Acquisition Access Period Research Articles

First Report of Bean Leafroll Luteovirus Infecting Pea in Italy.

Approximately 5,000 ha of processing peas (Pisum sativum L) are cultivated annually in the Po River Valley of northern Italy. During the 1998 growing season, affected pea plants in this region were observed that exhibited mild chlorosis and mottling, leaf rolling, and stunting symptoms. High aphid populations and disease levels of nearly 100% were observed in susceptible varieties. Samples from affected fields were analyzed for the presence of bean leafroll virus (BLRV). Nonviruliferous pea aphids (Acyrthosiphon pisum Harris) received a 48-h acquisition access period on symptomatic leaves. Aphids were then transferred to Puget pea and Diana faba bean for a 72-h inoculation access period. Leaf samples were also macerated in 0.05 M potassium phosphate pH 7.4, and inoculated mechanically to pea, faba bean, chickpea (Cicer arietinum L.), Chenopodium quinoa Willd., and C. amaranticolor Coste & Reyn. Symptoms typical of those observed in the original field plants appeared 10 to 14 days after aphid transmissions in both pea and faba bean inoculated with pea aphids. No symptoms were observed in any of the hosts that were inoculated mechanically. Total nucleic acid extracts from the original pea samples, and from leaf tissue of pea and faba bean plants inoculated with aphids, served as templates in reverse-transcriptase polymerase chain reaction assays. Primers BLR-V157 and BLR-C546, which flanked a 400-bp fragment, were designed with available sequence data for the coat protein gene of BLRV (1). An amplification product of the expected size was generated from symptomatic plants but not from healthy controls. Sequence analysis of the cloned fragments revealed a 99% nucleic acid homology with the published sequence for BLRV and an isolate obtained from alfalfa in Washington State (R. Larsen, unpublished). This is the first report of BLRV in Italy. Reference: (1) B. Brill et al. Nucleic Acids Res. 18:5544, 1990.

The Identification of the Vector Species of Iris Yellow Spot Tospovirus Occurring on Onion in Brazil.

In Brazil, tospoviruses have been reported in several horticultural and ornamental plants. In the northeast region of Brazil, a tospovirus has emerged as a devastating virus on onion cultures. Based on serology and the sequence of nucleocapsid (N) protein gene, this pathogen was identified as a strain of iris yellow spot tospovirus (IYSV) (1). This virus was first identified on iris and leek in The Netherlands and later on onion in Israel. For an effective integrated management of tospoviruses in Brazil, identification of IYSV vector is essential. Three thrips species, Thrips tabaci, Frankliniella schultzei, and F. occidentalis, that are major vegetable and floral crop pests in the Federal District, Brazil, were tested for their ability to transmit the virus by leaf disk assay (2). All thrips, up to 8 h old, were given an acquisition access period of 48 h at 25°C on IYSV-infected Nicotiana benthamiana plants in Tashiro-cages. Thrips were then reared on uninfected Datura stramonium detached leaves until the adult stage. These adults were transferred individually to microcentrifuge tubes containing an N. benthamiana leaf disk and were incubated for 48 h for virus inoculation. The leaf disks were then incubated 4 more days to allow development of the virus infection, and the presence of virus was evaluated by Dot-enzyme-linked immunosorbent assay (Dot-ELISA) with polyclonal antibodies against N protein of IYSV. Adult thrips were also used for direct inoculation to N. benthamiana plants, three thrips per plant. By the leaf disk assay, 45.8% (22 out of 48) of T. tabaci transmitted the virus, but F. schultzei (n = 48) and F. occidentalis (n = 32) did not transmit it. All plants (4 out of 4) directly inoculated by T. tabaci showed symptoms and infection by Dot-ELISA, while no plants inoculated with F. schultzei (n = 5) and F. occidentalis (n = 3) were positive, either by symptom observation or by Dot-ELISA. Only T. tabaci showed potential for a high capacity to transmit the IYSV onion isolate. In the field, considering the host preference of thrips, T. tabaci was considered the most important vector species of IYSV on onion.

