Studies on the transmission of velvet tobacco mottle virus by the mirid, Cyrtopeltis nicotianae

The minimum acquisition period of velvet tobacco mottle virus (VTMoV) by its mirid vector Cyrtopeltis nicotianae was about 1 min, with an increase in the rate of transmission (i.e. proportion of test plants infected) for acquisition periods up to 1000 min. Pre-acquisition starvation periods up to 18 h did not affect the rate of transmission. After an acquisition access period of 2 days, the minimum inoculation period was between 1 and 2 h and the rate of transmission increased with increasing inoculation time; when the acquisition access period was 1 h, or if vectors were fasted for 16 h after the 2 day acquisition, the rate of transmission was significantly lower. When mirids were transferred sequentially each day to a healthy plant after a 24 h acquisition feed, they transmitted intermittently for up to 10 days. Up to 50% of mirids transmitted after a moult and this was not due to the mirids probing the shed cuticles or exudates of infective insects. Mirids transmitted after a moult, following acquisition periods of 10, 100 or 1000 min. C. nicotianae transmitted solanum nodiflorum mottle virus (SNMV), sowbane mosaic virus (SoMV) and southern bean mosaic virus (SBMV), but not subterranean clover mottle virus (SCMoV), lucerne transient streak virus (LTSV), tobacco ringspot virus (TRSV), galinsoga mosaic virus (GMV), nor nicotiana velutina mosaic virus (NVMV). Tomato bushy stunt virus (TBSV) was transmitted to 1/58 test plants.     

Transmission characteristics of Spiroplasma citri and its effect on leafhopper vectors from the Circulifer tenellus complex

Several leafhopper variants of the Circulifer tenellus complex were collected in “citrus stubborn” affected areas in Israel. Two of these variants transmitted the Spiroplasma citri to Matthiola incana after being injected with the disease agent. The variant from Atriplex halimus was designated Circulifer tenellus-A (CTA) and the variant from Portulaca oleracea was designated Circulifer tenellus-? (CTP). Transmission characteristics were determined for both leafhoppers. A high rate of transmission (43.3%) was obtained by single CTA leafhoppers that were injected with the Amiad S. citri isolate from the Upper Galilee, compared with 7% transmission obtained with the CTP leafhoppers. The Gilgal S. citri isolate from the Jordan Valley, was not transmitted by either. Injection was more effective than acquisition access feeding to render the leafhopper infective for both CTA and CTP. The minimum acquisition access period needed for the CTA variant to transmit the Amiad isolate was 1 h. Longer AAPs did not necessarily result in a higher rate of transmission. The minimum incubation period was 6 days and the maximum was 32 days. The LP₅₀ calculated from the logarithmic curve y = 45.74Ln(x)–53.68 was 9.64 days. The minimum inoculation access period (IAP) was lh. The same transmission parameters for the CTP variant could not be determined, as no transmission was obtained even when groups of five-six insects were placed on a single plant.     

Beet curly top virus from phloem exudate of Amsinckia douglasiana

Amsinckia douglasiana when infected with beet curly top virus (BCTV) produces more exudate, which is highly infective, than any other known host. Attempts were made to purify BCTV from the phloem exudate of infected Amsinckia douglasiana by differential centrifugation and sucrose density gradient fractionation. A_260/280 ratio of virus preparations was 1.58; S values were 74 and 147. Infectivity was distributed among several fractions during density gradient purification. Electron microscopy revealed the presence of pentagonal and hexagonal particles of 14–23 nm, some of which were paired. The virus was insensitive to DNAse, RNAse, and trypsin when these enzymes were tested individually but infectivity was substantially decreased when the virus was subjected to trypsin treatment followed by DNAse, suggesting that DNA is associated with the beet curly top virus.     

