Life history of the bird cherry-oat aphid, Rhopalosiphum padi (Homoptera: Aphididae), on transgenic and untransformed wheat challenged with barley yellow dwarf virus

The life history of Rhopalosiphum padi (L.) was monitored on transgenic and untransformed (soft white winter wheat plants that were infected with Barley yellow dwarf virus (BLDV), noninfected, or challenged with virus-free aphids under laboratory conditions. Two transgenic soft white winter wheat genotypes (103.1J and 126.02) derived from the parental variety Lambert and expressing the barley yellow dwarf virus coat protein gene, and two untransformed varieties, virus-susceptible Lambert and virus-tolerant Caldwell, were tested. B. padi nymphal development was significantly longer on the transgenic genotypes infected with BYDV, compared with noninfected transgenic plants. In contrast, nymphal development on Lambert was significantly shorter on BYDV-infected than on noninfected plants. Nymphal development on noninfected Lambert was significantly longer than on noninfected transgenics. No significant difference in nymphal development period was detected between virus-infected and noninfected Caldwell. Aphid total fecundity, length of reproductive period, and intrinsic rate of increase were significantly reduced on BYDV-infected transgenic plants compared with BYDV-infected Lambert. In contrast, reproductive period, total adult fecundity, and intrinsic rate of increase on noninfected Lambert were significantly reduced compared with noninfected transgenics. Transgenic plants infected with BYDV were inferior hosts for R. padi compared with infected Lambert. However, noninfected transgenics were superior hosts for aphids than noninfected Lambert. Moderate resistance to BYDV, as indicated by a significantly lower virus titer, was detected in the transgenic genotypes compared with the untransformed ones. Results show for the first time that transgenic virus resistance in wheat can indirectly influence R. padi life history.     

Life history of the bird cherry-oat aphid, Rhopalosiphum padi, on transgenic and non-transformed wheat challenged with Wheat streak mosaic virus

The life history of the bird cherry-oat aphid, Rhopalosiphum padi (L.) (Hemiptera: Aphididae), was studied via laboratory assays on Wheat streak mosaic virus (WSMV)-infected and non-infected transgenic and non-transformed wheat [ Triticum aestivum L. (Poaceae)]. Although R. padi is not a WSMV vector, it is known to colonize WSMV-infected wheat plants. Two transgenic soft white winter wheat genotypes, 366-D03 and 366-D8, that express the WSMV coat protein gene, and the WSMV-susceptible non-transformed cultivar Daws were tested. All genotypes showed disease symptoms when infected with WSMV. Whereas plant height was significantly reduced on virus-infected compared to non-infected plants of all genotypes, virus-infected transgenic plants exhibited lower virus titer and lower disease rating scores than Daws. No significant effects of WSMV infection or genotypes were observed on the length of R. padi nymphal development period, nor on their pre-, and post-reproductive periods. Rhopalosiphum padi reproductive period was significantly longer on Daws infected with WSMV than on non-infected plants of this cultivar. In contrast, there were no significant differences in length of R. padi reproductive period between virus-infected and non-infected transgenic plants within a genotype. Rhopalosiphum padi daily fecundity was significantly lower and adult longevity significantly longer on virus-infected than on non-infected plants of all genotypes. Total aphid fecundity and intrinsic rate of increase were not significantly different among treatments. The percentage of winged aphids that developed was greater on WSMV-infected compared to non-infected plants within a genotype. Results indicate that both virus infection status of plants and wheat genotype influence the life history of R. padi.     

Transmission of tomato spotted wilt virus by Frankliniella occidentalism median acquisition and inoculation access period

To quantify the transmission of tomato spotted wilt virus (TSWV) by Frankliniella occidentalis, the median acquisition access period (AAP₅₀) and median inoculation access period (IAP₅₀) were determined. These parameters were established using transmission rates obtained after AAPs and in IAPs which both ranged from 5 to 2560 min. An AAP₅₀ of 106 min was found when larvae acquired virus from TSWV-infected Impatiens plants. IAP₅₀s of 58 or 137 min, respectively, were calculated when petunia or Datura stramonium leaf disks were used to test the inoculation efficiency of viruliferous thrips. The virus could successfully be acquired or inoculated in periods of 5 min. Transmission reached an optimum after an AAP of 21.3 h (AAP_opt) and in an IAP of 42.7 h (IAP_opt). These results show that TSWV can be acquired and transmitted efficiently by F. occidentalis in short feeding periods.     

