen concentration (≈103 CFU mL-1) and a lack of almond leaf scorch symptoms. Physiologically, an excellent plant water status was observed (RWC 82-89%) regardless of the stress conditions, which was associated with an increased proline content compared to that of the control plants, particularly in response to Xf stress (≈8-fold). The plant's response did not lead to a gene modulation that was specific to different stress factors but seemed more indistinct: upregulation of the LEA and DHN gene transcripts by Xf was observed, while the PR transcript was upregulated by drought stress. In addition, the genes encoding the transcription factors (TFs) were differentially induced by stress conditions. Filippo Ceo could be an excellent cultivar for coexistence with Xf subps. pauca, confirming its resistance to both water stress and the pathogen, although this similar health status was achieved differently due to transcriptional reprogramming that results in the modulation of genes directly or indirectly involved in defence strategies.

Complete genome assemblies of several Xylella fastidiosa subspecies multiplex strains reveals high phage content and novel plasmids

The Gram-negative bacterium Xylella fastidiosa (Xf) was originally found in the Americas, but has now been identified in more than 20 countries across America, Asia, and Europe. This plant pathogen is currently listed as a priority pest in Europe due to its socio-economic and ecological impacts. Within the three Xf subspecies fastidiosa, multiplex and pauca, subsp. multiplex displays a notably wider range of host plants than the other two subspecies. Comparative genomics may allow determination of how Xf subsp. multiplex adapts to new and diverse hosts and environments, so it is important that more genomes of this subspecies are defined. Twelve complete closed genomes sequences of Xf subsp. multiplex were obtained using a hybrid assembly approach combining Illumina and Oxford Nanopore technologies. The combined use of Canu and Unicycler assemblers enabled identification and closure of several plasmid sequences with high similarity to other plasmids described in strains of Xf subsp. fastidiosa and subsp. pauca. The analysis also revealed prophage sequences and contigs outside the chromosomes, annotated as phages. These new genomes, in conjunction with those existing in GenBank, will facilitate exploration of the evolutionary dynamics of Xf subsp. multiplex, its host adaptation mechanisms, and the potential emergence of novel strains of this important plant pathogen.

Xylella fastidiosa en Costa Rica: Impacto económico, relevancia agrícola y una visión general de la investigación nacional

Xylella fastidiosa is a phytopathogenic bacterium considered a quarantine pest in crops of agricultural importance. It is associated with various diseases in crops such as: coffee, citrus, grapes, olive and ornamental species. This bacterium has negative repercussions on the export market of Costa Rica, due to phytosanitary restrictions imposed especially on the phoenix palm (Phoenix spp.). In addition, its high dispersion capacity, the existence of genetic variants and its interaction with other microorganisms make X. fastidiosa a bacterium of interest to agriculture. Currently, different research efforts are being conducted to expand the understanding about X. fastidiosa and involve the general population in the subject. This article describes the economic and social impact of X. fastidiosa on national agricultural production. Information was collected through a bibliographic review and was complemented by contributions from professionals from the Center for Cell and Molecular Biology Research (CIBCM) who currently lead research on the subject.

Xylem-dwelling pathogen unaffected by local xylem vessel network properties in grapevines (Vitis spp.).

Xylella fastidiosa (Xf) is the xylem-dwelling bacterium associated with Pierce's disease (PD), which causes mortality in agriculturally important species, such as grapevine (Vitis vinifera). The development of PD symptoms in grapevines depends on the ability of Xf to produce cell-wall-degrading enzymes to break up intervessel pit membranes and systematically spread through the xylem vessel network. Our objective here was to investigate whether PD resistance could be mechanistically linked to xylem vessel network local connectivity. We used high-resolution X-ray micro-computed tomography (microCT) imaging to identify and describe the type, area and spatial distribution of intervessel connections for six different grapevine genotypes from three genetic backgrounds, with varying resistance to PD (four PD resistant and two PD susceptible). Our results suggest that PD resistance is unlikely to derive from local xylem network connectivity. The intervessel pit area (Ai) varied from 0.07 ± 0.01 mm2 mm-3 in Lenoir to 0.17 ± 0.03 mm2 mm-3 in Blanc do Bois, both PD resistant. Intervessel contact fraction (Cp) was not statically significant, but the two PD-susceptible genotypes, Syrah (0.056 ± 0.015) and Chardonnay (0.041 ± 0.013), were among the most highly connected vessel networks. Neither Ai nor Cp explained differences in PD resistance among the six genotypes. Bayesian re-analysis of our data shows moderate evidence against the effects of the traits analysed: Ai (BF01 = 4.88), mean vessel density (4.86), relay diameter (4.30), relay density (3.31) and solitary vessel proportion (3.19). Our results show that radial and tangential xylem network connectivity is highly conserved within the six different Vitis genotypes we sampled. The way that Xf traverses the vessel network may limit the importance of local network properties to its spread and may confer greater importance on host biochemical responses.

