Fastidiosa Transmission Research Articles

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.

Feeding behaviour and mortality of Philaenus spumarius exposed to insecticides and their impact on Xylella fastidiosa transmission.

Insecticides are essential, though controversial tools in modern pest management. Insecticides can slow the spread of key vector-borne plant pathogens, but often lead to inconsistent results given that insecticide use is generally focused on acute toxicity under no-choice conditions. Here, we analysed the lethal (survival) and sublethal (feeding behaviour) effects of six commercial products (acetamiprid, deltamethrin, spinosad, sulfoxaflor, pyrethrin and kaolin) on Philaenus spumarius, vector of the bacterium Xylella fastidiosa. Furthermore, we assessed the impact of insecticides displaying different degrees of acute toxicity against spittlebugs (highest to lowest: acetamiprid, pyrethrin and kaolin) on the transmission of X. fastidiosa by P. spumarius under both free-choice and no-choice conditions. Deltamethrin, acetamiprid and to a limited extent pyrethrin significantly altered the feeding behaviour of P. spumarius. Deltamethrin and acetamiprid were highly toxic against P. spumarius, but the mortality induced by exposure to pyrethrin was limited overall. By contrast, spinosad, sulfoxaflor and kaolin did not significantly impact P. spumarius feeding behaviour or survival. Under no-choice conditions, both pyrethrin and acetamiprid reduced the X. fastidiosa inoculation rate compared with kaolin and the control. On the other hand, pyrethrin reduced transmission, but acetamiprid failed to significantly affect bacterial inoculation under free-choice conditions. Pyrethrin was the only compound able to reduce X. fastidiosa transmission under both free-choice and no-choice conditions. Xylella fastidiosa management strategy based exclusively on the evaluation of insecticide acute toxicity under no-choice conditions would most likely fail to prevent, or slow, bacterial spread. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

Assessment of Nymphal Ecology and Adult Xylella fastidiosa Transmission Biology of Aphrophora nr. permutata (Hemiptera: Aphrophoridae) in California Vineyards.

Although Aphrophora nr. permutata (Hemiptera: Aphrophoridae) is a reported vector of the plant pathogen Xylella fastidiosa (Wells) (Xanthomonadales: Xanthomonadaceae), its ecology and role in Pierce's disease dynamics in coastal California vineyards are poorly understood. From 2016 to 2020, we surveyed the abundance of A. nr. permutata nymphs among potential host plants along the vineyard floor, the vineyard edges, and adjacent vegetation in vineyards in Napa and Sonoma county. In 2019 and 2020, vineyards adjacent to woodland habitat hosted larger A. nr. permutata populations than those next to riparian habitat, while in 2017 and 2018, the nymphal populations were similar among riparian and woodland sites. Among 2020 plant cover taxa, nymph abundance was positively associated with Helminthotheca echioides, Vicia sativa, and Daucus carota cover and negatively associated with Taraxacum officinale cover. In 2018 and 2019, we also tracked early-season occurrence and development of A. nr. permutata nymphs among potential host plants. Analyses showed a significant effect of site, year, and plant taxa on the first detection of nymphs and a significant effect of site and year on the estimated development time between first and fifth instars. In 2019, we conducted grapevine to grapevine X. fastidiosa transmission experiments with individuals and groups of five A. nr. permutata adults. In the transmission experiment, 5% (3 of 60) individual A. nr. permutata and 7.7% (1 of 13) of groups successfully transmitted X. fastidiosa. This study provides preliminary evidence of potential host plant associations with A. nr. permutata abundance and phenology that should be explored further with field and greenhouse-based approaches.

Plant defense against a pathogen drives nonlinear transmission dynamics through both vector preference and acquisition

