Vector Transmission Efficiency Research Articles

Is the Glycoprotein Responsible for the Differences in Dispersal Rates between Lettuce Necrotic Yellows Virus Subgroups?

Lettuce necrotic yellows virus is a type of species in the Cytorhabdovirus genus and appears to be endemic to Australia and Aotearoa New Zealand (NZ). The population of lettuce necrotic yellows virus (LNYV) is made up of two subgroups, SI and SII. Previous studies demonstrated that SII appears to be outcompeting SI and suggested that SII may have greater vector transmission efficiency and/or higher replication rate in its host plant or insect vector. Rhabdovirus glycoproteins are important for virus–insect interactions. Here, we present an analysis of LNYV glycoprotein sequences to identify key features and variations that may cause SII to interact with its aphid vector with greater efficiency than SI. Phylogenetic analysis of glycoprotein sequences from NZ isolates confirmed the existence of two subgroups within the NZ LNYV population, while predicted 3D structures revealed the LNYV glycoproteins have domain architectures similar to Vesicular Stomatitis Virus (VSV). Importantly, changing amino acids at positions 244 and 247 of the post-fusion form of the LNYV glycoprotein altered the predicted structure of Domain III, glycosylation at N248 and the overall stability of the protein. These data support the glycoprotein as having a role in the population differences of LNYV observed between Australia and New Zealand.

Lethal and Sublethal Effects of Flupyradifurone on Bemisia tabaci MED (Hemiptera: Aleyrodidae) Feeding Behavior and TYLCV Transmission in Tomato.

Pesticides primarily affect target organisms via direct toxicity, but may also alter the feeding behaviors of surviving individuals in ways that alter their effect on host plants. The latter impact is especially important when pests can transmit plant pathogens. The Mediterranean (MED) population of the sweetpotato whitefly Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) transmits Tomato yellow leaf curl virus (TYLCV), a pathogen that can be economically devastating in field and greenhouse cropping systems. We first assessed the impact of sublethal (LC15) and label concentrations of flupyradifurone, a butenolide-derived insecticide, on the feeding behavior of TYLCV-infected MED on tomato. We next measured the effect of flupyradifurone on plant TYLCV load, vector transmission efficiency, and MED survival. Both the LC15 and label flupyradifurone concentrations dramatically altered MED feeding and caused the near cessation of both salivation and phloem ingestion (necessary for viral transmission and acquisition, respectively). Both concentrations also significantly reduced plant TYLCV load, and the label rate of flupyradifurone sharply decreased TYLCV transmission while killing >99% of MED. As the first report of pesticide-induced changes in the feeding behavior of viruliferous Bemisia, our findings highlight the potential importance of chemically driven feeding cessation in the control of TYLCV and other Bemisia-transmitted plant pathogens.

Accumulation and transmission of alphasatellite, betasatellite and tomato yellow leaf curl virus in susceptible and Ty-1-resistant tomato plants

Begomoviruses (family Geminiviridae) are frequently associated with alphasatellites and betasatellites in the Old World. Tomato yellow leaf curl virus, one of the most damaging begomovirus species worldwide, was recently found associated with betasatellites in the eastern coast of the Mediterranean Sea, and in the Middle East region. Tomato yellow leaf curl virus (TYLCV)/betasatellite associations were shown to increase TYLCV virulence in experimental conditions. The sustainability of TYLCV/satellite associations in tomato was assessed here by estimating accumulation levels of satellites in comparison to TYLCV, vector transmission efficiency, and by testing how far the popular Ty-1 resistance gene used in most TYLCV-resistant tomato cultivars in the Mediterranean Basin is effective against betasatellites. Three satellites previously isolated from okra in Burkina Faso—of the species Cotton leaf curl Gezira betasatellite, Cotton leaf curl Gezira alphasatellite and Okra leaf curl Burkina Faso alphasatellite—were shown to accumulate at levels similar to, or higher than, the helper virus TYLCV-Mld in tomato plants from 32 to 150 days post inoculation (dpi). Cotton leaf curl Gezira betasatellite (CLCuGB) reduced TYLCV-Mld accumulation whereas alphasatellites did not. Transmission tests were performed with B. tabaci from plants infected with TYLCV-Mld/CLCuGB- or TYLCV-Mld/Okra leaf curl Burkina Faso alphasatellite. At 32 dpi, both satellites were transmitted to more than 50% of TYLCV-infected test plants. Betasatellite transmission, tested further with 150 dpi source plants was successful in more than 30% of TYLCV-infected test plants. Ty-1 resistant tomato plants co-infected with TYLCV (–Mld or –IL) and CLCuGB exhibited mild leaf curling and mosaic symptoms at the early stage of infection associated with a positive effect on TYLCV-IL accumulation, while resistant plants infected with TYLCV only, were asymptomatic. Together with previous experimental studies, these results further emphasize the potential risk of betasatellites to tomato cultivation, including with Ty-1 resistant cultivars.