Host finding and probing behavior by Philaenus spumarius on olive varieties with a different degree of susceptibility to Xylella fastidiosa

Abundance on and access time to the host plant are the pivotal factors in Xylella fastidiosa transmission to olive by the meadow spittlebug Philaenus spumarius. Therefore, olive varieties suitable for the vectors, i.e., plants providing all the necessary cues to the insect for their location, settling and acceptance, and devoid of antixenotic defenses, could be more susceptible to infection than varieties non- or less suitable for the vector. Here we evaluated whether a bacterium-susceptible olive variety, Ogliarola Salentina, could be a more suitable host for P. spumarius than the two resistant varieties Leccino and FS-17. We carried out: (i) an evaluation of between-hosts and within-host preference; (ii) an insect survival analysis; (iii) an Electrical Penetration Graph (EPG)-assisted analysis of the probing behavior; (iv) light microscopy of the tissues the spittlebugs had access to; (v) an analysis of the xylem sap primary metabolites. In choice tests, the insect exhibited a significant preference for Ogliarola Salentina. In addition, spittlebugs displayed longer xylem sap ingestion bouts on the bacterium-susceptible variety compared to resistant genotypes, possibly because of differences in the xylem sap chemical profile rather than xylem anatomy. Spittlebugs preference for Ogliarola over both Leccino and FS-17 could be a relevant and so far overlooked component of the low disease prevalence in these two olive varieties reported in Southern Italian olive orchards. Overall, our data point toward the importance of incorporating studies on vector-plant interaction and host traits of resistance to the vector in research on genotypes resistant to X. fastidiosa.

Diversity, Composition, and Specificity of the Philaenus spumarius Bacteriome.

Philaenus spumarius (Linnaeus, 1758) (Hemiptera, Aphrophoridae) was recently classified as a pest due to its ability to act as a vector of the phytopathogen Xylella fastidiosa. This insect has been reported to harbour several symbiotic bacteria that play essential roles in P. spumarius health and fitness. However, the factors driving bacterial assemblages remain largely unexplored. Here, the bacteriome associated with different organs (head, abdomen, and genitalia) of males and females of P. spumarius was characterized using culturally dependent and independent methods and compared in terms of diversity and composition. The bacteriome of P. spumarius is enriched in Proteobacteria, Bacteroidota, and Actinobacteria phyla, as well as in Candidatus Sulcia and Cutibacterium genera. The most frequent isolates were Curtobacterium, Pseudomonas, and Rhizobiaceae sp.1. Males display a more diverse bacterial community than females, but no differences in diversity were found in distinct organs. However, the organ shapes the bacteriome structure more than sex, with the Microbacteriaceae family revealing a high level of organ specificity and the Blattabacteriaceae family showing a high level of sex specificity. Several symbiotic bacterial genera were identified in P. spumarius for the first time, including Rhodococcus, Citrobacter, Halomonas, Streptomyces, and Providencia. Differences in the bacterial composition within P. spumarius organs and sexes suggest an adaptation of bacteria to particular insect tissues, potentially shaped by their significance in the life and overall fitness of P. spumarius. Although more research on the bacteria of P. spumarius interactions is needed, such knowledge could help to develop specific bacterial-based insect management strategies.

The Exometabolome of Xylella fastidiosa in Contact with Paraburkholderia phytofirmans Supernatant Reveals Changes in Nicotinamide, Amino Acids, Biotin, and Plant Hormones.