AbstractHost defense against vector‐borne plant pathogens is a critical component of integrated disease management. However, theory predicts that traits that confer tolerance or partial resistance can, under certain ecological conditions, enhance the spread of pathogens and spillover to more susceptible populations or cultivars. A key component driving such epidemic risk appears to be variation in host‐selection behavior of vectors based on infection status of the host. While recent theory has further emphasized the importance of infection‐induced host‐selection behavior by insect vectors for plant disease epidemiology, experimental tests on the relationship between vector host‐selection preference and transmission are lacking. We test how host plant defense—conferred by the PdR1 gene complex—mediates vector host‐selection preference and transmission of the pathogenic bacterium Xylella fastidiosa among grapevine cultivars. We confirmed that PdR1 confers resistance against X. fastidiosa by reducing both pathogen population size and disease severity. We found that vector transmission rates to new hosts exhibited unimodal dynamics over the course of infection when both susceptible and resistant were infected and acted as sources of the pathogen. Transmission from susceptible plants initially increased and then declined as insect vectors avoided severely diseased plants. While transmission from PdR1‐resistant plants also initially increased and then declined as well, this was not due to avoidance by vectors, although the exact mechanism remains unclear. We show that (1) vector preference changes over the course of disease progression, (2) vector preference is clearly important but a poor predictor of transmission, and (3) the post‐latent incubation period—in which plant hosts are infectious but asymptomatic—is likely a key period for vector transmission of X. fastidiosa. Our results suggest that, consistent with theory, defensive traits lengthen the duration of the incubation period, increasing X. fastidiosa transmission. However, defensive traits may over the long‐term ultimately reduce spread possibly through induced resistance. Vector host‐selection preference, host resistance, and transmission are clearly dynamic, changing over the course of disease progression. Understanding these dynamics is critical for broader insights into the epidemiology of vector‐borne plant pathogens, theory development, and deploying disease‐resistant cultivars in an effective and sustainable manner.

First sharpshooter species proven as vectors of Xylella fastidiosa subsp. multiplex in Prunus salicina trees in Brazil

Plum leaf scald (PLS), caused by the vector-borne bacterium Xylella fastidiosa, is a major obstacle to the expansion of plum crops in Brazil. In other affected crops, this pathogen is naturally transmitted mainly by sharpshooter leafhoppers (Hemiptera: Cicadellidae: Cicadellinae). Despite the importance of PLS, the vector species responsible for bacterial spread in plums remain unknown. This study, therefore, aimed to determine the X. fastidiosa transmission ability of three sharpshooter species commonly found in plum orchards in southern and southeastern Brazil: Macugonalia cavifrons (Stål), Macugonalia leucomelas (Walker), and Sibovia sagata (Signoret). After a 72-h acquisition access period to plum trees infected with X. fastidiosa subsp. multiplex (sequence type 67), the insects were transferred for an inoculation access period of 96 h to 10–20 healthy Prunus salicina plants for each sharpshooter species, using four insects per plant. Bacterial infection in the plants was verified 7 months after inoculation by polymerase chain reaction. Results showed that the three sharpshooter species were able to transmit X. fastidiosa to plums, with mean transmission efficiencies by single insects of 16%, 14%, and 18% for M. cavifrons, M. leucomelas, and S. sagata, respectively. To our knowledge, this study is the first to identify vector species and prove the transmission of a PLS strain of X. fastidiosa subsp. multiplex by sharpshooters in Brazil.

Seasonal Abundance and Infectivity of Philaenus spumarius (Hemiptera: Aphrophoridae), a Vector of Xylella fastidiosa in California Vineyards.

The meadow spittlebug, Philaenus spumarius (Linnaeus) (Hemiptera: Aphrophoridae), is a vector of the plant pathogen Xylella fastidiosa; however, its role in recent outbreaks of Pierce's disease of grapevine (PD) in California is unclear. While the phenology and ecology of P. spumarius can help determine its contributions to PD epidemics, both remain poorly described in the North Coast vineyards of California. We assessed the phenology of P. spumarius in the region. Spittlemasses were first observed in February or March, while the emergence of adult spittlebugs did not occur until April or May depending on the year. Analysis of sweep and trap data from 2016 to 2018 revealed significant effects of survey month, vineyard site, and year on adult abundance in sweep and trap surveys. Spittlebug adults were present in the vineyards from April until December, with the greatest number of adults by sweep net in May or June, whereas adults on traps peaked between July and November. Analysis of natural infectivity in groups of field-collected spittlebug adults showed significant difference in transmission rates among months. Spittlebugs successfully transmitted Xylella fastidiosa (Wells) (Xanthomonadales: Xanthomonadaceae) to potted grapevines between July and December. The greatest risk of X. fastidiosa transmission by P. spumarius was in December (60%) followed by October (30%). However, the infectivity patterns of the meadow spittlebug did not align with the historical paradigm of California North Coast PD. We discuss alternative hypotheses in which P. spumarius could play a role in the epidemiology of this disease.

XadA2 Adhesin Decreases Biofilm Formation and Transmission of Xylella fastidiosa subsp. pauca.