Maize rough dwarf virus (Reoviridae) in its planthopper vector Laodelphax striatellus in relation to vector infectivity

The infectivity of females of the planthopper vector Laodelphax striatellus given access to maize rough dwarf virus (MRDV) infected plants was assessed for up to 55 days from the end of the access period. A 3-day inoculation access period was used, and this avoided intermittent transmission. Maximum infectivity was reached c. 30 days after acquisition access and the proportion of transmitter insects then remained constant. There was no difference in the efficiency of female L. striatellus in acquiring MRDV as third instar nymphs or as adults when compared in transmission tests 24, 30, 35 and 40 days after access to the virus. ELISA tests for MRDV subviral particles (SVPs) discriminated between individual viruliferous and non-viruliferous insects from the 30th day after access. Of the viruliferous (ELISA positive) insects about 30% did not transmit MRDV and the proportion remained similar from 30 to 55 days after access. None of the non-transmitter insects tested in serial transfer transmission tests was positive in ELISA. The concentration of SVPs detected by ELISA in the transmitter hoppers continued to increase exponentially, even after maximum infectivity was reached.     

Chickpea chlorotic dwarf virus,a new leafhopper-transmitted geminivirus of chickpea in India1

A disease of chickpea in India, characterised by chlorosis, severe stunting and phloem browning, was shown to be caused by a geminivirus. This virus was transmitted by the leafhopper Orosius orientalis from chickpea to chickpea and several other plant species. A method for purification of this virus was devised and a polyclonal antiserum produced. The majority of the purified particles were geminate. The size of the coat protein was shown to be 32 kD and the nucleic acid was shown to be circular ssDNA of 2900 nucleotides. By immunosorbent electron microscopy this virus was shown to be unrelated to the leafhopper-transmitted geminiviruses known to infect dicotyledons such as beet curly top, bean summer death and tobacco yellow dwarf viruses. On the basis of particle morphology, leafhopper transmission, host range and serology this virus was considered to be a new, hitherto undescribed, geminivirus and was named chickpea chlorotic dwarf virus.     

Acquisition and Transmission of two Begomoviruses by the B and a non-B Biotype of Bemisia tabaci from Zhejiang, China

Acquisition and transmission was studied of Tomato yellow leaf curl China virus (TYLCCNV) and Tobacco curly shoot virus (TbCSV) by the B and a non‐B biotype (China‐ZHJ‐1) of Bemisia tabaci from Zhejiang, China. The frequency of TYLCCNV and TbCSV detection by PCR in whitefly adults increased with increasing length of feeding on virus‐infected plants. The virus DNA was detected by PCR in 40% of the B biotype adults tested after a period of 30 min access to infected plants and in all adults after a 12‐h period of access. All ZHJ‐1 adults acquired TYLCCNV and TbCSV after a 48‐h period of access to the virus‐infected plants. Viruliferous B and ZHJ‐1 adults retained TYLCCNV DNA for their entire life when placed on healthy cotton plants. Viruliferous ZHJ‐1 adults retained TbCSV DNA for their entire life when placed on healthy cotton plants but the B biotype adults did not. Transmission of TYLCCNV was achieved with one B or ZHJ‐1 adult per plant, and the probability of transmission reached 100% when the number of adults was increased to 10 per plant. The efficiency for TYLCCNV transmission to healthy plants by adults of both B and ZHJ‐1 was much higher than that for TbCSV.     

Relations of carrot red leaf and carrot mottle viruses with their aphid vector, Cavariella aegopodii

Studies with Scottish isolates of carrot red leaf (CRLV) and carrot mottle (CMotV) viruses confirmed the dependency of CMotV on CRLV for transmission by the aphid Cavariella aegopodii. CMotV was transmitted by aphids only when the two viruses were present in the same source plant, and its transmission was not assisted by anthriscus yellows virus, which acts as a helper for parsnip yellow fleck virus. Some test plants became infected with CRLV alone, and a few with CMotV alone. In winter, aphid transmission of CRLV and CMotV was greatly increased when the source plants received supplementary lighting whereas the CMotV infectivity of sap was not increased. C. aegopodii acquired CRLV and CMotV after minimum acquisition access times of 30 min and inoculated them after minimum inoculation access times of 2 min. There was a minimum latent period of 7–18 h. The viruses were retained by the aphid after moulting and are therefore circulative in the vector, but were not transmitted to progeny insects. Aphids allowed 24 h to acquire the viruses continued to transmit them for at least 12 days, but some aphids allowed 6 h or less for virus acquisition ceased to transmit after 3 or 4 days. CRLV is considered a tentative member of the luteovirus group.     