Microbial competition within plant tissues affects invading pathogens' fitness. Metabolomics is a great tool for studying their biochemical interactions by identifying accumulated metabolites. Xylella fastidiosa, a Gram-negative bacterium causing Pierce's disease (PD) in grapevines, secretes various virulence factors including cell wall-degrading enzymes, adhesion proteins, and quorum-sensing molecules. These factors, along with outer membrane vesicles, contribute to its pathogenicity. Previous studies demonstrated that co-inoculating X. fastidiosa with the Paraburkholderia phytofirmans strain PsJN suppressed PD symptoms. Here, we further investigated the interaction between the phytopathogen and the endophyte by analyzing the exometabolome of wild-type X. fastidiosa and a diffusible signaling factor (DSF) mutant lacking quorum sensing, cultivated with 20% P. phytofirmans spent media. Liquid chromatography-mass spectrometry (LC-MS) and the Method for Metabolite Annotation and Gene Integration (MAGI) were used to detect and map metabolites to genomes, revealing a total of 121 metabolites, of which 25 were further investigated. These metabolites potentially relate to host adaptation, virulence, and pathogenicity. Notably, this study presents the first comprehensive profile of X. fastidiosa in the presence of a P. phytofirmans spent media. The results highlight that P. phytofirmans and the absence of functional quorum sensing affect the ratios of glutamine to glutamate (Gln:Glu) in X. fastidiosa. Additionally, two compounds with plant metabolism and growth properties, 2-aminoisobutyric acid and gibberellic acid, were downregulated when X. fastidiosa interacted with P. phytofirmans. These findings suggest that P. phytofirmans-mediated disease suppression involves modulation of the exometabolome of X. fastidiosa, impacting plant immunity.

Does the superfluous killing exhibited by spiders mask the functional response parameters? A case study from the perspective of biological control

AbstractThe meadow spittlebug Philaenus spumarius L. (Hemiptera: Aphrophoridae) is a xylem‐sap feeder and the most abundant and widespread vector of Xylella fastidiosa (Xanthomonadales: Xanthomonadaceae) throughout Europe. Control strategies for the disease are based on containment and eradication measures regulated by the European Union. Within the context of biological control, the use of natural enemies aims at limiting the insect vector population in agroecosystems. Spiders are generalist predators with potential as biological control agents occupying all the habitats within crops. Crab spiders (Thomisidae) could be potential natural enemies against P. spumarius. The functional response (FR) of a predator represents the intake rate as a function of food availability and allows estimating feeding behaviour parameters such as the prey handling time and attack rate. However, spiders often kill more prey than they consume, unveiling their capacity as natural enemies. We assessed and compared the FR of Xysticus acerbus (Thomisidae) fed on P. spumarius in the laboratory considering (1) the total number of dead individuals and (2) only the consumed ones. We found that although both FRs were of type‐II, they significantly differed. The values of attack rate and handling time for consumed were 0.1 and 3.6, respectively, while for dead, the attack rate was 0.3 and 1.1, being both significantly different. Moreover, the number of overkilled individuals reached a plateau at the highest prey densities. Xysticus acerbus could be a potential natural enemy of P. spumarius. Nevertheless, we suggest that the effect of including the killed but not consumed prey individuals in FR studies using wasteful killing predators should be considered and further studied.

Detection and Characterization of Xylella fastidiosa subsp. fastidiosa in Rabbiteye Blueberry in South Carolina.

Xylella fastidiosa causes bacterial leaf scorch in southern highbush (Vaccinium corymbosum interspecific hybrids) and is also associated with a distinct disease phenotype in rabbiteye blueberry (V. virgatum) cultivars in the southeastern United States. Both X. fastidiosa subsp. fastidiosa and X. fastidiosa subsp. multiplex have been reported to cause problems in southern highbush blueberry, but so far only X. fastidiosa subsp. multiplex has been reported in rabbiteye cultivars in Louisiana. In this study, we report detection of X. fastidiosa in rabbiteye blueberry plants in association with symptoms of foliar reddening and shoot dieback. High throughput sequencing of an X. fastidiosa-positive plant sample and comparative analyses identified the strain in one of these plants as being X. fastidiosa subsp. fastidiosa. We briefly discuss the implications of these findings, which may spur research into blueberry as a potential inoculum source that could enable spread to other susceptible fruit crops in South Carolina.

