Direct Evidence of Egestion and Salivation of Xylella fastidiosa Suggests Sharpshooters Can Be "Flying Syringes"

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

BACKGROUND AND AIMS

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.

METHODS

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).

KEY RESULTS

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).

CONCLUSIONS

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.

Wolbachia infection and genetic diversity of Italian populations of Philaenus spumarius, the main vector of Xylella fastidiosa in Europe

Philaenus spumarius is a cosmopolitan species that has become a major threat to European agriculture being recognized as the main vector of the introduced plant pathogen Xylella fastidiosa, the agent of the “olive quick decline syndrome”, a disease which is devastating olive orchards in southern Italy. Wolbachia are bacterial symbionts of many insects, frequently as reproductive parasites, sometime by establishing mutualistic relationships, able to spread within host populations. Philaenus spumarius harbors Wolbachia, but the role played by this symbiont is unknown and data on the infection prevalence within host populations are limited. Here, the Wolbachia infection rate was analyzed in relation to the geographic distribution and the genetic diversity of the Italian populations of P. spumarius. Analysis of the COI gene sequences revealed a geographically structured distribution of the three main mitochondrial lineages of P. spumarius. Wolbachia was detected in half of the populations sampled in northern Italy where most individuals belonged to the western-Mediterranean lineage. All populations sampled in southern and central Italy, where the individuals of the eastern-Mediterranean lineage were largely prevalent, were uninfected. Individuals of the north-eastern lineage were found only in populations from the Alps in the northernmost part of Italy, at high altitudes. In this area, Wolbachia infection reached the highest prevalence, with no difference between north-eastern and western-Mediterranean lineage. Analysis of molecular diversity of COI sequences suggested no significant effect of Wolbachia on population genetics of P. spumarius. Using the MLST approach, six new Wolbachia sequence types were identified. Using FISH, Wolbachia were observed within the host’s reproductive tissues and salivary glands. Results obtained led us to discuss the role of Wolbachia in P. spumarius, the factors influencing the geographic distribution of the infection, and the exploitation of Wolbachia for the control of the vector insect to reduce the spread of X. fastidiosa.

Fluid dynamic simulations at the interface of the blue-green sharpshooter functional foregut and grapevine xylem sap with implications for transmission of Xylella fastidiosa

Xylella fastidiosa is a multi-continental, lethal, plant pathogenic bacterium that is transmitted by sharpshooter leafhoppers (Insecta: Hemiptera: Cicadellidae: Cicadellinae) and adult spittlebugs (Hemiptera: Aphrophoridae). The bacterium forms biofilms in plant xylem and the functional foregut of the insect. These biofilms serve as sources of inoculum for insect acquisition and subsequent inoculation to a healthy plant. In this study, 3D fluid dynamic simulations were performed for bidirectional cibarial propulsion of xylem sap through tube-like grapevine xylem and an anatomically accurate model of the functional foregut of the blue-green sharpshooter, Graphocephala atropunctata. The analysis supports a model of how fluid dynamics influence X. fastidiosa transmission. The model supports the hypothesis that X. fastidiosa inoculation is mostly driven by detachment of bacteria from the foregut due to high-velocity flow during egestion (outward fluid flow from the stylets). Acquisition occurs by fluid dynamics during both egestion and ingestion (fluid uptake through the stylets and swallowing). These simulation results are supported by previously reported X. fastidiosa colonization patterns in the functional foregut and sharpshooter stylet probing behaviors. The model indicates that xylem vessel diameter influences drag forces imposed on xylem wall-adherent bacteria; thus, vessel diameter may be an important component of the complex transmission process. Results from this study are directly applicable to development of novel grapevine resistance traits via electropenetrographic monitoring of vector acquisition and inoculation behaviors.