Co-infection with a wheat rhabdovirus causes a reduction in Mal de Río Cuarto virus titer in its planthopper vector.

Mal de Río Cuarto virus (MRCV, Fijivirus, Reoviridae) causes one of the most important diseases in maize (Zea mays L.) in Argentina and has been detected in mixed infections with a rhabdovirus closely related to Maize yellow striate virus. In nature both viruses are able to infect maize and several grasses including wheat, and are transmitted in a persistent propagative manner by Delphacodes kuscheli Fennah (Hemiptera: Delphacidae). This work describes the interactions between MRCV and rhabdovirus within their natural vector and the consequences of such co-infection regarding virus transmission and symptom expression. First- and third-instar D. kuscheli nymphs were fed on MRCV-infected wheat plants or MRCV-rhabdovirus-infected oat plants, and two latency periods were considered. Transmission efficiency and viral load of MRCV-transmitting and non-transmitting planthoppers were determined by real-time quantitative polymerase chain reaction analysis (RTqPCR). Vector transmission efficiency was related to treatments (life stages at acquisition and latency periods). Nevertheless, no correlation between transmission efficiency and type of inoculum used to infect insects with MRCV was found. Treatment by third-instar nymphs 17 days after Acquisition Access Period was the most efficient for MRCV transmission, regardless of the type of inoculum. Plants co-infected with MRCV and rhabdovirus showed the typical MRCV symptoms earlier than plants singly infected with MRCV. The transmitting planthoppers showed significantly higher MRCV titers than non-transmitting insects fed on single or mixed inocula, confirming that successful MRCV transmission is positively associated with viral accumulation in the insect. Furthermore, MRCV viral titers were higher in transmitting planthoppers that acquired this virus from a single inoculum than in those that acquired the virus from a mixed inoculum, indicating that the presence of the rhabdovirus somehow impaired MRCV replication and/or acquisition. This is the first study about interactions between MRCV and a rhabdovirus closely related to Maize yellow striate virus in this insect vector (D. kuscheli), and contributes to a better understanding of planthopper-virus interactions and their epidemiological implications.

Plant infection by two different viruses induce contrasting changes of vectors fitness and behavior.

Insect-vectored plant viruses can induce changes in plant phenotypes, thus influencing plant-vector interactions in a way that may promote their dispersal according to their mode of transmission (i.e., circulative vs. noncirculative). This indirect vector manipulation requires host-virus-vector coevolution and would thus be effective solely in very specific plant-virus-vector species associations. Some studies suggest this manipulation may depend on multiple factors relative to various intrinsic characteristics of vectors such as transmission efficiency. In anintegrative study, we tested the effects of infection of the Brassicaceae Camelina sativa with the noncirculative Cauliflower mosaic virus (CaMV) or the circulative Turnip yellows virus (TuYV) on the host-plant colonization of two aphid species differing in their virus transmission efficiency: the polyphagous Myzus persicae, efficient vector of both viruses, and the Brassicaceae specialist Brevicoryne brassicae, poor vector of TuYV and efficient vector of CaMV. Results confirmed the important role of virus mode of transmission as plant-mediated effects of CaMV on the two aphid species induced negative alterations of feeding behavior (i.e., decreased phloem sap ingestion) and performance that were both conducive for virus fitness by promoting dispersion after a rapid acquisition. In addition, virus transmission efficiency may also play a role in vector manipulation by viruses as only the responses of the efficient vector to plant-mediated effects of TuYV, that is, enhanced feeding behavior and performances, were favorable to their acquisition and further dispersal. Altogether, this work demonstrated that vector transmission efficiency also has to be considered when studying the mechanisms underlying vector manipulation by viruses. Our results also reinforce the idea that vector manipulation requires coevolution between plant, virus and vector.