Exploring the xylem-sap to unravel biological features of Xylella fastidiosa subspecies pauca ST53 in immune, resistant and susceptible crop species through metabolomics and in vitro studies.

Xylella fastidiosa subsp. pauca ST53 (Xfp) is a pathogenic bacterium causing one of the most severe plant diseases currently threatening the olive-growing areas of the Mediterranean, the Olive Quick Decline Syndrome (OQDS). The majority of the olive cultivars upon infections more or less rapidly develop severe desiccation phenomena, while few are resistant (e.g. Leccino and FS17), being less impacted by the infections. The present study contributes to elucidating the basis of the resistance phenomenon by investigating the influence of the composition of the xylem sap of plant species on the rate of bacterial multiplication. Xylem saps from Xfp host and non-host species were used for growing the bacterium in vitro, monitoring bacterial growth, biofilm formation, and the expression of specific genes. Moreover, species-specific metabolites, such as mannitol, quinic acid, tartaric acid, and choline were identified by non-targeted NMR-based metabolomic analysis in olive, grapevine, and citrus. In general, the xylem saps of immune species, including grapevine and citrus, were richer in amino acids, organic acids, and glucose. The results showed greater bacterial growth in the olive cultivar notoriously susceptible to Xfp (Cellina di Nardò), compared to that recorded in the resistant cultivar Leccino. Conversely, higher biofilm formation occurred in Leccino compared to Cellina di Nardò. Using the xylem saps of two Xfp-immune species (citrus and grapevine), a divergent bacterial behavior was recorded: low planktonic growth and biofilm production were detected in citrus compared to the grapevine. A parallel evaluation of the expression of 15 genes showed that Xfp directs its molecular functions mainly to virulence. Overall, the results gained through this multidisciplinary study contribute to extending the knowledge on the host-pathogen interaction, while confirming that the host response and resistance mechanism have a multifactorial basis, most likely with a cumulative effect on the phenotype.

Knowledge of xylem feeders (Hemiptera: Auchenorrhyncha) in Albanian olive orchards in readiness for a potential outbreak with Xylella fastidiosa invasion

Summary Since Xylella fastidiosa first appeared in southern Italy, it has become essential to manage and limit its transmission. In areas unaffected by the bacterium, such as Albania, it is necessary to assess the risk of its spread, primarily by understanding the ecology of its potential vectors. A survey of 14 olive orchards was conducted over a period of 12 months from August 2015 and July 2016 in two Albanian counties (Vlora and Tirana) where olive production is important. The aim was to investigate the presence of Auchenorrhyncha species, with particular attention to xylem feeders, and Auchenorrhyncha species belonging to the families Aphrophoridae, Cercopidae, Cicadellidae, Issidae, Flatidae were found in both counties. In Vlora, four out of 11 collected species were Aphrophoridae and Cercopidae, whereas in Tirana, three species belonging to these families were captured. In both counties, the most abundant xylem feeder species was Philaenus spumarius. In Vlora, P. spumarius was found more frequently on olive trees than in Tirana. Philaenus spumarius and Neophilaenus campestris adults were most frequent in August, September, and October 2015, while both species were almost absent from December 2015 to March 2016. Our sampling demonstrated that Vlora region has a greater number of Auchenorrhyncha species than Tirana. Furthermore, molecular tests (real-time LAMP) showed the absence of X. fastidiosa in the tested individuals. This survey constitutes an important update on the X. fastidiosa situation in Albania, showing that the country is still free of X. fastidiosa, at least in the surveyed areas.

Histological analysis of <i>Xylella fastidiosa</i> infection in <i>Quercus pyrenaica</i> in Northern Portugal