In vitro antiviral activity of sulfated Auricularia auricula polysaccharides

Total Auricularia auricula polysaccharide (AAP(t)) was prepared by extracting and removing the proteins. Column chromatography was used to further graded it into AAP(1) and AAP(2). Three AAPs were modified by chlorosulfonic acid-pyridine method to obtain three sulfated AAPs (sAAPs), sAAP(t), sAAP(1) and sAAP(2), respectively. Three sAAPs and Newcastle disease virus (NDV) were added into cultivation system of chicken embryo fibroblast (CEF) in three manners, pre-, post- and simultaneous-adding polysaccharide with NDV respectively, taking three non-modified AAPs as control. Their anti-viral activities were compared by MTT method. The results showed that sAAPs and AAPs at a certain concentration could significantly inhibit the cellular infectivity of NDV in three manners. The effects of sAAPs were better than that of AAPs. It indicated that sulfated modification could enhance the antiviral activity of AAP. sAAP(1) and sAAP(t) possessed stronger activity and would be as the component of a new-type antiviral drug.     

Seasonal abundance of aphids (Homoptera: Aphididae) in wheat and their role as barley yellow dwarf virus vectors in the South Carolina coastal plain

Aphid (Homoptera: Aphididae) seasonal flight activity and abundance in wheat, Triticum aestivum L., and the significance of aphid species as vectors of barley yellow dwarf virus were studied over a nine-year period in the South Carolina coastal plain. Four aphid species colonized wheat in a consistent seasonal pattern. Greenbug, Schizaphis graminum (Rondani), and rice root aphid, Rhopalosiphum rufiabdominalis (Sasaki), colonized seedlingwheat immediately after crop emergence, with apterous colonies usually peaking in December or January and then declining for the remainder of the season. These two aphid species are unlikely to cause economic loss on wheat in South Carolina, thus crop managers should not have to sample for the subterranean R. rufiabdominalis colonies. Bird cherry-oat aphid, Rhopalosiphum padi (L.), was the second most abundant species and the most economically important. Rhopalosiphum padi colonies usually remained below 10/row-meter until peaking in February or March. Barley yellow dwarf incidence and wheat yield loss were significantly correlated with R. padi peak abundance and aphid-day accumulation on the crop. Based on transmission assays, R. padi was primarily responsible for vectoring the predominant virus serotype (PAV) we found in wheat. Pest management efforts should focus on sampling for and suppressing this aphid species. December planting reduced aphid-day accumulation and barley yellow dwarf incidence, but delayed planting is not a practical management option. English grain aphid, Sitobion avenae (F.), was the last species to colonize wheat each season, and the most abundant. Sitobion avenae was responsible for late-season virus transmission and caused direct yield loss by feeding on heads and flag leaves during an outbreak year.     

A highly conserved mechanism of regulated ribosome stalling mediated by fungal arginine attenuator peptides that appears independent of the charging status of arginyl-tRNAs

The Arg attenuator peptide (AAP) is an evolutionarily conserved peptide involved in Arg-specific negative translational control. It is encoded as an upstream open reading frame (uORF) in fungal mRNAs specifying the small subunit of Arg-specific carbamoyl phosphate synthetase. We examined the functions of the Saccharomyces cerevisiae CPA1 and Neurospora crassa arg-2 AAPs using translation extracts from S. cerevisiae, N. crassa, and wheat germ. Synthetic RNA containing AAP and firefly luciferase (LUC) sequences were used to program translation; analyses of LUC activity indicated that the AAPs conferred Arg-specific negative regulation in each system. The AAPs functioned either as uORFs or fused in-frame at the N terminus of LUC. Mutant AAPs lacking function in vivo did not function in vitro. Therefore, trans-acting factors conferring AAP-mediated regulation are in both fungal and plant systems. Analyses of ribosome stalling in the fungal extracts by primer extension inhibition (toeprint) assays showed that these AAPs acted similarly to stall ribosomes in the region immediately distal to the AAP coding region in response to Arg. The regulatory effect increased as the Arg concentration increased; all of the arginyl-tRNAs examined appeared maximally charged at low Arg concentrations. Therefore, AAP-mediated Arg-specific regulation appeared independent of the charging status of arginyl-tRNA.     