Tracing the Geminivirus-Whitefly Transmission Pathway by Polymerase Chain Reaction in Whitefly Extracts, Saliva, Hemolymph, and Honeydew

ABSTRACT A membrane feeding system and polymerase chain reaction (PCR) were used to track squash leaf curl virus (SLCV) DNA in whole whitefly body extracts and in saliva, honeydew, and hemolymph of its whitefly vector, Bemisia tabaci, and a whitefly nonvector, Trialeurodes vaporariorum. SLCV ingestion was monitored by PCR in whiteflies that were given acquisition access periods (AAPs) ranging from 0.5 to 96 h on virus-infected plants. SLCV detection by PCR in whole body extracts was considered reflective of virus ingestion. As whiteflies were given longer AAPs, the number of whiteflies that ingested SLCV increased. SLCV DNA was detected in honeydew of vector and nonvector whiteflies, indicating that virions, viral DNA, or both passed unimpeded through the digestive system. SLCV DNA was detected in saliva and hemolymph of B. tabaci, but not in these fractions from nonvector whiteflies, despite virus ingestion by both. Although vector and nonvector whiteflies both ingested SLCV, only in the vector, B. tabaci, did virus cross the gut barrier, enter the hemolymph, or pass into the salivary system. These results suggest that digestive epithelia of nonvector whiteflies did not permit SLCV passage from the gut to hemocoel, whereas virus effectively crossed the analogous gut barrier in vector whiteflies.

Biology of the Transmission of Peach Mosaic Virus by Eriophyes insidiosus (Acari: Eriophyidae).

Experiments were undertaken to elucidate the characteristics of the transmission of peach mosaic virus (PMV) by Eriophyes insidiosus. Transmission efficiency by single E. insidiosus was as high as 17%. The minimum inoculation access period was between 3 and 6 h. E. insidiosus acquired the virus after a minimum acquisition access period of 3 days. No latent period was demonstrated. While most plant viruses which are transmitted by eriophyid mites are transmitted in a persistent mode, our data are more consistent with a semipersistent model.

Immunofluorescent Localization of Tobacco Ringspot Nepovirus in the Vector Nematode Xiphinema americanum

ABSTRACT An indirect immunofluorescent technique was developed to localize tobacco ringspot nepovirus (TRSV) in the vector nematode Xiphinema americanum sensu stricto. A population of this nematode that efficiently transmitted TRSV was given an acquisition access period of 10 days on TRSV-infected cucumber. Treatment of fragments of viruliferous nematodes with a polyclonal antiserum against TRSV followed by fluorescein isothiocyanate-conjugated goat anti-rabbit immunoglobulin G resulted in virus-specific bright fluorescence only in the lumen of the stylet extension and esophagus. Virus-specific fluorescent signals were observed in the virus-retention region of 44% of the nematode fragments examined. The percentage of nematodes labeled with virus-specific fluorescence increased as the acquisition access period increased from 0 to 22 days; the increase paralleled the increase in the transmission efficiency of the nematode population. Visualization of the entire virus-retention region of individual nematodes within a population of vector or nonvector nematodes provides a rapid and simple means of monitoring specific attachment of plant viruses.

Behavioral Response and Virus Vector Ability of Myzus persicae (Homoptera: Aphididae) Probing on Pepper Plants Treated with Aphicides