Susceptibility to Xylella fastidiosa and functional xylem anatomy in Olea europaea: revisiting a tale of plant-pathogen interaction

Xylella fastidiosa is a xylem-limited bacterium causing the Olive Quick Decline Syndrome, which is currently devastating the agricultural landscape of Southern Italy. The bacterium is injected into the xylem vessels of leaf petioles after the penetration of the insect vector's stylet. From here, it is supposed to colonize the xylem vasculature moving against water flow inside conductive vessels. Widespread vessel clogging following the bacterial infection and causing the failure of water transport seemed not to fully supported by the recent empirical xylem anatomical observations in infected olive trees. We tested the hypothesis that the higher susceptibility to the X. fastidiosa's infection in Cellina di Nardò compared with Leccino is associated to the higher vulnerability to air embolism of its larger vessels. Such hypothesis is motivated by the recognized ability of X. fastidiosa in degrading pit membranes and also because air embolism would possibly provide microenvironmental conditions more favourable to its more efficient aerobic metabolism. We revised the relevant literature on bacterium growth and xylem physiology, and carried out empirical field, mid-summer measurements of xylem anatomy and native embolism in olive cultivars with high (Cellina di Nardò) and low susceptibility (Leccino) to the infection by X. fastidiosa. Both cultivars had similar shoot mass traits and vessel length (~80 cm), but the highly susceptible one had larger vessels and a lower number of vessels supplying a given leaf mass. Native air embolism reduced mean xylem hydraulic conductance by ~58 % (Cellina di Nardò) and ~38 % (Leccino). The higher air-embolism vulnerability of the larger vessels in Cellina di Nardò possibly facilitates the X. fastidiosa's infection compared to Leccino. Some important characteristics of the vector-pathogen-plant interactions still require deep investigations acknowledging both the pathogen metabolic pathways and the biophysical principles of xylem hydraulics.

Impact of the United States Department of Agriculture, Agricultural Research Service on Plant Pathology: 2015-2020

There is an increasing need to supply the world with more food as the population continues to grow. Research on mitigating the effects of plant diseases to improve crop yield and quality can help provide more food without increasing the land area devoted to farming. National Program 303 (NP 303) within the U.S. Department of Agriculture, Agricultural Research Service is dedicated to research across multiple fields in plant pathology. This review article highlights the research impact within NP 303 between 2015 and 2020, including case studies on wheat and citrus diseases and the National Plant Disease Recovery System, which provide specific examples of this impact.

Functional foregut anatomy of the blue-green sharpshooter illustrated using a 3D model

Sharpshooter leafhoppers (Hemiptera: Cicadellidae: Cicadellinae) are important vectors of the plant pathogenic bacterium Xylella fastidiosa Wells et al. (Xanthomonadales: Xanthomonadaceae). This pathogen causes economically significant diseases in olive, citrus, and grapes on multiple continents. Bacterial acquisition and inoculation mechanisms are linked to X. fastidiosa biofilm formation and fluid dynamics in the functional foregut of sharpshooters, which together result in egestion (expulsion) of fluids likely carrying bacteria. One key X. fastidiosa vector is the blue–green sharpshooter, Graphocephala atropunctata (Signoret, 1854). Herein, a 3D model of the blue–green sharpshooter functional foregut is derived from a meta-analysis of published microscopy images. The model is used to illustrate preexisting and newly defined anatomical terminology that is relevant for investigating fluid dynamics in the functional foregut of sharpshooters. The vivid 3D illustrations herein and supplementary interactive 3D figures are suitable resources for multidisciplinary researchers who may be unfamiliar with insect anatomy. The 3D model can also be used in future fluid dynamic simulations to better understand acquisition, retention, and inoculation of X. fastidiosa. Improved understanding of these processes could lead to new targets for preventing diseases caused by X. fastidiosa.

Targeted Mutations in Xylella fastidiosa Affect Acquisition and Retention by the Glassy-Winged Sharpshooter, Homalodisca vitripennis (Hemiptera: Cicadellidae)