A study of the mode of transmission of maize streak virus by Cicadulina mbila using an enzyme-linked immunosorbent assay

Two batches of Cicadulina mbila were given two distinct acquisition access periods (AAP) (3 h and 50 h) on maize plants infected with maize streak virus (MSV). Infectivity assays on susceptible maize were carried out 1, 3, 10, 17, 26 and 35 days after the AAP. Transmission efficiency was significantly higher for C. mbila subjected to the 50-h AAP. At the same time as the infectivity assays, the amount of MSV in each leafhopper was determined by an indirect double antibody sandwich (IDAS) ELISA. There were more ELISA-positive insects after the 50-h AAP than after the 3-h AAP. In the group given a 3-h AAP, only 7% of the insects tested between day 1 and 35 were found to be positive by ELISA. In contrast, after the 50-h AAP, the majority of C. mbila were positive, yet a decrease in ELISA-positive insects was noticed from day 17 onwards. Using a calibration curve obtained with purified virus, as little as 0.15 ng of MSV per insect could be measured by the IDAS-ELISA. A mean value of 0.36 ng of MSV per C. mbila was found 3 days after the 50–h acquisition, whereas 14 days later there was only 0.20 ng of virus per insect. For comparison, when leafhoppers were kept on infected maize, they displayed substantial accumulation of MSV up to an average of 3.83 ng of MSV per insect after 35 days of continuous acquisition. The amount of virus per insect detected in females was usually greater than the amount detected in males. Our results suggest that MSV does not multiply in C. mbila and contribute to the understanding of the persistence of transmission efficiency in the absence of virus multiplication.     

Ingestion, transmission, and persistence of Chino del tomate virus (CdTV), a New World begomovirus, by Old and New World biotypes of the whitefly vector Bemisia tabaci

Two whitefly biotypes of Bemisia tabaci, from either the Eastern or Western Hemisphere, respectively, were compared with respect to their competency to ingest and their efficiency to transmit the New World begomovirus, Chino del tomate virus (CdTV). The AZ A biotype of B.tabaci originates from the arid southwestern USA and northwestern Mexico, while the B biotype has an origin in the Middle East or Northern Africa. The ability of these two vector biotypes to ingest and subsequently to transmit CdTV were evaluated for an acquisition‐access period (AAP) that ranged from 0 to 72 h, followed by a 48 h inoculation‐access period (IAP). Individual adult whiteflies were monitored for CdTV ingestion using polymerase chain reaction (PCR) to detect the viral coat protein gene (AV1 ORF), and transmission efficiency (frequency) was determined by allowing potentially viruliferous whiteflies access to tomato seedlings following each experimental AAP. PCR results for individual adult whiteflies indicated that CdTV was ingested from infected tomato plants by both biotypes 93% of the time. Transmission frequencies by both vector biotypes increased with longer AAPs. However, the AZ A biotype transmitted CdTV 50% of the time, compared to only 27% for the B biotype. Evidence that virus was ingested with equal competency by the A and B biotypes confirmed that both vectors were capable of ingesting CdTV from tomato at the same frequency, even when the AAP was 0.5 h. Consequently, either the acquisition and/or transmission stages of the pathway, rather than ingestion competency, were responsible for differences in vector‐mediated transmissibility. Detection frequency of CdTV, after 48 h AAP, by PCR in single females of AZ B biotype was significantly higher than males.     