Establishing alighting preferences and species transmission differences for Pea seed-borne mosaic virus aphid vectors

Pea seed-borne mosaic virus (PSbMV) infection causes a serious disease of field pea (Pisum sativum) crops worldwide. The PSbMV transmission efficiencies of five aphid species previously found landing in south-west Australian pea crops in which PSbMV was spreading were studied. With plants of susceptible pea cv. Kaspa, the transmission efficiencies of Aphis craccivora, Myzus persicae, Acyrthosiphon kondoi and Rhopalosiphum padi were 27%, 26%, 6% and 3%, respectively. Lipaphis erysimi did not transmit PSbMV in these experiments. The transmission efficiencies found for M. persicae and A. craccivora resembled earlier findings, but PSbMV vector transmission efficiency data were unavailable for A. kondoi, R. padi and L. erysimi. With plants of partially PSbMV resistant pea cv. PBA Twilight, transmission efficiencies of M. persicae, A. craccivora and R. padi were 16%, 12% and 1%, respectively, reflecting putative partial resistance to aphid inoculation. To examine aphid alighting preferences over time, free-choice assays were conducted with two aphid species representing efficient (M. persicae) and inefficient (R. padi) vector species. For this, alatae were set free on multiple occasions (10–15 repetitions each) amongst PSbMV-infected and mock-inoculated pea or faba bean (Vicia faba) plants. Following release, non-viruliferous R. padi alatae exhibited a general preference for PSbMV-infected pea and faba bean plants after 30min–4h, but preferred mock-inoculated plants after 24h. In contrast, non-viruliferous M. persicae alatae alighted on mock-inoculated pea plants preferentially for up to 48h following their release. With faba bean, M. persicae preferred infected plants at the front of assay cages, but mock-inoculated ones their backs, apparently due to increased levels of natural light there. When preliminary analyses were performed to detect PSbMV-induced changes in the volatile organic compound profiles of pea and faba bean plants, higher numbers of volatiles representing a range of compound groups (such as aldehydes, ketones and esters) were found in the headspaces of PSbMV-infected than of mock-inoculated pea or faba bean plants. This indicates PSbMV induces physiological changes in these hosts which manifest as altered volatile emissions. These alterations could be responsible for the differences in alighting preferences. Information from this study enhances understanding of virus-vector relationships in the PSbMV-pea and faba bean pathosystems.

Aphid density and community composition differentially affect apterous aphid movement and plant virus transmission

1. Although many vector-borne pathogens are transmitted by an array of vector species, most studies do not account for the potential effects of species interactions. 2. By manipulating conspecific and heterospecific vector density in small experimental mesocosms, this study disentangled the impact of vector density and community composition on vector movement and plant virus transmission in the potato virus Y system. 3. The following predictions were tested: (i) increasing aphid density will increase aphid movement and virus transmission; (ii) adding low-efficiency vectors and thereby decreasing the average transmission efficiency of the vector assemblage will decrease virus transmission; and (iii) aphid movement and the average vector transmission efficiency will mediate the effect of aphid density and community composition on virus transmission. 4. It was found that initial density positively affected aphid movement, but had no effect on virus transmission, and that conspecific density was more important than heterospecific density. Conversely, community composition affected both aphid movement and virus transmission. These effects were driven by species identity, rather than species richness per se. 5. The results of this study emphasise the importance of accounting for vector behaviour, and analysing it within the context of the wider vector assemblage.