Xylella fastidiosa is a vector-borne bacterium that causes diseases in many plants of economic interest. The bacterium–vector initial interactions involve bacterial membrane-bound adhesins that mediate cell attachment to the foregut of insect vectors. We investigated the role of the afimbrial adhesin XadA2 in the binding and biofilm formation of X. fastidiosa subsp. pauca to vector surfaces in vitro, as well as its potential to disrupt pathogen transmission. We showed that XadA2 has binding affinity for polysaccharides on sharpshooter hindwings, used as a proxy for the interactions between X. fastidiosa and vectors. When in a medium without carbon sources, the bacterium used wing components, likely chitin, as a source of nutrients and formed a biofilm on the wing surface. There was a significant reduction in X. fastidiosa biofilm formation and cell aggregation on vector wings in competition assays with XadA2 or its specific antibody (anti-XadA2). Finally, pathogen acquisition and transmission to plant were significantly reduced when the vectors acquired X. fastidiosa from an artificial diet supplemented with anti-XadA2. These results show that XadA2 is important in mediating bacterial colonization in the insect and that it could be used as a target for blocking X. fastidiosa transmission.

Review of the EPG Waveforms of Sharpshooters and Spittlebugs Including Their Biological Meanings in Relation to Transmission of Xylella fastidiosa (Xanthomonadales: Xanthomonadaceae).

Electropenetrography (EPG) is one of the most rigorous methods to study stylet probing behaviors of piercing-sucking insects whose mouthparts move invisibly inside hosts. EPG is particularly useful for identifying vector behaviors that control transmission (acquisition, retention, and inoculation) of plant pathogens, comparing those behaviors among vector species, and aiding in development of novel vector and disease management tactics. Xylella fastidiosa (Wells et al.) is a gram-negative, invasive bacterium native to the Americas, where it is the causal agent of lethal scorch-type diseases such as Pierce's disease of grapevines. Xylella fastidiosa is transmitted by sharpshooter leafhoppers (Hemiptera: Cicadellidae: Cicadellinae) and spittlebugs (Hemiptera: Aphrophoridae). Despite over 75 yr of study, details of the inoculation mechanism of X. fastidiosa were unknown until the advent of EPG research with sharpshooters. Herein, the following topics are presented: 1) review of key EPG principles and waveforms published to date, emphasizing sharpshooters and spittlebugs; 2) summary of present understanding of biological meanings of sharpshooter waveforms; 3) review of mechanisms of transmission for X. fastidiosa illuminated by EPG; and 4) recommendations of the most useful waveform categories for EPG use in future, quantitative comparisons of sharpshooter stylet probing on various treatments such as infected versus uninfected plants, resistant varieties, or insecticide treatments. In addition, new work on the functional anatomy of the precibarial valve is discussed in the context of X. fastidiosa transmission and EPG waveforms. Also, the first block diagram of secondary, signal-processing circuits for the AC-DC EPG is published, and is discussed in relation to EPG signals appearances and meanings.

Feeding behavior in relation to spittlebug transmission of Xylella fastidiosa

Here, we provide the first insights into the transmission dynamics of the bacterium Xylella fastidiosa by the meadow spittlebug Philaenus spumarius, gathered through DC EPG (electrical penetration graph)-assisted transmission tests and comparative observations of the probing and feeding behavior of infective versus non-infective vectors on healthy olive plants. Bacterial cells binding to P. spumarius’ foregut occurred at a very low rate and in a time as short as 15 min spent by the insect in xylem ingestion or activities interspersed with xylem ingestion (interruption during xylem ingestion and resting). P. spumarius inoculation of bacterial cells into the xylem was exclusively associated with an early (ca. 2 to 7 min after the onset of the first probe) and occasional behavior, provisionally termed waveform Xe, presumably related to egestion regulated by pre-cibarial valve fluttering. Infective spittlebugs compared to non-infective ones exhibited: (i) longer non-probing and shorter xylem ingestion; (ii) longer duration of single non-probing events; (iii) fewer sustained ingestions (ingestion longer than 10 min) and interruptions of xylem activity (N); and (iv) longer time required to perform the first probe. These observations suggest difficulties in feeding of infective P. spumarius probably caused by the presence of X. fastidiosa within the foregut. Overall, our data indicate that likely short time—few minutes—is required for X. fastidiosa transmission by P. spumarius; thus, vector control strategies should aim at preventing spittlebug access to the host plant. Furthermore, our findings represent an important contribution for further research on the disruption of spittlebug–bacterium interactions.