Xylella fastidiosa (Wells) is a xylem-limited bacterium that causes Pierce's disease of grapevines. The bacterium is transmitted by insect vectors such as the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar). Experiments were conducted to compare the role of selected X. fastidiosa genes on 1) bacterial acquisition and retention in GWSS foreguts, and 2) transmission to grapevines by GWSS. Bacterial genotypes used were: mutants Xf-ΔpilG, Xf-ΔpilH, Xf-ΔgacA, and Xf-ΔpopP; plus wild type (WT) as control. Results showed that Xf-ΔpilG had enhanced colonization rate and larger numbers in GWSS compared with WT. Yet, Xf-ΔpilG exhibited the same transmission efficiency as WT. The Xf-ΔpilH exhibited poor acquisition and retention. Although initial adhesion, multiplication, and retention of Xf-ΔpilH in GWSS were almost eliminated compared with WT, the mutation did not reduce transmission success in grapevines. Overall, Xf-ΔgacA showed colonization rates and numbers in foreguts similar to WT. The Xf-ΔgacA mutation did not affect initial adhesion, multiplication, and long-term retention compared with WT, and was not significantly diminished in transmission efficiency. In contrast, numbers of Xf-ΔpopP were variable over time, displaying greatest fluctuation from highest to lowest levels. Thus, Xf-ΔpopP had a strong, negative effect on initial adhesion, but adhered and slowly multiplied in the foregut. Again, transmission was not diminished compared to WT. Despite reductions in acquisition and retention by GWSS, transmission efficiency of genotypes to grapevines was not affected. Therefore, in order to stop the spread of X. fastidiosa by GWSS using gene-level targets, complete disruption of bacterial colonization mechanisms is required.

The Probing Behavior Component of Disease Transmission in Insect-Transmitted Bacterial Plant Pathogens

Insects can be effective vectors of plant diseases and this may result in billions of dollars in lost agricultural productivity. New, emerging or introduced diseases will continue to cause extensive damage in afflicted areas. Understanding how the vector acquires the pathogen and inoculates new hosts is critical in developing effective management strategies. Management may be an insecticide applied to kill the vector or a host plant resistance mechanism to make the host plant less suitable for the vector. In either case, the tactic must act before the insect performs the key behavior(s) resulting in either acquisition or transmission. This requires knowledge of the timing of behaviors the insect uses to probe the plant and commence ingestion. These behaviors are visualized using electropenetrography (EPG), wherein the plant and insect become part of an electrical circuit. With the tools to define specific steps in the probing process, we can understand the timing of acquisition and inoculation. With that understanding comes the potential for more relevant testing of management strategies, through insecticides or host plant resistance. The primary example will be Candidatus Liberibacter asiaticus transmitted by Diaphorina citri Kuwayama in the citrus agroecosystem, with additional examples used as appropriate.

EPG combined with micro-CT and video recording reveals new insights on the feeding behavior of Philaenus spumarius

The meadow spittlebug Philaenus spumarius plays a key role in the transmission of the bacterium Xylella fastidiosa to olive in Apulia (South Italy). Currently, available data on P. spumarius feeding behavior is limited, and a real-time observation of the different steps involved in stylet insertion, exploratory probes, and ingestion, has never been carried out. Therefore, we performed an EPG-assisted characterization of P. spumarius female feeding behavior on olive, in order to detect and analyze the main EPG waveforms describing their amplitude, frequency, voltage level, and electrical origin of the traces during stylet penetration in plant tissues. Thereafter, each of the main waveforms was correlated with specific biological activities, through video recording and analysis of excretion by adults and excretion/secretion by nymphs. Furthermore, the specific stylet tips position within the plant tissues during each of the waveforms observed was assessed by microcomputer tomography (micro-CT). Additional EPG-recordings were carried out with males of P. spumarius on olive, in order to assess possible sex-related differences. P. spumarius feeding behavior can be described by five main distinct waveforms: C (pathway), Xc (xylem contact/pre-ingestion), Xi (xylem sap ingestion), R (resting), N (interruption within xylem phase). Compared to males, females require shorter time to begin the first probe, and their Xi phase is significantly longer. Furthermore, considering the single waveform events, males on olive exhibit longer np and R compared to females.

Xylella fastidiosa: an examination of a re-emerging plant pathogen

Xylella fastidiosa is a Gram-negative bacterial plant pathogen with an extremely wide host range. This species has recently been resolved into subspecies that correlate with host specificity. This review focuses on the status of X. fastidiosa pathogenic associations in plant hosts in which the bacterium is either endemic or has been recently introduced. Plant diseases associated with X. fastidiosa have been documented for over a century, and much about what is known in the context of host-pathogen interactions is based on these hosts, such as grape and citrus, in which this pathogen has been well described. Recent attention has focused on newly emerging X. fastidiosa diseases, such as in olives.