Inoculation and acquisition of maize bushy stunt mycoplasma by its leafhopper vector Dalbulus maidis

Maize bushy stunt mycoplasma (MBSM), a mycoplasma-like organism, is transmitted in a persistent manner by the corn leafhopper, Dalbulus maidis, to maize (Zea mays). The influence of the duration of acquisition access and inoculation access periods on the transmission of MBSM by D. maidis was investigated. The proportion of plants infected by D. maidis increased significantly from 0 to 0.51 as the inoculation access time to a plant increased from 10 min to 72 h (X²= 101.5, P < 0.001). Likewise, the proportion of insects acquiring MBSM from infected plants increased from 0 to 0.19 as the acquisition access time to the source plant increased from 10 min to 72 h (X²= 53.2, P < 0.001). The data were fitted to a loglinear regression model. No significant association was found between the sex of the insects and vector ability.     

Acquisition and transmission of corn stunt spiroplasma by its leafhopper vector Dalbulus maidis

As the acquisition access period of Dalbulus maidis on infected maize increased from 15 min to 7 days, the incubation period of corn stunt spiroplasma (CSS) in the insect decreased from 27 days to 8 days and the final proportion of transmitting insects increased from 5% to 100%. After 7 days access the median incubation period (IPsO) was 14.3 days (IP₅₀ females = 12.9 days: IP₅₀ males =16.8 days), while the proportion of transmitting insects increased from 4. 3% (9 days after the start of acquisition access) to a maximum of 93% (after 22 days), before decreasing. Females started transmitting significantly earlier and a greater proportion transmitted each day than males, until day 22 when both sexes transmitted equally. Of the insects which transmitted CSS, 29% did so continuously until death; 66% failed to transmit during the last 1–3 days, and 5% transmitted intermittently towards the end of their life. During daily transfer, females were more likely to infect plants consecutively (up to 25) than males, and females infected the higher proportion of test plants. As the transmission access period was increased from 1 h to 72 h, the proportion of transmitting insects increased from 22.5% to 97.3% and the incubation period in maize decreased.     

Aphid retention of maize dwarf mosaic virus (potyvirus): epidemiological implications

In total, 17 589 aphids were assayed for rate of loss of inoculativity and maximum retention times of maize dwarf mosaic (MDMV). The Standard-treatment, involved acquisition access to MDMV-infected tissue followed by confinement of active aphids in Petri dishes. In addition various aphid immobilisation treatments were used to prevent probing on solid surfaces after acquisition access to simulate conditions experienced by wind-borne aphids when aloft. Immobilisation treatments, using nitrogen or argon gases at 25°C, or cold treatments at 6°C after acquisition access greatly increased the efficiency of MDMV transmission by greenbugs, Schizaphis graminum, in an experimental design where insects were individually assayed for transmission over a 7 h period. Further tests in which groups of greenbugs were assayed for MDMV transmission revealed that MDMV strains may be retained for over 21 h, regardless of post-acquisition access treatment. Experiments with other aphid vectors of MDMV (Dactynotus ambrosiae, Macrosiphum euphorbiae, Rhopalosiphum maidis and Myzus persicae) also demonstrated MDMV retention times exceeding 18 h. These results show that the rate at which aphids lose MDMV inoculativity is lower when solid surface probing behaviour is denied, and that MDMV retention times are longer than those previously published. The findings are discussed in relation to the epidemiology of nonpersistent viruses and their dispersal over great distances.     