Transmission of Xylella fastidiosa by naturally infected Philaenus spumarius (Hemiptera, Aphrophoridae) to different host plants

AbstractThe recent establishment of Xylella fastidiosa subspecies pauca in the southern Italian region of Apulia threatens agricultural crops and the environment. Olive is an important and widespread ancient crop in Italy and, so far, the most impacted host. The meadow spittlebug Philaenus spumarius (Hemiptera, Aphrophoridae) has been identified as a vector of X. fastidiosa in southern Italy; this species is one of the most common potential vectors in Europe. To generate disease management strategies, data on X. fastidiosa transmission by P. spumarius are necessary. Therefore, we carried out transmission experiments by using field‐collected spittlebugs in 2014 and 2015 (5 and 11 collection dates, respectively), and transferring groups of insects immediately on to recipient plants. Various host plant species were tested: olive, oleander, sweet orange, grapevine and the stone fruit rootstock GF677 (Prunus persica × Prunus amygdalus). Xylella fastidiosa was detected in all the host plants after insect plant access except for grapevine; infections to sweet orange and stone fruit were not systemic. In 2015, estimates of insect X. fastidiosa infectivity were obtained; the number of PCR‐positive P. spumarius on each plant was positively correlated with the plant infection status. The proportion of P. spumarius infected with X. fastidiosa ranged from 25% to 71% during the entire survey period. The number of X. fastidiosa cells detected in P. spumarius heads ranged from 3.5 × 10 to 4.0 × 102 (CFU equivalents), which is lower than that reported for leafhopper vectors in the Americas. These data show that field‐collected P. spumarius have high rates of X. fastidiosa infection and are competent vectors.

Transmission of Xylella fastidiosa to Grapevine by the Meadow Spittlebug.

There is little information available on Xylella fastidiosa transmission by spittlebugs (Hemiptera, Cercopoidea). This group of insect vectors may be of epidemiological relevance in certain diseases, so it is important to better understand the basic parameters of X. fastidiosa transmission by spittlebugs. We used grapevines as a host plant and the aphrophorid Philaenus spumarius as a vector to estimate the effect of plant access time on X. fastidiosa transmission to plants; in addition, bacterial population estimates in the heads of vectors were determined and correlated with plant infection status. Results show that transmission efficiency of X. fastidiosa by P. spumarius increased with plant access time, similarly to insect vectors in another family (Hemiptera, Cicadellidae). Furthermore, a positive correlation between pathogen populations in P. spumarius and transmission to plants was observed. Bacterial populations in insects were one to two orders of magnitude lower than those observed in leafhopper vectors, and population size peaked within 3 days of plant access period. These results suggest that P. spumarius has either a limited number of sites in the foregut that may be colonized, or that fluid dynamics in the mouthparts of these insects is different from that in leafhoppers. Altogether our results indicate that X. fastidiosa transmission by spittlebugs is similar to that by leafhoppers. In addition, the relationship between cell numbers in vectors and plant infection may have under-appreciated consequences to pathogen spread.

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

Xylella fastidiosa is unique among insect-transmitted plant pathogens because it is propagative but noncirculative, adhering to and multiplying on the cuticular lining of the anterior foregut. Any inoculation mechanism for X. fastidiosa must explain how bacterial cells exit the vector's stylets via the food canal and directly enter the plant. A combined egestion-salivation mechanism has been proposed to explain these unique features. Egestion is the putative outward flow of fluid from the foregut via hypothesized bidirectional pumping of the cibarium. The present study traced green fluorescent protein-expressing X. fastidiosa or fluorescent nanoparticles acquired from artificial diets by glassy-winged sharpshooters, Homalodisca vitripennis, as they were egested into simultaneously secreted saliva. X. fastidiosa or nanoparticles were shown to mix with gelling saliva to form fluorescent deposits and salivary sheaths on artificial diets, providing the first direct, conclusive evidence of egestion by any hemipteran insect. Therefore, the present results strongly support an egestion-salivation mechanism of X. fastidiosa inoculation. Results also support that a column of fluid is transiently held in the foregut without being swallowed. Evidence also supports (but does not definitively prove) that bacteria were suspended in the column of fluid during the vector's transit from diet to diet, and were egested with the held fluid. Thus, we hypothesize that sharpshooters could be true "flying syringes," especially when inoculation occurs very soon after uptake of bacteria, suggesting the new paradigm of a nonpersistent X. fastidiosa transmission mechanism.