Low and high affinity amino acid H+-cotransporters for cellular import of neutral and charged amino acids

Amides and acidic amino acids represent the major long distance transport forms of organic nitrogen. Six amino acid permeases (AAPs) from Arabidopsis mediating transport of a wide spectrum of amino acids were isolated. AAPs are distantly related to plasma membrane amino acid transport systems N and A and to vesicular transporters such as VGAT from mammals. A detailed comparison of the properties by electrophysiology after heterologous expression in Xenopus oocytes shows that, although capable of recognizing and transporting a wide spectrum of amino acids, individual AAPs differ with respect to specificity. Apparent substrate affinities are influenced by structure and net charge and vary by three orders of magnitude. AAPs mediate cotransport of neutral amino acids with one proton. Uncharged forms of acidic and basic amino acids are cotransported with one proton. Since all AAPs are differentially expressed, different tissues may be supplied with a different spectrum of amino acids. AAP3 and AAP5 are the only transporters mediating efficient transport of the basic amino acids. In vivo competition shows that the capability to transport basic amino acids in planta might be overruled by excess amides and acidic amino acids in the apoplasm. With the exception of AAP6, AAPs do not recognize aspartate; only AAP6 has an affinity for aspartate in the physiologically relevant range. This property is due to an overall higher affinity of AAP6 for neutral and acidic amino acids. Thus AAP6 may serve a different role either in cooperating with the lower affinity systems to acquire amino acids in the low concentration range, as a system responsible for aspartate transport or as an uptake system from the xylem. In agreement, a yeast mutant deficient in acidic amino acid uptake at low aspartate concentrations was complemented only by AAP6. Taken together, the AAPs transport neutral, acidic and cationic amino acids, including the major transport forms, i.e. glutamine, asparagine and glutamate. Increasing proton concentrations strongly activate transport of amino acids. Thus the actual apoplasmic concentration of amino acids and the pH will determine what is transported in vivo, i.e. major amino acids such as glutamine, asparagine, and glutamate will be mobilized preferentially.     

Comparison of transmission abilities of four Cicadulina species vectors of maize streak virus from Nigeria

Four Cicadulina species [Cicadulina arachidis China, Cicadulina dabrowskii Webb, Cicadulina mbila (Naude), and Cicadulina storeyi China (Homoptera: Cicadellidae)] found during field surveys in 1997–1999 across five ecological zones in Nigeria were reared in screenhouses and females were used in a study to compare their abilities to transmit Maize streak virus (Geminiviridae: genus Mastrevirus) from maize to maize using the susceptible variety Pool 16. The results showed that for both acquisition access feeding periods (AAP) and inoculation access feeding periods (IAP), transmission efficiencies decreased in the following order: C. storeyi > C. mbila > C. arachidis > C. dabrowskii. The transmission efficiencies of these vectors increased with longer feeding periods, as the means of susceptible test plants that showed Mastrevirus symptoms for both acquisition and IAPs were higher for 24 and 48 h than for 1 and 2 h for all four species studied. The epidemiological implications of these differences in transmission abilities are discussed.     

Screening for Barley Yellow Dwarf Luteovirus Resistance in Barley on the Basis of Virus Movement

The movement of barley yellow dwarf luteovirus (BYDV) was evaluated in susceptible and resistant barley and bread wheat genotypes. After leaf inoculation, the virus infected the root system and the growing point of susceptible earlier than resistant, barley genotypes. No difference in virus movement occurred in resistant and susceptible wheat genotypes. It was possible to reliably differentiate susceptible from resistant genotypes when root extracts of 41 barley genotypes were tested by DAS-ELISA 3 or 4 days after inoculation at the oneleaf stage. When barley plants inoculated at the two- or three-leaf stage were assayed by tissue-blot ELISA on nitrocellulose membrane, virus was detected in the phloem vessels of the growing points of the susceptible, but not of the resistant genotype, 4–6 days after inoculation. Our results thus suggest that screening for BYDV resistance in barley could be done quickly and cheaply especially when assays are made by the tissue-blot test.     

Identification of highly transformable wheat genotypes for mass production of fertile transgenic plants.