Curly Top of Cultivated Plants and Weeds and Report of a Unique Curtovirus from Iran

The incidence of curly top disease on cultivated plants and weeds was investigated in Kerman Province (southeastern Iran) from October 2003 to November 2004. A total of 1186 samples were collected in fields of sugar beet and other crops as well as within commercial plastic houses. Curtovirus infection of four field crops, three vegetables and 11 weeds was verified by indirect enzyme‐linked immunosorbent assay (ELISA) using a polyclonal antibody. An undescribed curtovirus, tentatively designated Iranian beet curly top virus (IBCTV), was isolated from three symptomatic beet samples collected randomly in widely separated regions of south‐eastern, southern and central Iran and used for molecular studies. A 672 bp segment of the coat protein (CP) gene of each isolate was amplified by PCR and sequenced. The results showed that the three isolates shared 98.5–98.7% nucleotide homology with each other but only 72.1–76.5% with other members of the genus Curtovirus. IBCTV was also detected by PCR using specific primers in other samples of sugar beet, tomato, spinach, turnip and several weed species collected in different parts of Iran. These results indicated that IBCTV is the dominant curtovirus in Iran.     

Trade-off between foliage and tuber resistance to Phthorimaea operculella in wild potatoes

Beet leafhopper, Circulifer tenellus (Baker) (Homoptera: Cicadellidae), is the only known North American vector of beet curly top virus (Geminiviridae), which causes major economic losses in a number of crops including sugar beet, tomato, beans, and peppers. Beet curly top virus is a phloem-limited, persistently transmitted, circulative geminivirus. The strain/species of curly top virus used in this study is the CFH strain, also referred to as beet severe curly top virus (BSCTV). The direct current (DC) electrical penetration graph technique was used to determine the specific stylet penetration behavior associated with inoculation of BSCTV. Viruliferous leafhoppers were allowed to feed on healthy 3–4-week-old sugar beet plants until specific electrical penetration graph waveforms were produced, at which point feeding was artificially terminated. A series of comparisons between leafhoppers that produced different combinations of waveforms clearly implicated waveform D1 as the only waveform correlated with inoculation of BSCTV. All successful inoculations contained waveform D1, and 56 out of 64 leafhoppers that produced waveform D1 successfully inoculated test plants. Eighty-five leafhoppers did not produce waveform D1 and none of these inoculated BSCTV. While the occurrence of waveform D1 appears to be necessary for BSCTV inoculation, there was no correlation between duration of waveform D1 and inoculation success rate. The correlation of waveform D1 and BSCTV inoculation found in this study implies that waveform D1 is associated with phloem salivation.     

Relations of the semi-persistent viruses, parsnip yellow fleck and anthriscus yellows, with their vector, Cav arietta aegopodii

Studies were made of the relations of parsnip yellow fleck virus (PYFV) and its helper virus, anthriscus yellows (AYV), with their aphid vector, Cavariella aegopodii. Apterous insects were more efficient vectors than alates; apterous nymphs were as efficient as apterous adults. C. aegopodii never transmitted PYFV in the absence of AYV, but aphids carrying both viruses infected some test plants with one or other virus alone. C. aegopodii that fed first on a source of AYV and then on a source of PYFV transmitted both viruses to test plants, but aphids that fed on the sources in the reverse order transmitted only AYV. Test plants receiving some aphids from a source of AYV, and others from a source of PYFV, became infected only with AYV. C. aegopodii acquired AYV or the AYV/PYFV complex from plants in a minimum acquisition access time (AAT) of 10–15 mm and inoculated the viruses to test plants in a minimum inoculation access time (IAT) of 2 min. Increasing either AAT or IAT, or both, to 1 h or longer increased the frequency of transmission of each virus. Starving the insects before the acquisition feed on AYV or AYV/PFYV sources did not affect transmission. Aphids already carrying AYV acquired PYFV from plants in a minimum AAT of only 2 min; they acquired and inoculated PYFV in a minimum total time of 12 min. The data suggest that AYV is confined to deeply lying tissues whereas PYFV is distributed throughout the leaf. C. aegopodii transmitted both PYFV and AYV in a semi-persistent manner: the aphids retained both viruses for up to 4 days but lost them on moulting. Neither virus was passed to progeny of viruliferous adults. Earlier results suggesting that AYV is a persistent virus may have been caused by contamination of the AYV culture with carrot red leaf virus.     