TAXONOMY

Bacteria; Gammaproteobacteria; family Xanthomonadaceae; genus Xylella; species fastidiosa.

MICROBIOLOGICAL PROPERTIES

Gram-negative rod (0.25-0.35 × 0.9-3.5 μm), non-flagellate, motile via Type IV pili-mediated twitching, fastidious.

HOST RANGE

Xylella fastidiosa has a broad host range that includes ornamental, ecological and agricultural plants belonging to over 300 different species in 63 different families. To date, X. fastidiosa has been found to be pathogenic in over 100 plant species. In addition, it can establish non-symptomatic associations with many plants as a commensal endophyte. Here, we list the four distinct subspecies of X. fastidiosa and some of the agriculturally relevant diseases caused by them: X. fastidiosa ssp. fastidiosa causes Pierce's disease (PD) of grapevine (Vitis vinifera); X. fastidiosa ssp. multiplex causes almond leaf scorch (ALS) and diseases on other nut and shade tree crops; X. fastidiosa ssp. pauca causes citrus variegated chlorosis (CVC) (Citrus spp.), coffee leaf scorch and olive quick decline syndrome (OQDS) (Olea europaea); X. fastidiosa ssp. sandyi causes oleander leaf scorch (OLS) (Nerium oleander). Significant host specificity seemingly exists for some of the subspecies, although this could be a result of technical biases based on the limited number of plants tested, whereas some subspecies are not as stringent in their host range and can infect several plant hosts.

DISEASE SYMPTOMS

Most X. fastidiosa-related diseases appear as marginal leaf necrosis and scorching of the leaves. In the case of PD, X. fastidiosa can also cause desiccation of berries (termed 'raisining'), irregular periderm development and abnormal abscission of petioles. In olive trees affected with OQDS, leaves exhibit marginal necrosis and defoliation, and overall tree decline occurs. Plants with ALS and OLS also exhibit the characteristic leaf scorch symptoms. Not all X. fastidiosa-related diseases exhibit the typical leaf scorch symptoms. These include CVC and Phony Peach disease, amongst others. In the case of CVC, symptoms include foliar wilt and interveinal chlorosis on the upper surfaces of the leaves (similar to zinc deficiency), which correspond to necrotic, gum-like regions on the undersides of the leaves. Additional symptoms of CVC include defoliation, dieback and hardening of fruits. Plants infected with Phony Peach disease exhibit a denser, more compact canopy (as a result of shortened internodes, darker green leaves and delayed leaf senescence), premature bloom and reduced fruit size. Some occlusions occur in the xylem vessels, but there are no foliar wilting, chlorosis or necrosis symptoms . USEFUL WEBSITES: http://www.piercesdisease.org/; https://pubmlst.org/xfastidiosa/; http://www.xylella.lncc.br/; https://nature.berkeley.edu/xylella/; https://ec.europa.eu/food/plant/plant_health_biosecurity/legislation/emergency_measures/xylella-fastidiosa_en.

Certain applied electrical signals during EPG cause negative effects on stylet probing behaviors by adult Lygus lineolaris (Hemiptera: Miridae)

This study is the first to fully evaluate whether electrical signals applied to large insects during electropenetrography (EPG; also called electrical penetration graph) negatively affect insect behavior. During EPG, electrical signals are applied to plants, and thus to the gold-wire-tethered insects feeding on them. The insect completes an electrical circuit whose changes in voltage reflect the insect's stylet probing/penetration behaviors, recorded as waveform output. For nearly 50 years of EPG science, evidence has supported that there are no or negligible effects on tiny insects from applied electricity during EPG. Recently however, EPG studies of large-bodied hemipterans such as heteropterans and sharpshooter leafhoppers have been published. The wider stylet diameters of such large insects cause them to have lower inherent resistances to applied signals compared with smaller insects, conveying more electrical current. The present study asked whether such increased currents would affect insect stylet probing, by comparing Lygus lineolaris behaviors on pin-head cotton squares using an AC-DC electropenetrograph. Effects of AC or DC applied signals were separately examined in two factorial studies, each comparing four input resistor (Ri) levels (10⁶, 10⁷, 10⁸ and 10⁹ Ω) and four applied voltage levels (2, 60, 150 and 250 mV). Results showed that changes in both probing and non-probing behaviors were indeed caused by changing signal type, Ri level, or applied voltage. Negative effects on feeding were numerically greater overall for DC than AC applied signals, perhaps due to muscular tetany from DC; however, AC versus DC could not be statistically tested. Results strongly support the need for flexible Ri and applied voltage levels and types, to tailor instrument settings to the size and special needs of each insect subject. Our findings will facilitate further EPG studies of Lygus spp., such as host plant resistance or insecticidal assays/bioassays to assess mode of action and appropriate dosage. It is hoped that this study will also inform EPG studies of similar, large heteropterans in the future.