Background matching behaviour and pathogen acquisition: plant site preference does not predict the bacterial acquisition efficiency of vectors

Many insect-borne pathogens are heterogeneously distributed within their hosts: therefore, a vector’s within-plant distribution may be a predictor of its exposure to pathogens. In this study, we set out to quantify plant site preference, in the context of background matching, and investigated its effect on acquisition of a bacterial pathogen by its leafhopper vectors. The two green-coloured species, Graphocephala atropunctata and Draeculacephala minerva, preferred green plant tissue and artificial backgrounds whereas the brown-coloured Homalodisca vitripennis preferred brown stem tissue and backgrounds. Within-plant feeding site did not predict either the acquisition success or the number of plant-pathogenic Xylella fastidiosa cells acquired by the vectors; an 86% mortality for G. atropunctata was reported on the lignified stem tissue. Overall, H. vitripennis acquired significantly more cells than G. atropunctata. A novel artificial diet-based transmission system was used to further illustrate that the observed between-species difference in the number of cells acquired was independent of vector-host plant interactions. H. vitripennis, a less efficient vector of the bacterium X. fastidiosa on grapevines, acquired more bacterial cells than G. atropunctata, possibly due to its larger size. Contrary to previous assumptions, pathogen acquisition efficiency by the vectors did not explain their reported differences in inoculation. Vector interactions with the host during the inoculation stage should be evaluated as another determinant of X. fastidiosa transmission efficiency.

Correlations of cibarial muscle activities of Homalodisca spp. sharpshooters (Hemiptera: Cicadellidae) with EPG ingestion waveform and excretion

Fluid flow into and out of the stylets of xylem-ingesting sharpshooters (Hemiptera: Cicadellidae: Cicadellinae) is powered by muscles of the cibarial pump. Such fluid flow is crucial for transmission of Xylella fastidiosa, the Pierce’s Disease bacterium, yet has not been rigorously studied via electrical penetration graph (EPG) technology. We correlated EPG waveforms with electromyographically (EMG) recorded muscle potentials from the cibarial dilator muscles, which power the piston-like cibarial diaphragm. There was a 1:1 correspondence of each cycle of cibarial muscle contraction/relaxation with each plateau of EPG waveform C. Results definitively showed that the C waveform represents active ingestion, i.e. fluid flow is propelled by cibarial muscle contraction. Moreover, each C waveform episode represents muscular diaphragm uplift, probably combined with a “bounce” from cuticular elasticity, to provide the suction that pulls fluid into the stylets. Fine structure of the EPG ingestion waveform represents directionality of fluid flow, supporting the primary role of streaming potentials as the electrical origin of the C waveform. Rhythmic bouts of cibarial pumping were generally correlated with sustained production of excretory droplets. However, neither the onset nor cessation of ingestion was correlated with onset or cessation of excretion, respectively. Volume of excreta is an inexact measure of ingestion. Implications for using EPG to understand the mechanism of X. fastidiosa transmission are discussed.

Reduction of Xylella fastidiosa Transmission Through Pecan Scion Wood by Hot-Water Treatment.

Pecan (Carya illinoinensis) bacterial leaf scorch disease, caused by the bacterium Xylella fastidiosa, causes leaf loss and reduction in yield of pecans. One of the ways that the pathogen infects newly developing trees is by graft-transmission through infected scion wood. Submersion of pecan scion wood in 46°C water for 30 min greatly reduced transmission of X. fastidiosa following grafting. During a 2-year study with potted rootstock grafted to either hot-water-treated or nontreated scion wood collected from limbs of 'Cape Fear' pecan infected with X. fastidiosa, the pathogen was detected in 21% of the trees that developed from the nontreated scion wood and 0.7% of the trees from the hot-water-treated scions. The hot-water treatment of 46°C for 30 min did not affect graft success. Likewise, scion diameter had no effect on success of grafting or on the efficacy of hot-water treatment. Similar hot-water treatments have been efficacious in elimination of X. fastidiosa and some other pathogens from grapevine cuttings. The use of the hot-water treatment demonstrated in this report could be helpful to individual pecan growers and nurseries that use scion wood that may be infected with X. fastidiosa to reduce the occurrence of pecan bacterial leaf scorch disease in new trees.