Grapevine Genotype Susceptibility toXylella fastidiosadoes not Predict Vector Transmission Success

For vector-borne diseases, interactions between vector, host, and pathogen can influence patterns of disease spread. In particular, previous studies suggest that host genotype may influence disease dynamics because of differences in susceptibility to the pathogen and, therefore, subsequent vector transmission efficiency from these plants. We tested this hypothesis by using the pathogenic bacterium Xylella fastidiosa, the etiological agent of Pierce's disease in grapevines, and its leafhopper vector Homalodisca vitripennis (Germar). Pathogen infection level and transmission efficiency among several widely cultivated red and white wine, table, and raisin grape cultivars, were compared with the expectation that vector transmission rate would differ among cultivars, because of underlying differences in susceptibility to infection. The 14 grapevine genotypes evaluated showed significant differences among cultivars in the populations of X. fastidiosa that developed in petioles. 'Flame seedless' hosted the highest bacterial populations, between 1.81 and 2.05 times higher than the least susceptible 'Merlot', 'Crimson seedless', 'Grenache Noir', and 'Rubired'. Although the transmission rate of X. fastidiosa by H. vitripennis varied substantially (zero to 33%), it was not significantly different among cultivars. These results suggest that either the relationship between vine infection level and transmission is weaker than previously reported, or innate differences in vector preference among cultivars confounded any effects of vine susceptibility to infection.

Vector Transmission Efficiency of Liberibacter by Bactericera cockerelli (Hemiptera: Triozidae) in Zebra Chip Potato Disease: Effects of Psyllid Life Stage and Inoculation Access Period

Successful transmission of plant pathogens by insects depends on the vector inoculation efficiency and how rapidly the insect can effectively transmit the pathogen to the host plant. The potato psyllid, Bactericera cockerelli (Sulc), has recently been found to transmit "Candidatus Liberibacter solanacearum," a bacterium associated with zebra chip (ZC), an emerging and economically important disease of potato in several parts of the world. Currently, little is known about the epidemiology of ZC and its vector's inoculation capabilities. Studies were conducted in the field and laboratory to 1) assess transmission efficiency of potato psyllid nymphs and adults; 2) determine whether psyllid inoculation access period affects ZC incidence, severity, and potato yield; and 3) determine how fast the psyllid can transmit liberibacter to potato, leading to ZC development. Results showed that adult potato psyllids were highly efficient vectors of liberibacter that causes ZC and that nymphs were less efficient than adults at transmitting this bacterium. It was also determined that inoculation access period had little influence on overall ZC disease incidence, severity, and resulting yield loss. Moreover, results showed that exposure of a plant to 20 adult potato psyllids for a period as short as 1 h resulted in ZC symptom development. Furthermore, it was shown that a single adult potato psyllid was capable of inoculating liberibacter to potato within a period as short as 6 h, thereby inducing development of ZC. This information will help in developing effective management strategies for this serious potato disease.

VAPA, an Innovative “Virus-Acquisition Phenotyping Assay” Opens New Horizons in Research into the Vector-Transmission of Plant Viruses

Host-to-host transmission—a key step in plant virus infection cycles—is ensured predominantly by vectors, especially aphids and related insects. A deeper understanding of the mechanisms of virus acquisition, which is critical to vector-transmission, might help to design future virus control strategies, because any newly discovered molecular or cellular process is a potential target for hampering viral spread within host populations. With this aim in mind, an aphid membrane-feeding assay was developed where aphids transmitted two non-circulative viruses [cauliflower mosaic virus (CaMV) and turnip mosaic virus] from infected protoplasts. In this assay, virus acquisition occurs exclusively from living cells. Most interestingly, we also show that CaMV is less efficiently transmitted by aphids in the presence of oryzalin—a microtubule-depolymerising drug. The example presented here demonstrates that our technically simple “virus-acquisition phenotyping assay” (VAPA) provides a first opportunity to implement correlative studies relating the physiological state of infected plant cells to vector-transmission efficiency.