First finding of a dual-meaning X wave for phloem and xylem fluid ingestion: Characterization of Scaphoideus titanus (Hemiptera: Cicadellidae) EPG waveforms

The leafhopper Scaphoideus titanus (Hemiptera: Cicadomorpha: Cicadellidae), an invasive deltocephaline species introduced into Europe from North America, is the vector of the most important phytoplasma disease in European viticulture, flavescence dorée. In this first electropenetrography (EPG) study of S. titanus, we characterized its feeding waveforms and defined their biological meanings. Four typical waveform phases (pathway, X wave, sustained ingestion, and interruption) and four families within those phases (A, B, C, and N) were characterized using DC EPG technology. We proposed biological meanings for these waveforms based on excreta pH-ingestion correlations, presence of X waves, and comparison with previous AC, DC, and AC-DC EPG waveforms conducted on Cicadomorpha. We observed that sustained (i.e., >10min) ingestion by a deltocephaline leafhopper can occur from both xylem and phloem vascular cells. Waveform C2x represented ingestion of xylem fluid, and two waveforms represented behaviors when stylets were inserted into phloem sieve elements: C2p variant 1 (C2p-1), which may represent salivation (perhaps simultaneous with ingestion), and C2p variant 2 (C2p-2), which represented active ingestion. Furthermore, we found that the EPG-recorded X wave has a dual meaning by occurring prior to sustained ingestion from either phloem or xylem. This X wave was very similar in appearance to the model X wave of sharpshooters, an entirely different leafhopper subfamily, Cicadellinae. All cicadellines are obligate xylem-ingesters. Such a "dual-meaning X wave" will provide insights into how the feeding tactics of S. titanus relate to other sheath-feeding hemipterans, and will provide support for future research to clarify the role of this leafhopper as a vector of plant pathogens.

Mixed xylem and phloem sap ingestion in sheath-feeders as normal dietary behavior: Evidence from the leafhopper Scaphoideus titanus

In phytophagous piercing-sucking insects, salivary sheath-feeding species are often described as xylem- or phloem-sap feeding specialists. Because these two food sources have very different characteristics, two feeding tactics are often associated with this supposed specialization. Studying the feeding behavior of insects provides substantial information on their biology, ecology, and evolution. Furthermore, study of feeding behavior is of primary importance to elucidate the transmission ability of insects that act as vectors of plant pathogens. In this study, we compared the durations of ingestion performed in xylem versus phloem by a leafhopper species, Scaphoideus titanus Ball, 1932. This was done by characterizing and statistically analyzing electrical signals recorded using the electropenetrography technique, derived from the feeding behaviors of males and females. We identified three groups of S. titanus based on their feeding behavior: 1) a group that reached the phloem quickly and probed for a longer time in phloem tissue than the other groups, 2) a group that reached the xylem quickly and probed for a longer time in xylem tissue than the other groups, and 3) a group where individuals did not ingest much sap. In addition, the numbers and durations of waveforms representing ingestion of xylem and phloem saps differed significantly depending on the sex of the leafhopper, indicating that the two sexes exhibit different feeding behaviors. Males had longer phloem ingestion events than did females, which indicates that males are greater phloem feeders than females. These differences are discussed, specifically in relation to hypotheses about evolution of sap feeding and phytoplasma transmission from plant to plant.