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.     

The relation between cowpea golden mosaic and its vector, Bemisia tabaci

Adult whiteflies (Bemisia tabaci) acquired the agent of cowpea golden mosaic (CGM-A) within 7 min and transmitted during 2 min inoculation access. Nymphs also acquired CGM-A and retained it through the pupal stage. Efficiency of transmission reached 90% when the acquisition period was increased to 36 h. The minimum latent period of CGM-A in B. tabaci was 8 h, and the median latent period (LP50) about 12 h. Transmission was generally intermittent and declined throughout the period of 21 days which was the maximum time insects remained infective. Female whiteflies were more efficient vectors than males. B. tabaci acquired CGM-A from plants inoculated only 60 h previously. The epidemiological implications of these findings are discussed.     

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.     

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.     

The effect of pre-acquisition starvation on aphid transmission of potyviruses during observed and electrically recorded stylet penetrations

The effect of pre-acquisition starvation on stylet penetration behaviour by the aphidMyzus persicae (Sulz.) and the consequent non-persistent transmission of the potyviruses beet mosaic virus (BMV) and potato virus Y (PVY) were investigated. Visual observations indicated that starved aphids initiated penetrations earlier and penetrated for shorter periods than non-starved insects. Tethering with a fine gold wire did not affect these observations with either starved or non-starved aphids, but starvation caused increased PVY and BMV acquisition efficiency, regardless of tethering. Tethered aphids were then made part of an electrical circuit and their stylet activities investigated in detail. Electrically recorded aphids also acquired and inoculated both potyviruses more efficiently when starved, and these acquisitions and inoculations were associated with stylet punctures of plant cell membranes. However, starvation did not affect the occurrence of electrically recorded membrane puncture, suggesting that non-behavioural factors may contribute to the enhancement of virus transmission by pre-acquisition starvation.     

Effect of sucrose supplementation by stem injection on the development of soybean plants

Over the past half decade several stem injection methods have been developed for cereal plants. These methods allow researchers to administer solutions to cereal plants to study their effects on plant physiology. However, little work has been done to extend this technique to non-cereals An experiment was conducted to test an injection technique that could be suitable for soybean plants (Glycine max [L.] Merr,), and to study the effect of long-term injection of sucrose on the growth of soybean plants. An injection set-up comprising a supporting stand and a fluid injection system was established. Pressure was applied to the plunger of a 5 ml syringe using ceramic bricks to force test solutions into the plants. Solutions of 0, 150, and 300 g sucrose l^-1 were injected into soybean plants for 8 weeks starting at the seedling VC stage. Distilled water had the greatest uptake rate, followed by the 150, and then the 300 g sucrose l^-1 solutions. The overall average uptake during the injection period was 77.3 ml. Average sucrose uptake values were 11.8 and 13.5 g per plant for the 150 and 300 g sucrose l^-1 treatments. This represented approximately 65% of the total dry weight of the plants. Sucrose injection increased leaf area and pod number relative to the control plants. Nodule numbers were lower for sucrose injected treatments, but their dry weights were higher than the control. Above-soil dry matter was higher for the plants injected with 300 g sucrose l^-1 than those injected with water. The injection system tested was able to administer concentrated solutions into soybean plants for most of their period of growth and development. The sucrose supplementation had positive effects on soybean growth but suppressed photosynthesis.Key words: Soybean, sucrose, injection.      

Cell membrane punctures during epidermal penetrations by aphids: consequences for the transmission of two potyviruses

Electrical penetration graphs (EPGs) were used to investigate aphid stylet activities during brief penetrations and the importance of these events for the nonpersistent transmission of two potyviruses, beet mosaic virus (BMV) and potato virus Y (PVY). Stylet puncture of a cell membrane, recorded as a potential drop, was correlated with the acquisition and inoculation of PVY and acquisition of BMV by Myzus persicae.