Behavioral response of the green peach aphid, Myzus persicae (Sulzer), to 3 different insecticides (cypermethrin, pirimicarb, and imidacloprid) was assessed on pepper plants, Capsicum annuum L., using electrical monitoring of aphid probing behavior. This technique enabled us to assess stylet penetration pathways as well as cell membrane punctures (potential drops). The effect of these insecticide treatments on potato virus Y (PVY) transmission by M. persicae also was tested. Pirimicarb and imidacloprid did not significantly affect probing behavior or PVY transmission efficiency when aphids were allowed a 10-min acquisition access period on infected insecticide-treated plants. Conversely, cypermethrin affected aphid behavior as well as PVY transmission efficiency. Aphids probing on cypermethrin-treated plants produced fewer (1.7 versus 3.3) and shorter (41 versus 152 s) penetrations than those probing on untreated plants. They also produced a lower number of potential drops (1.3 versus 4.2). Moreover, cypermethrin caused a paralysis of aphids within 2.5 min of exposure to the treated plants. Aphids were not able to subsequently inoculate healthy plants. However, when the acquisition access period was shorter than 2.5 min, aphids were able to efficiently transmit PVY despite the cypermethrin treatment of the virus source plant. The possible existence of an additional deterrent effect caused by cypermethrin on M. persicae and its implications in PVY epidemiology is discussed.

Long-term association of tomato yellow leaf curl virus with its whitefly vector Bemisia tabaci: effect on the insect transmission capacity, longevity and fecundity.

The association between tomato yellow leaf curl geminivirus (TYLCV, Israeli isolate) and its insect vector, the whitefly Bemisia tabaci, was investigated. Insects that emerged during a 24 h period were caged with TYLCV-infected plants for a 48 h acquisition access period, then with egg-plants--a TYLCV non-host--for the rest of their lives. While TYLCV DNA was associated with the whiteflies during their entire adult life, the amount of capsid protein rapidly decreased and was not detectable in the insect after approximately 12 days of age. The ability of the infected whiteflies to transmit TYLCV to tomato test plants steadily decreased with age but did not disappear completely. Transmission by viruliferous insects decreased from 100% to 10-20% during their adult lifetime, compared with a decrease from 100% to 50% for non-viruliferous insects. The association of TYLCV with adult B. tabaci led to a reduction of 17-23% in their life expectancy compared with insects that had not acquired the virus, and to a 40-50% decrease in the mean number of eggs laid. These results suggest that TYLCV has some features reminiscent of an insect pathogen.

Vectoring ability of aphid clones of Rhopalosiphum padi (L.) and Sitobion avenae (Fabr.) and their capacity to retain barley yellow dwarf virus

SummaryVectoring ability of four aphid clones, Rp‐M and Rp‐R26 of Rhopalosiphum padi and Sa‐R1 and Sa‐V of Sitobion avenae, to transmit barley yellow dwarf (PAV, MAV and RPV) luteoviruses (BYDV) was compared in controlled conditions. Significant differences between highly efficient vectors (HEV), Rp‐M and Sa‐Rl, and poorly efficient vectors (PEV), Rp‐R26 and Sa‐V, were found in transmission of their specific viruses with acquisition and inoculation access periods (AAP, IAP) of 5 days. BYD‐RPV was occasionally transmitted by both clones of S. avenae. None of 150 tested apterous adults of the Rp‐R26 transmitted BYD‐MAV, while 10% of transmission was observed from those of the Rp‐M in a parallel test. An improved ELISA and immuno‐PCR were adapted to test for viruses in aphids. The results obtained by the improved ELISA indicated there was a good correlation between virus detection in single aphids of HEV clones after a 5 day AAP and virus transmission by them. In contrast, the percentages of virus‐carrying aphids of PEV clones were generally higher than those of their transmission rates. BYD‐MAV and BYD‐RPV were also detected by the improved ELISA in single aphids of their PEV clones, with the exception of BYD‐RPV in those of Sa‐V. However, after a 2‐day IAP, the improved ELISA in most cases failed to detect these viruses in single aphids of PEV clones.Detection by immuno‐PCR demonstrated that all three viruses could be acquired and retained by the aphids of both HEV and PEV clones. But, as visualised from electrophoretic bands, after the 2‐day IAP the amplified products from aphid extracts of PEV clones were reduced. The detection in a batch of nine aphids by the improved ELISA revealed that virus content in PEV clones decreased more rapidly than that in HEV clones during transmission. Thus, the difference in transmission efficiency of the aphid clones within species was not caused by an inability to acquire virus, but was determined by variation in vectoring ability between them. This was due to differences in ability to prevent the passage of virions from haemocoel to salivary duct and/or different capacities for the retention of BYDV.