Sustainable Management of Plant Quarantine Pests: The Case of Olive Quick Decline Syndrome

The disease outbreak of Xylella fastidiosa subsp. pauca strain CoDiRO (Complesso del Disseccamento Rapido dell’Olivo) in Salento (Apulia, South Italy) associated with severe cases of olive quick decline syndrome may represent not just a new disease paradigm, but a challenge for policy formulation and science communication in plant pathology. Plant health management can be achieved by applying a technocratic model, in which objective science is thought to directly inform policy-making, or via decisionistic or inclusive models, in which scientific considerations drive risk assessment. Each could be applied to X. fastidiosa and CoDiRO strain management, thanks to consistent literature related to pathogen/host interactions, hosts, vectors, and diagnostic tools, reviewed here. However, consensus among stakeholders seems to be necessary in order to avoid plant health management failures or gridlocks, due to environmental, economic, and social implications in the X. fastidiosa threat. Here we discuss the role of consensus in building scientific opinion, reporting different approaches of governance after severe disease outbreaks in Europe. These case studies, and the available risk analysis for Xylella strains, should drive policy formulations towards more cooperative networks.

PAMPs, PRRs, effectors and R-genes associated with citrus-pathogen interactions

BACKGROUND

Recent application of molecular-based technologies has considerably advanced our understanding of complex processes in plant-pathogen interactions and their key components such as PAMPs, PRRs, effectors and R-genes. To develop novel control strategies for disease prevention in citrus, it is essential to expand and consolidate our knowledge of the molecular interaction of citrus plants with their pathogens.

SCOPE

This review provides an overview of our understanding of citrus plant immunity, focusing on the molecular mechanisms involved in the interactions with viruses, bacteria, fungi, oomycetes and vectors related to the following diseases: tristeza, psorosis, citrus variegated chlorosis, citrus canker, huanglongbing, brown spot, post-bloom, anthracnose, gummosis and citrus root rot.

Latency and Persistence of 'Candidatus Liberibacter asiaticus' in Its Psyllid Vector, Diaphorina citri (Hemiptera: Liviidae)

Although 'Candidatus Liberibacter asiaticus' (Las) is a major pathogen associated with citrus huanglongbing (HLB), some characteristics of transmission by the psyllid vector Diaphorina citri are not fully understood. We examined the latent period and persistence of transmission of Las by D. citri in a series of experiments at 25°C, in which third-instar psyllid nymphs and 1-week-old adults were confined on infected citrus for an acquisition access period (AAP), and submitted to sequential inoculation access periods (IAPs) on healthy citrus seedlings. The median latent period (LP₅₀, i.e., acquisition time after which 50% of the individuals can inoculate) of 16.8 and 17.8 days for psyllids that acquired Las as nymphs and adults, respectively, was determined by transferring single individuals in 48-h IAPs. Inoculation events were intermittent and randomly distributed over the IAPs, but were more frequent after acquisition by nymphs. A minimum latent period of 7 to 10 days was observed by transferring groups of 10 psyllids in 48-h IAPs, after a 96-h AAP by nymphs. Psyllids transmitted for up to 5 weeks, when submitted to sequential 1-week IAPs after a 14-day AAP as nymphs. The long latent period and persistence of transmission are indirect evidences of circulative propagation of Las in D. citri.

Colonization and Intrusive Invasion of Potato Psyllid by 'Candidatus Liberibacter solanacearum'

Previous studies have shown that the fastidious bacterial plant pathogen 'Candidatus Liberibacter solanacearum' (CLso) is transmitted circulatively and propagatively by the potato psyllid (PoP) Bactericera cockerelli. In this study, the temporal and spatial interrelationships between CLso PoP were investigated by scanning electron microscopy of the digestive system of PoP immature and adult instars and salivary glands of adults post CLso ingestion. CLso biofilms were not detectable on the outer midgut surface of the first and second instars; however, for third to fifth instars and teneral and mature adults, biofilms were observed in increasing numbers in each successive developmental stage. In adult PoP midguts, CLso cells were observed between the basal lamina and basal epithelial cell membranes; in basal laminar perforations, on the outer basal laminar surface, and in the ventricular lumen, epithelial cytosol, and filter chamber periventricular space. CLso were also abundantly visible in the salivary gland pericellular spaces and in the epidermal cell cytosol of the head. Collectively, these results point to an intrusive, systemic invasion of PoP by CLso that employs an endo/exocytosis-like mechanism, in the context of a propagative, circulative mode of transmission.