Vector Transmission of <I>Xylella fastidiosa</I>: Applying Fundamental Knowledge to Generate Disease Management Strategies

Xylella fastidiosa is a xylem-limited bacterium transmitted to plants by xylem sap-feeding insects. This pathogen has a wide host range, causing disease in crops such as grape, citrus, almond, and coffee; ornamental plants; and trees. Sharpshooter leafhoppers are the major vectors of X. fastidiosa to crops of economic importance. Transmission characteristics include the lack of a latent period, no transstadial or transovarial transmission, persistence in adults, and multiplication in the foregut of vectors. Various factors influence vector transmission of X. fastidiosa, including the distribution and density of bacterial populations in host plants, insect host range and plant preference, season of inoculation, and climatic conditions. The ecology of vectors can affect epidemics, as demonstrated by the large increase in Pierce’s disease of grapevine incidence in California after the introduction of Homalodisca coagulata (Say). Disease control strategies should incorporate basic knowledge of transmission biology, vector ecology, and other interactions involved in X. fastidiosa diseases. We discuss basic aspects of X. fastidiosa transmission by vectors, the ecology of insects in relation to transmission and disease spread, and how basic research can be applied to the development of management strategies for a X. fastidiosa disease.

Vector Transmission of Xylella fastidiosa to Dormant Grape.

Homalodisca coagulata (Say) is a sharpshooter leafhopper vector of the bacterial plant pathogen Xylella fastidiosa. Introduced into California about 15 years ago, this insect triggered recent outbreaks of Pierce's disease of grapevine in the state. H. coagulata has been observed feeding on dormant grapevines during the winter, raising the possibility of X. fastidiosa transmission during that season. We tested whether H. coagulata can acquire X. fastidiosa from and inoculate the bacterium to dormant grape (Vitis vinifera) in the laboratory and in the field. Usually, >90% of H. coagulata survived on dormant plants in the laboratory and field. Field experiments showed that H. coagulata can inoculate X. fastidiosa into dormant plants, yet field acquisition experiments did not result in transmission. Transmission to dormant plants during the winter is a potential problem in California vineyards adjacent to citrus groves or other habitats with large overwintering populations of H. coagulata. Because dormant plants have positive root pressure, our findings provide evidence that X. fastidiosa transmission does not require negative pressure in plant xylem to be inoculated into plants.

Laboratory-Based Monitoring of an Insect-Transmitted Plant Pathogen System

winged sharpshooter, Homalodisca coagulata (Say), is challenging to study, not only because the bacterium is fastidious, but also because the insect does not survive well in captivity, and it will not feed from an artificial diet (i.e., parafilm sachet). We developed a simple and efficient transmission cycle for the study of X. fastidiosa transmission by H. coagulata, allowing collection of sufficient transmission data in 1 week. Specific numbers of cells can be detected both in the plant tissue and within the insect vector by realtime PCR (7). Real-time PCR is more sensitive than traditional PCR and is a single step process, thus reducing the possibility of error and speeding up detection time. A real-time PCR protocol for detecting the citrus variegated chlorosis strain of X. fastidiosa has already been established as a superior technique compared to traditional PCR (8). Real-time PCR was performed in a Rotor-Gene™ 3000 (Corbett Research, Mortlake, NSW, Australia) using iQ™ Supermix (Bio-Rad Laboratories, Hercules, CA, USA) in 20-µL reactions with 525 nM of each X.

Feeding site preference of Dilobopterus costalimai Young and Oncometopia facialis (Signoret) (Hemiptera: Cicadellidae) on citrus plants

The efficiency of Xylella fastidiosa transmission by sharpshooters on citrus plants is low and varies with the vector species. The feeding behavior of the sharpshooter vector related to X. fastidiosa acquisition and inoculation is one of the probable conditioning factors in transmission efficiency. The objective of this study was to assess the behavior of two sharpshooter species, Dilobopterus costalimai Young and Oncometopia facialis (Signoret), in selection of citrus nursery trees and feeding places and to ascertain the influence of sprouting on plant attractiveness to the vectors. Free choice tests were performed with the two species in observation chambers (63 x 63 x 120 cm), releasing 40 individuals per chamber. The number of dead insects were assessed as well as those that chose, or not, one of the treatments, at 3, 15, 21, 24, 39, 45 and 48h after the release. For D. costalimai the leaves were important at the start of host selection, and were substituted over time by the secondary branch. D. costalimai preferred to stay on the secondary leaf nervures followed by the secondary branch. O. facialis showed no difference in preference for staying on the leaves (central and secondary nervures) and secondary branch on citrus plants, and this varied with the time of day. The presence of sprouting in the nursery trees increased the attractiveness of the plant to the two sharpshooter species raising the chances of X. fastidiosa acquisition or inoculation. So, control measures should be adopted against these insects in periods of greater vegetation in orange orchards.