Vector within‐host feeding preference mediates transmission of a heterogeneously distributed pathogen

1. Ecological theory predicts that vector preference for certain host species or discrimination between infected versus uninfected hosts impacts disease incidence. However, little information exists on the extent to which vector within‐host feeding preference mediates transmission. This may be particularly important for plant pathogens, such as sharpshooter transmission of the bacterium Xylella fastidiosa, which are distributed irregularly throughout hosts.2. We documented the within‐host distribution of two vector species that differ in transmission efficiency, the leafhoppers Draeculacephala minerva and Graphocephala atropunctata, and which are free to move throughout entirely caged alfalfa plants. The more efficient vector D. minerva fed preferentially at the base of the plant near the soil surface, whereas the less efficient G. atropunctata preferred overwhelming the top of the plant.3. Next we documented X. fastidiosa heterogeneity in mechanically inoculated plants. Infection rates were up to 50% higher and mean bacterial population densities were 100‐fold higher near the plant base than at the top or in the taproot.4. Finally, we estimated transmission efficiency of the two leafhoppers when they were confined at either the base or top of inoculated alfalfa plants. Both vectors were inefficient when confined at the top of infected plants and were 20–60% more efficient when confined at the plant base.5. These results show that vector transmission efficiency is determined by the interaction between leafhopper within‐plant feeding behaviour and pathogen within‐plant distribution. Fine‐scale vector and pathogen overlap is likely to be a requirement generally for efficient transmission of vector‐borne pathogens.

Temperature mediates vector transmission efficiency: inoculum supply and plant infection dynamics

AbstractClimate, particularly environmental temperature, frequently plays an important role in disease epidemiology. This study investigated the role of environmental temperature on transmission of the generalist plant pathogen Xylella fastidiosa by its leafhopper vectors. In this system temperature is known to influence both vector performance and feeding rate, yet the implications for pathogen transmission have not been documented. Experiments were conducted over a range of temperatures to document effects on transmission efficiency of the California native Graphocephala atropunctata (blue–green sharpshooter) and the invasive Homalodisca vitripennis (glassy‐winged sharpshooter). Inoculation efficiency of H. vitripennis was positively related to temperature. Graphocephala atropunctata mortality and transmission responded non‐linearly to temperature, with the highest rates of both at the highest temperature. The experiment also evaluated whether differences in inoculum supply contributed to plant infection level using quantitative PCR. Although total X. fastidiosa population within G. atropunctata was not related to plant infection, the number of infectious vectors was a strong predictor of plant infection level–suggesting that the number of inoculation events is important in the development of systemic infection of X. fastidiosa in grapevines. These results, along with existing evidence from the literature, point to wide‐ranging impacts of climate on the epidemiology of X. fastidiosa diseases.

Co-Infection by Two Criniviruses Alters Accumulation of Each Virus in a Host-Specific Manner and Influences Efficiency of Virus Transmission

Tomato chlorosis virus (ToCV), and Tomato infectious chlorosis virus (TICV), family Closteroviridae, genus Crinivirus, cause interveinal chlorosis, leaf brittleness, and limited necrotic flecking or bronzing on tomato leaves. Both viruses cause a decline in plant vigor and reduce fruit yield, and are emerging as serious production problems for field and greenhouse tomato growers in many parts of the world. The viruses have been found together in tomato, indicating that infection by one Crinivirus sp. does not prevent infection by a second. Transmission efficiency and virus persistence in the vector varies significantly among the four different whitefly vectors of ToCV; Bemisia tabaci biotypes A and B, Trialeurodes abutilonea, and T. vaporariorum. Only T. vaporariorum can transmit TICV. In order to elucidate the effects of co-infection on Crinivirus sp. accumulation and transmission efficiency, we established Physalis wrightii and Nicotiana benthamiana source plants, containing either TICV or ToCV alone or both viruses together. Vectors were allowed to feed separately on all virus sources, as well as virus-free plants, then were transferred to young plants of both host species. Plants were tested by quantitative reverse-transcription polymerase chain reaction, and results indicated host-specific differences in accumulation by TICV and ToCV and alteration of accumulation patterns during co-infection compared with single infection. In N. benthamiana, TICV titers increased during co-infection compared with levels in single infection, while ToCV titers decreased. However, in P. wrightii, titers of both TICV and ToCV decreased during mixed infection compared with single infection, although to different degrees. Vector transmission efficiency of both viruses corresponded with virus concentration in the host in both single and mixed infections. This illustrates that Crinivirus epidemiology is impacted not only by vector transmission specificity and incidence of hosts but also by interactions between viruses and efficiency of accumulation in host plants.