The efficiency of wheat biolistic transformation systems strongly depends on the bombardment parameters, the condition of the donor plant, and the plant genotype chosen for the transformation process. This paper analyzes the transformation efficiency of the 129 wheat sister lines generically called 'Bobwhite', originally obtained from the cross 'Aurora'//'Kalyan'/'Bluebird 3'/'Woodpecker'. A number of factors influencing the transformation were examined, such as the ability to produce embryogenic callus, regeneration in selection medium, and overall transformation performance. Of the 129 genotypes evaluated, eight demonstrated transformation efficiencies above 60% (60 independent transgenic events per 100 immature embryos bombarded). Among the eight genotypes identified, we studied agronomic characteristics such as earliness to identify the most adaptable line(s) for different lab conditions. 'Bobwhite' SH 98 26 was identified as a super-transformable wheat line.     

Taxonomic differences shape the responses of freshwater aerobic anoxygenic phototrophic bacterial communities to light and predation

Aerobic anoxygenic phototrophic (AAP) bacteria are a phylogenetically diverse and ubiquitous group of prokaryotes that use organic matter but can harvest light using bacteriochlorophyll a. Although the factors regulating AAP ecology have long been investigated through field surveys, the few available experimental studies have considered AAPs as a group, thus disregarding the potential differential responses between taxonomically distinct AAP assemblages. Here, we used sequencing of the pufM gene to describe the diversity of AAPs in 10 environmentally distinct temperate lakes, and to investigate the taxonomic responses of AAP communities in these lakes when subjected to similar experimental manipulations of light and predator removal. The studied communities were clearly dominated by Limnohabitans AAP but presented a clear taxonomic segregation between lakes presumably driven by local conditions, which was maintained after experimental manipulations. Predation reduction (but not light exposure) caused significant compositional shifts across most assemblages, but the magnitude of these changes could not be clearly related to changes in bulk AAP abundances or taxonomic richness of AAP assemblages during experiments. Only a few operational taxonomic units, which differed taxonomically between lakes, were found to respond positively during experimental treatments. Our results highlight that different freshwater AAP communities respond differently to similar control mechanisms, highlighting that in‐depth knowledge on AAP diversity is essential to understand the ecology and potential role of these photoheterotrophs.     

Evaluation of Gossypium species for resistance to cotton leaf curl Burewala virus

Cotton leaf curl disease (CLCuD), caused by cotton leaf curl Burewala virus (CLCuBV), has emerged as a major threat to cotton production in Pakistan. Resistance to CLCuBV was evaluated in cultivated and wild cotton genotypes representing six Gossypium species by visual symptom scoring and virus assessment using PCR tests. Considerable variation in responses was observed when using whitefly and graft transmission to inoculate Gossypium genotypes with CLCuBV in field and greenhouse experiments. Under field evaluation, all cultivated genotypes of Gossypium hirsutum and three genotypes of G. barbadense were susceptible. Eleven genotypes that represented six wild and cultivated Gossypium species were considered to be highly resistant as they were free from infection. Similar results were obtained when these genotypes were tested using whitefly transmission. To verify these findings, 132 cultivated and wild genotypes were tested by graft inoculation. All G. hirsutum genotypes (116 cultivated, 1 wild, 1 transgenic Coker-312 and 1 non-transgenic Coker-312), three G. barbadense genotypes and one G. thurberi genotype were highly susceptible and exhibited symptoms 9–12 days after grafting. Four genotypes of G. arboreum and one genotype of G. anomalum did not express symptoms but had a detectable level of virus. One genotype of G. herbaceum and three wild genotypes of G. hirsutum showed mild symptoms (severity indexes of 1–2) and exhibited delayed disease development. These genotypes were classified as moderately resistant to resistant. Resistant genotypes that were identified in this study will be useful sources for exploitation of breeding programmes aimed at developing CLCuBV-resistant varieties and increasing genetic diversity.     