Reverse-Transcription Polymerase Chain Reaction Detection of Citrus Tristeza Virus in Aphids.

A rapid and simple reverse-transcription polymerase chain reaction (RT-PCR) method was developed for the detection of citrus tristeza virus (CTV) in three aphid species. Seven CTV isolates from a worldwide isolate collection were used for aphid acquisition feeding by three aphid species. These included the most efficient CTV vector, the brown citrus aphid, Toxoptera citricida; the melon aphid, Aphis gossypii; and the green peach aphid, Myzus persicae, a non-vector for CTV. A short procedure for nucleic acid extraction from single or groups of aphids was developed. Nucleic acid extracts from 1, 3, 5, and 10 aphids with acquisition-access periods of 24 and 48 h were reverse transcribed and amplified using primers for the coat protein gene of the Florida B3 (T-36) isolate of CTV. PCR-amplified fragments of approximately 670 bp were obtained from all the isolates tested and the amplified product from the aphids fed on citrus infected with isolate B3 was confirmed as the CTV coat protein gene by digesting with various restriction enzymes. This technique will be useful in investigations of CTV-vector-plant interactions and CTV epidemiology.

Occurrence of Squash Mosaic Comovirus Infecting Summer Squash (Cucurbita pepo) in South Dakota and Transmission by Diabroticabarberi.

Summer squash (Cucurbita pepo L.) exhibiting strong mosaic symptoms and leaf deformations were found in Brookings County during the summers of 1994 and 1995. Four isolates were mechanically transmitted to C. pepo cv. Early Prolific Straightneck. Ouchterlony double diffusion tests (conducted both in Arkansas and South Dakota) and protein-A enzyme-linked immunosorbent assays (ELISAs) with squash mosaic virus (SqMV) antisera provided by R. C. Gergerich were positive for SqMV. Pumpkin mosaic virus, a strain of SqMV, was previously reported infecting pumpkin in South Dakota (1). The pumpkin strain was differentiated from other strains of SqMV by its ability to infect lentil (Lens esculenta Moench), garden pea (Pisum sativum L.), and fenugreek (Trigonella foenum-graecum L.). The South Dakota isolates in this study did not infect these hosts, although the Arkansas isolate was found to infect Pisum sativum cvs. Little Marvel and Dwarf Gray Sugar Pod when tested serologically. The presence of adult Diabrotica undecimpunctata howardii Barber and D. barberi (Smith and Lawrence) was noted on infected plants in the area. Transmission tests were conducted with D. barberi provided by the USDA-ARS Northern Grain Insect Research Laboratory, Brookings, SD. The beetles had been previously reared on artificial diet media and germinated corn seed. Individual beetles were given a 48-h acquisition access period on infected or healthy detached squash leaves in petri dishes followed by a 48-h inoculation access period on Early Prolific Straightneck squash plants (1 beetle per plant). Beetles were removed at the end of this period, and the plants were grown under greenhouse conditions for 3 weeks. Plants were assayed for SqMV by protein-A ELISA. Transmission percentages of SqMV were as follows: SqMV (Arkansas isolate), 16.7%; and four South Dakota isolates, 12.5, 11.4, 14.6, and 5.1%. Beetles fed on healthy plants did not transmit virus. The rate of transmission found in this study indicates that D. barbari serves as an efficient vector for the transmission of SqMV in field plantings. Reference: (1) W. N. Stoner. Proc. S. D. Acad. Sci. 56:178, 1977.

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

Summary.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.