<abstract> <p><italic>Quercus pyrenaica</italic> Willd thrives in the intermediate zone between the Mediterranean sclerophyllous and the temperate deciduous forest. In December 2022, the presence of the bacteria <italic>Xylella fastidiosa (Xf)</italic> was confirmed in samples collected from a <italic>Quercus pyrenaica</italic> located in Sabrosa, Vila Real, Portugal. Following <italic>Xf</italic> infection, the transport of water and nutrients is hindered due to the occlusion of xylem vessels. This loss of hydraulic conductivity may lead to vessel blockage and subsequent embolism formation. The objective of this study was to investigate the interaction between <italic>Xf</italic> and <italic>Quercus pyrenaica</italic> tissues, as well as the mechanism by which the bacteria can spread through the plant's xylem vessels, ultimately resulting in the formation of vascular plugs. At the time of the sample collection (10 months post-detection), symptoms of Bacterial Leaf Scorch (BLS) began to appear. Examination of xylem vessels using both light and scanning electron microscopy (SEM) revealed the presence of various types of occlusions, predominantly tyloses. Additionally, fibrillar networks, gums, starch grains, and crystals were observed. The stem vessels exhibited significantly more occlusions compared to the leaves. Furthermore, individual bacterial cells were observed to be attached to the vessel wall. This implies that occlusions were primarily induced by tyloses and gums as a defensive response to the invasion of vascular pathogens, in addition to the pathogen itself. This study highlights the presence of starch grains in stems, which may function as a refilling mechanism, thereby preventing the loss of hydraulic conductivity in plants and potentially acting as a means to entrap the bacteria. These mechanisms exemplify the constitutive defense systems of the plant against <italic>Xf</italic>. Understanding the interaction between <italic>Xylella fastidiosa</italic> and <italic>Quercus pyrenaica</italic> is crucial, given that the latter species occupies nearly 95% of the natural distribution area of Portugal.</p> </abstract>

A decade of monitoring surveys for Xylella fastidiosa subsp. pauca in olive groves in Apulia (Italy) reveals a low incidence of the bacterium in the demarcated areas

AbstractIn October 2013, Xylella fastidiosa subsp. pauca (Xfp) was reported as being associated with a decline of olive groves in Salento (Apulia, Italy) and named as “olive quick decline syndrome” (OQDS). Due to its quarantine status, a series of mandatory actions were established in the Apulia region to impede its further spread in the territory. Upon the demarcation of “containment” and “buffer” zones, monitoring surveys were carried out in these areas for sampling olive trees and other host plants that showed symptoms putatively attributable to OQDS. The eradication of plants that tested positive for Xfp, including also all the asymptomatic ones in a radius of 100 or 50 m, was performed over 10 years. An analysis, based on the official data of such surveillance activities over the period of a decade, was performed. The data show that the Xfp incidence in the “containment” and “buffer” zones is very low, especially during the last three campaigns from 2020–2021 to 2022–2023, when the bacterium was detected in a range of 0.06%–0.70% of the sampled plants. In most of the sampled trees that showed symptoms of OQDS, the bacterium was not detected. Based on these data and according to epidemiological models that verified the negligible role of asymptomatic olive trees in the spreading of OQDS, we propose to eliminate the rule requiring the uprooting of all host plants that surround one Xfp‐positive tree in a radius of 50 m. Such an implementation could save many healthy centennial and monumental olive trees and the remarkable landscape that they contribute to.

Worldwide potential insect vectors of Xylella fastidiosa and assessment of their importance with a focus on Morocco

Abstract In Morocco, the climate conditions are favorable for the establishment and the spread of Xylella fastidiosa (X.f). However, the successful establishment of the bacterium depends on many factors; mainly: bacterial subspecies and sequence type, host plants susceptibility, feeding behavior and transmission efficiency of the insect vectors. Knowledge of the relationship between the bacterium–affected crop agro-ecosystem and potential insect vectors is of crucial importance. In this work, we list the tritrophic interaction X.f-host plants-insect vectors that occur worldwide in order to apply it to the current situation in Morocco and for risk analysis on the bacterium in the country. Two most relevant X.f subspecies of the bacterium (in terms of impact on crops) were considered, namely, subsp. fastidiosa and subsp. pauca. Based on the international literature and public databases, the majority of the X.f-insect vectors are comprised in two families: Cicadellidae and Aphrophoridae. Among all cicadellid species recorded, a high number had the capacity to transmit X.f to hosts in America while this ability is null for other regions (except Graphocephala versuta Say (Hemiptera: Cicadellidae) recorded in Algeria). In Morocco, none of the cicadellid genera reported worldwide as vectors of X.f, have been so far reported, whereas many species of spittlebugs and leafhoppers are present. Philaenus tesselatus Melichar (Hemiptera: Aphrophoridae) is highly abundant in Morocco and could play a role as potential vector in case the bacterium is introduced in the country. With regard to the X.f hosts, citrus, olive, almond and grapevine, forest agroecosystems and oleander are considered the main susceptible species present in Morocco.