Acquisition, Retention and Transmission of Faba Bean Necrotic Yellows Virus by Two of its Aphid Vectors, Aphis craccivora (Koch) and Acyrthosiphon pisum (Harris)

Faba bean necrotic yellows virus (FBNYV) belongs to a new group of plant viruses that have unusually small isometric virions and a multipartite ssDNA genome. It is the causal agent of some virus diseases affecting several food and fodder legumes in west Asia and north Africa. FBNYV is persistently transmitted by various aphid species of which Aphis craccivora appears to be the most significant natural vector. In attempts to obtain a better understanding of factors involved in FBNYV spread under field conditions, the interactions of the virus with A. craccivora and Acyrthosiphon pisum were studied. The two species were efficient vectors and very similar in their minimum acquisition (AAP) and minimum inoculation access feeding periods which ranged from 15 to 30min and 5–15min, respectively. Following an AAP of 72 h and daily serial transfers of individual aphids to single plants, many individuals retained and transmitted the virus throughout their life span (up to 32 days) but at erratic efficiencies. In this persistence experiment A. pisum was a more efficient vector than A. craccivora. For both aphid species no decrease in transmission efficiency was observed, suggesting that nymphs acquired large amounts of FBNYV virions which were not depleted in their hemocoel during the experiment. Based on log-pro-bit analysis, median latency period (LPso) values of 108.8h and 105.0h were calculated for FBNYV in A. craccivora and A. pisum , respectively. FBNYV was not lost during moults and was not passed on to the par-thenogenetic offspring by viruliferous adults. Aphids which acquired FBNYV as adults were strikingly poor vectors as compared to nymphs.     

Preliminary evidence of recovery from Tomato spotted wilt virus infection in Frankliniella occidentalis individuals

In this study we analysed the ability of individual thrips to transmit Tomato spotted wilt virus (TSWV) in a population of Frankliniella occidentalis over their lifespan as adults (about 10 days). In three experiments a total of 636 thrips were individually tested for their transmission capacity through leaf disc assays using four inoculation access periods (IAPs). Almost half of the transmitting thrips maintained the capacity to infect leaf discs in each of the four IAPs, confirming the persistent propagative nature of the transmission modality. Nevertheless, a relevant number of thrips (9.25% of transmitter thrips) was able to transmit in the early phases of their adult life (for the first two IAPs), but did not transmit the virus for the remainder of their lifetime. We compared the virus titer of these individuals at the end of the fourth IAP with that of individuals that maintained transmission ability in the four IAPs and showed a statistically significant difference. This difference could be evidence for recovery from TSWV infection in individual thrips.     

The impact of transgenic wheat expressing GNA (snowdrop lectin) on the aphids Sitobion avenae, Schizaphis graminum, and Rhopalosiphum padi

This study investigated the impact of transgenic wheat expressing Galanthus nivalis agglutinin (GNA), commonly known as snowdrop lectin, on three wheat aphids: Sitobion avenae (F.), Schizaphis graminum (Rondani), and Rhopalosiphum padi (L.). We compared the feeding behavior and the life-table parameters of aphids reared on GNA transgenic wheat (test group) and those aphids reared on untransformed wheat (control group). The results showed that the feeding behaviors of S. avenae and S. graminum on GNA transgenic wheat were affected. Compared with the control group, they had shorter initial probing period, longer total nonprobing period, shorter initial and total phloem sap ingestion phase (waveform E2), shorter duration of sustained ingestion (E (pd) > 10 min), and lower percentage of phloem phase of the total observation time. Moreover, S. graminum made more probes and had a longer total duration of extracellular stylet pathway (waveform C). The fecundity and intrinsic rate of natural increase (r(m)) of S. avenae and S. graminum on the transgenic wheat were lowered in the first and second generations, however, the survival and lifespan were not affected. The effects of the GNA expressing wheat on S. graminum and S. avenae were not significant in the third generation, suggesting rapid adaptation by the two aphid species. Despite the impact we found on S. avenae and S. graminum, transgenic GNA expressing wheat did not have any effects on R. padi.     