Biotypes of the castor bean whitefly Trialeurodes ricini (Misra) (Hom., Aleyrodidae) in Egypt: biochemical characterization and efficiency of geminivirus transmission

AbstractThe newly introduced castor bean whitefly (CBW) Trialeurodes ricini (Misra) has become an important pest which is widely spread all over Egypt. Biochemical studies were conducted to distinguish the different T. ricini biotypes and to determine the biotype economic threshold based on its ability to transmit plant viruses. Forty‐five samples of T. ricini adults were collected from castor bean (Ricinus communis) at different geographical locations representing the Egyptian Delta and Nile valley governorates. Esterase banding patterns were investigated to evaluate the extent of variation among the tested populations of the castor bean whitefly. Electrophoresis of homogenates of individual T. ricini females showed that there are five different biotypes (C1–C5). The predominant biotype (C1 type) was found in about 80% of the tested locations. In transmission studies, the polymerase chain reaction (PCR) was used to detect geminiviruses in nucleic acid extracted from the different castor bean whitefly biotypes. PCR revealed that the five biotypes of T. ricini are viruliferous. To evaluate the efficiency of the predominant T. ricini biotype for transmitting the tomato yellow leaf curl virus (TYLCV), acquisition and inoculation access periods were determined. PCR indicated that 50% of whitefly adults collected from castor bean, acquired the virus after 10 h exposure to TYLCV infected tomato. Experimentally, the rate of transmission increased as the inoculation access period was lengthened and reached a maximum after 4 h. The obtained symptomatological results were also confirmed by PCR.

Propagation of tomato spotted wilt virus in Frankliniella occidentalis does neither result in pathological effects nor in transovarial passage of the virus

AbstractThe effect of tomato spotted wilt virus (TSWV) on Frankliniella occidentalis Pergande (Thysanoptera; Thripidae) following a 6‐hour acquisition access period on infected plants was investigated. No statistically significant differences were observed among viruliferous, non‐viruliferous and control thrips with respect to developmental time, reproduction rate and survival. Thrips larvae, exposed or non‐exposed to TSWV, developed from egg to adult in 13.1 and 13.2 days, respectively. Exposed females produced an average of 28.3 larvae whereas control thrips produced 22.3 larvae and longevity was 13.4 and 12.5 days, respectively. None of these values were significantly different. Population reproductive statistics, net reproductive rate (R0), mean generation time (T) and intrinsic rate of increase (rm) were calculated from the life fertility tables. R0 and rm were higher for viruliferous thrips as compared to non‐viruliferous and non‐exposed thrips. Virus transmission studies revealed that viruliferous thrips were able to transmit virus until death and that TSWV was not transovarially transmitted.

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

Summary.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 (AAPopt) and in an IAP of 42.7 h (IAPopt). These results show that TSWV can be acquired and transmitted efficiently by F. occidentalis in short feeding periods.

VARIABILITY AMONG APHID CLONES OF RHOPALOSIPHUM PADI L. AND SITOBION AVENAE FABR. (HOMOPTERA: APHIDIDAE) IN TRANSMISSION OF THREE PAV ISOLATES OF BARLEY YELLOW DWARF VIRUSES

AbstractVariability in vectoring efficiency among six clones of Rhopalosiphum padi L. and five clones of Sitobion avenae Fabr. in transmission of three French PAV isolates (PAV-RG, PAV-2t, and PAV-13t) of barley yellow dwarf viruses (BYDVs) on seedlings of barley (Hordeum vulgare L.) cv. Plaisant was determined. All the clones could transmit these three isolates, but their vectoring efficiency was significantly different: 91% and 56% transmission efficiencies were obtained from the most efficient clones (Rp-M and Sa-R1) but only 21% and 9% from the least efficient clones (Rp-R26 and Sa-V) with 5-day acquisition and inoculation access periods (AAP, IAP). A significant difference in overall transmission between apterous adults and winged aphids of the tested clones was also found. In most cases, apterous adults were more efficient than were winged aphids. The overall transmission efficiency of apterous adults was 1.5 and 1.7 times that of winged ones for R. padi and S. avenae, respectively. The transmissibility of PAV-RG and PAV-2t isolates was similar within each of the tested aphid clones, but that of the PAV-13t isolate differed, particularly for the poorly efficient vector clones of S. avenae. Temperature could significantly influence the vectoring efficiency of the tested clones of R. padi, but the influence was greater on Rp-R26. High temperature (25 °C) facilitated more transmission than did low temperature (14 °C), and high temperature for AAP played a more important role than it did for IAP.