Direct real-time PCR detection of Plum pox virus in field surveys in Ontario

A new procedure is described for the detection of Plum pox virus (PPV) in samples from large-scale field tests. The new direct real-time polymerase chain reaction (drtPCR) procedure was based on crude supernatants collected from peach (Prunus persica) leaves macerated in a buffer that was specially developed for this purpose and named “direct pathogen extract buffer.” Specific TaqMan primers and probes were designed for PPV detection: two sets specific to PPV D and M strains, respectively, and one for universal detection of PPV strains D, M, C, EA, and W. These primer and probe sets can be used singly or for duplex differentiation of D and M strains. Using Prunus spp. tissue infected with 30 known fruit tree viruses, the universal primer and probe set correctly identified PPV-infected and PPV-free samples, with no nonspecific cross-reactions. Based on endpoint analysis of the drtPCR reaction, a threshold cyle value of 36 was suggested as the maximum threshold to declare a sample positive for PPV. A comparative analysis comparing drtPCR and ELISA using 12 200 field samples revealed that drtPCR was approximately 100- to 1000-fold more sensitive than ELISA and was able to detect PPV at an earlier stage of infection than ELISA. The drtPCR is a valuable tool for PPV diagnosis and may also be applicable to other studies, including pathogen population dynamics and vector transmission efficiency.

Substitution of conserved cysteine residues in wheat streak mosaic virus HC-Pro abolishes virus transmission by the wheat curl mite

Substitutions in the amino-proximal region of wheat streak mosaic virus (WSMV) HC-Pro were evaluated for effects on transmission by the wheat curl mite (Aceria tosichella Keifer). Alanine substitution at cysteine residues 16, 46 and 49 abolished vector transmission. Although alanine substitution at Cys(20) had no effect, substitution with arginine reduced vector transmission efficiency. Random substitutions at other positions (Lys(7) to Asn, Asn(19) to Ile, and Arg(45) to Lys) did not affect vector transmission. These results suggest that a zinc-finger-like motif (His(13)-X2-Cys(16)-X29-Cys(46)-X2-Cys(49)) in WSMV HC-Pro is essential for vector transmission.

Acquisition and Retention ofXylella fastidiosaby an Efficient Vector,Graphocephala atropunctata

The blue-green sharpshooter (BGSS), Graphocephala atropunctata (Signoret), is an efficient vector of Xylella fastidiosa Wells et al, the causal bacterium of Pierce's disease of grapevine. In experiments to assess changes over time in vector transmission efficiency and populations of bacteria in BGSS, 81 of 120 BGSS that were fed for 12 h on infected grapevine acquired the bacterium. Individual infective BGSS were exposed 4 times (1 h, and 7, 21, and 60 days after acquisition) to uninfected grapevines for 3-h inoculation feeding periods to determine the efficiency of transmission to grapevine. After each inoculation efficiency test, some BGSS were tested by culturing for X. fastidiosa to determine the bacterial population in the vector heads and bodies. Transmission efficiency increased from 56 to 92% between 1 h and 7 days post-acquisition, and remained high thereafter (91% at 21 days and 76% at 60 days). Bacterial population size in the heads of the inoculative vectors also increased between 1 h and 7 days, but did not consistently increase further. Xylella fastidiosa was not recovered by culture from about 40% of the inoculative insects. Populations below the detection limit (100 bacteria) in the BGSS heads were enough for efficient transmission