Diuraphis noxia and Rhopalosiphum padi (Hemiptera: Aphididae) interactions and their injury on resistant and susceptible cereal seedlings

Interspecific interactions between the symptomatic (chlorosis-eliciting) Russian wheat aphid, Diuraphis noxia (Mordvilko), and the asymptomatic (nonchlorosis-eliciting) bird cherry-oat aphid, Rhopalosiphum padi (L.), on four cereal genotypes were examined by simultaneous infestations. Four cereals (i.e., Diuraphis noxia-susceptible 'Arapahoe' wheat and 'Morex' barley, and D. noxia-resistant 'Halt' wheat and 'Border' oat) and four infestations (i.e., control, D. noxia, R. padi, and D. noxia/R. padi) were used in the research. Whereas D. noxia biomass confirmed D. noxia resistance among the cereals, R. padi biomass indicated that the D. noxia-resistant cereals did not confer R. padi resistance. D. noxia biomass was significantly lower in D. noxia/R. padi infestation than that in D. noxia infestation on all cereals, except Border oat, which indicated an antagonistic effect of R. padi on D. noxia. All aphid infestations caused a significant plant biomass reduction in comparison with the control. In comparison with D. noxia infestation, D. noxia/R. padi caused a significant plant biomass reduction on all cereals, except Morex barley. Although D. noxia biomass in D. noxia/R. padi infestation was significantly less than that in D. noxia infestation, leaf chlorophyll reduction was the same between D. noxia/R. padi and D. noxia infestations, which suggested that the asymptomatic R. padi enhanced the D. noxia-elicited leaf chlorophyll loss. The regression between chlorophyll content and aphid biomass indicated that the asymptomatic R. padi in the D. noxia/R. padi infestation enhanced chlorophyll loss, but interspecific aphid interaction on plant biomass varied among the cereals.     

Rate of Loss of Infectivity of Maize Dwarf Mosaic Virus by Schizaphis graminum Rondani After Different Acquisition Access Periods

The rate of loss of infectivity for greenbugs, Schizaphis graminum Rondani, carrying maize dwarf mosaic virus (MDMV) was similar for adult apterae and alates regardless of the acquisition access period (AAP) given. The rate was rapid during the first 2 h post-acquisition and then decreased. Weighted least squares linear regression of log % transmission were plotted against retention-time intervals to obtain regression slopes for each of the AAP groups tested (1, 3, 17, and 30 min). There were no significant differences among the regression slopes using F-test comparisons. In serial transmission trials, transmission frequencies among aphids allowed 1-min or 30-min timed acquisition probes followed by 30-see or 1-min timed inoculation probes were not significantly different when statistical tests that quantify categorical data were applied. This suggests little difference in the amount of virus obtained by these groups of aphids. Transmission and retention data are discussed in relation to the two principal theories of the mechanism of non-persistent virus transmission by aphids. Transmission of MDMV by the greenbug by a two mechanism process (stylet-borne and ingestion-egestion) is not confirmed.     

Acquisition and transmission of selected CTV isolates by Aphis gossypii

Citrus tristeza virus (CTV) is a severe threat to the citrus industry. Disease symptoms and severity may vary depending on the CTV isolates. These are responsible for the decline of trees grafted on sour orange rootstock, or stem pitting on some citrus commercial cultivars regardless of rootstock. In the Calabria region (Italy), CTV was first reported on cultivars imported from other countries. However, recent observations suggested that natural spread of CTV was occurring and a study was needed to determine the epidemiological status and aphid transmission of CTV in Calabria. The role played by local A. gossypii in the spread of CTV was analyzed in the laboratory using various viral acquisition, inoculation periods with three different CTV isolates. Single aphid vectors acquired CTV after a minimum of 30 min acquisition access period (AAP) and were able to transmit the virus after a 60 min inoculation access period (IAP) to healthy plants. A minimum of four aphid vectors were needed to reach 50% transmission probability. The results suggested that the three tested strains are transmitted by A. gossypii in a semi-persistent mode. The results demonstrated that local A. gossypii population can acquire and transmit efficiently the tested virus isolates with serious implications on the virus spread.