A Survey of Xylella fastidiosa in the U.S. State of Virginia Reveals Wide Distribution of Both Subspecies fastidiosa and multiplex in Grapevine.

Global travel and trade in combination with climate change are expanding the geographic distribution of plant pathogens. The bacterium Xylella fastidiosa is a prime example. Native to the Americas, it has spread to Europe, Asia, and the Middle East. To assess the risk that pathogen introductions pose to crops in newly invaded areas, it is key to survey their diversity, host range, and disease incidence in relation to climatic conditions where they are already present. We performed a survey of X. fastidiosa in grapevine in Virginia using a combination of quantitative PCR, multilocus sequencing, and metagenomics. We also analyzed samples from deciduous trees with leaf scorch symptoms. X. fastidiosa subspecies fastidiosa was identified in grapevines in all regions of the state, even in Northern Virginia, where the temperature was below -9°C for 10 days per year on average in the years preceding sampling. Unexpectedly, we also found for the first time grapevine samples infected with X. fastidiosa subspecies multiplex (Xfm). The Xfm lineage found in grapevines had been previously isolated from blueberries in the Southeastern United States and was distinct from that found in deciduous trees in Virginia. The obtained results will be important for risk assessment of X. fastidiosa introductions in other parts of the world.

Diversity and composition of the bacterial communities associated with the Australian spittlebugs Bathyllus albicinctus and Philagra parva (Hemiptera: Aphrophoridae).

Spittlebugs and froghoppers (Hemiptera: Cercopoidea) are insects feeding on xylem, which potentially can cause significant economic damage worldwide by transmitting plant pathogenic bacteria such as Xylella fastidiosa. Australia and New Zealand are currently free from X. fastidiosa, but they are home to at least 45 native spittlebug species. Among these, the Australian natives Bathyllus albicinctus (Erichson, 1842) and Philagra parva (Donovan, 1805) are particularly widespread and can be found across southern and eastern Australia, with B. albicinctus also in New Zealand. The potential that both species might be capable of vectoring Xylella fastidiosa poses a substantial biosecurity risk if the bacterium were to invade these regions. In this study, we examined 87 spittlebug nymphs collected across 12 different host plant species, in five locations in Victoria, Australia. Our objective was to explore the factors influencing bacterial communities within and between these widespread spittlebug species, considering geographic location, insect phylogenetics, and host plant associations. We employed COI barcoding to assess insect genetic variation and 16S high throughput sequencing (HTS) metabarcoding to analyse bacterial microbiome diversity across various host plants. Our findings revealed minimal genetic divergence among spittlebug individuals in the same species, highlighting conspecificity despite conspicuous morphological divergences. On the other hand, we recorded significant variation in bacterial communities harboured by Bathyllus albicinctus nymphs feeding on different plants, even when these were collected within close proximity to each other. Therefore, host plant association appeared to shape the bacterial communities of spittlebugs more than insect genetic divergence or geographical location. These diverse bacterial communities could potentially facilitate transmission of plant pathogenic bacteria, underscoring the risk of widespread transmission among numerous plant hosts through insect-plant interactions. This study emphasizes the critical need to understand these complex interactions, particularly in the context of biosecurity.

Seasonal Dynamics and Distribution of Xylella fastidiosa in Infected Almond Trees.

This research focused on studying the dynamics of the bacterial pathogen Xylella fastidiosa in almond trees across different developmental stages. The objective was to understand the seasonal distribution and concentration of X. fastidiosa within almond trees. Different tree organs, including leaves, shoots, branches, fruits, flowers, and roots, from 10 X. fastidiosa-infected almond trees were sampled over 2 years. The incidence and concentration of X. fastidiosa were determined using qPCR and isolation. Throughout the study, X. fastidiosa was consistently absent from fruits, flowers, and roots, whereas it was detected in leaves as well as in shoots and branches. We demonstrate that the absence of X. fastidiosa in the roots is likely linked to the inability of this isolate to infect the peach-almond hybrid rootstock GF677. X. fastidiosa incidence in shoots and branches remained consistent throughout the year, whereas in leaf petioles, it varied across developmental stages, with lower detection during the early and late stages of the season. Similarly, viable X. fastidiosa cells were isolated from shoots and branches at all developmental stages, but no successful isolations were achieved from leaf petioles during the vegetative and nut growth stage. Studying the progression of almond leaf scorch symptoms in trees with initial infections showed that once symptoms emerged on one branch, symptomless branches were likely already infected by the bacterium. Therefore, selectively pruning symptomatic branches is unlikely to cure the tree. This study enhances our understanding of X. fastidiosa dynamics in almond trees and may have practical applications for its detection and control.

A Study of the Toxic Effect of Plant Extracts against Philaenus spumarius (Hemiptera: Aphrophoridae).

The meadow spittlebug Philaenus spumarius (Hemiptera: Aphrophoridae) is distributed in several habitats worldwide and has been recently recognized as the main vector of Xylella fastidiosa subsp. pauca. This bacterium has been associated with olive quick decline syndrome (OQDS) in the Salento Peninsula (Italy) and is responsible for extensive desiccation and die-off of olive trees. Current OQDS management strategies include the control of P. spumarius populations, mainly through the removal of weed hosts and insecticide treatments. In addition to the mandatory phytosanitary measures, the demand for new strategies compatible with integrated pest management is increasing. In this study, laboratory biological assays were performed to assess the potential toxic effect of vegetal formulations against P. spumarius adults. Two formulations were tested at different concentrations: Form A, an emulsion of 10% hot pepper-infused oil (Capsicum annuum subspecies Cayenna in olive oil) and Arabic gum in an aqueous solution of extracts of Salvia guaranitica, and Form B, an aqueous solution of extracts of Taxus baccata. Both Form A and B showed to be toxic against P. spumarius compared to the water control. The mean percentage of spittlebug mortality obtained with Form A one hour after treatments was dose-dependent; the lethal dose values were 0.13% (LD25), 0.36% (LD50), and 0.85% (LD75). At the same time, no significant differences in mortality rate were observed between the 0.75% treatment and the treatments with deltamethrin (about 90%). The mean percentage of spittlebug mortality obtained with Form B ranged from 21% to 53% one hour after treatment, but these values were significantly lower than those obtained with deltamethrin. The effectiveness of Form A on the P. spumarius population was also evaluated in the field. The averages of captures in the three experimental blocks were 1.8/trap for treated and 7.7/trap for untreated plots, and the spittlebug populations significantly decreased after treatments. Based on these results and the literature data, we hypothesize that the effectiveness of Form A is the result of the synergistic effect of all its components. No symptoms of phytotoxicity were recorded on olive trees treated with Form A, and the number of P. spumarius specimens collected on these plants was much lower than on untreated plants. These results suggest the potential use of Form A in the protection of olive trees. This vegetal formulation can thus be considered as a valid alternative to chemical insecticides for the control of the main vector of X. fastidiosa and could be integrated into a sustainable management system for OQDS.

Xylella fastidiosa, Possible New Threat to Chestnut (Castanea sativa Mill.) in Italy

This is the first worldwide report of X. fastidiosa (Xf) subsp. pauca on Castanea sativa and the first characterization of Xf infection on this species. Plants located in three sites (in a long-term affected area in Apulia) were monitored for symptoms and bacterial concentrations in spring and summer, while microscopic analyses were performed to evaluate the pathogen distribution in the xylem vessels. All chestnut plants appeared asymptomatic but the Xf subsp. pauca strain “De Donno” was present at low concentrations (1.14 × 104 and 1.56 × 103 cfu mL−1 in April and June) and with a low incidence (respectively, 38% and 30%). The FISH-CLSM (Fluorescent In Situ Hybridization—Confocal Laser Scanning Microscope) analysis showed evident Xf occlusions but in a low percentage (9.2 ± 3.4%); these data can explain the lack of symptoms on the canopy. Furthermore, through a bibliographic analysis it emerged that Philaenus spumarius and Neophilaenus campestris, two Xf vectors present in Europe, are suckling feeding insects on chestnut trees and could be involved in the bacterial transmission to this species. Asymptomatic Xf host species can play a considerable role in new outbreak emergence or in the expansion of existing ones. So, it is essential to identify them to plan more effective monitoring activities.