Host range, vector relations and serological relationships of cotton leaf curl virus from southern India

SummaryIn 1989 to 1991, leaf curl disease was observed in cotton (Gossypium bar‐badense cv. Local) grown in kitchen gardens in five districts in Karnataka State, India, and in 1994 it was recorded in G. hirsutum cv. Sharada in two districts. Symptoms consist of leaf curling, vein thickening, leaf enations, and stunting and distortion of plants. The disease is caused by cotton leaf curl virus (CLCuV‐K), which was transmitted by the whitefly Bemisia tabaci to 24 plant species in six families. Hosts include bean (Phaseolus vulgaris), pepper, tobacco, tomato and several weeds, almost all of which developed leaf curl, with or without vein thickening. CLCuV‐K was transmitted from cotton to cotton by adult B. tabaci after an acquisition access period of 1 h, could be inoculated in 5 min, had a minimum latent period of 8 h and was retained by viruliferous insects for up to 9 days. Female B. tabaci transmitted more frequently than males. CLCuV‐K is a whitefly‐transmitted geminivirus. It reacted with two out of 17 monoclonal antibodies (MAbs) raised to African cassava mosaic virus and five out of 10 MAbs raised to Indian cassava mosaic virus. CLCuV‐K isolates from different locations in Karnataka had similar epitope profiles. As judged by these profiles, CLCuV‐K is closely related to Indian tomato leaf curl virus from Karnataka, is distinguishable from several other whitefly‐transmitted geminiviruses found in India and is still more distantly related to those, including cotton leaf crumple virus from the USA, found in other continents.CLCuV‐K infected all cultivars tested of G. barbadense and one of six cultivars of G. hirsutum but none of G. arboreum or G. herbaceum.

The effects of azadirachtin on the acquisition and inoculation of potato leafroll virus by Myzus persicae

The roots of Nicotiana clevelandii (Gray) seedlings infected with potato leafroll virus (PLRV) were immersed in 0, 50, 100 or 150 ppm azadirachtin solutions for 84 h. Adult apterous peach-potato aphids, Myzus persicae (Sulzer), were placed onto the seedlings for an acquisition access period (AAP) of 72 h starting 12 h after root immersion or 2 days after the seedlings had been re-potted in compost. The aphids were then placed onto indicator plants to assess PLRV acquisition. Although azadirachtin did not affect aphid survival, PLRV infection of the indicator plants was recorded less frequently when inoculated by aphids which had spent the AAP on azadirachtin-treated plants. The ability to produce viable nymphs on indicator plants was severely reduced when the aphids had spent the AAP on seedlings with their roots immersed in azadirachtin. This effect was also detected after the aphids had spent the AAP on azadirachtin-treated seedlings re-potted after treatment. The concentrations required to prevent PLRV acquisition from infected seedlngs were not visibly phytotoxic, but a higher concentration of azadirachtin (500 ppm) caused short-term (wilting) and long-term (deformation) phytotoxicity. Immersing the roots of uninfected N. clevelandii seedlings in 500 ppm azadirachtin solution did not prevent them becoming infected with PLRV after exposure to viruliferous aphids in ‘choice’ and ‘nochoice’ experiments.

Transmission efficiency of three isolates of maize stripe tenuivirus in relation to virus titre in the planthopper vector

Isolates of maize stripe tenuivirus (MSt V) from Florida (US), Costa Rica (CR), and Nigeria, Africa (AF), were transmitted to maize plants by the planthopper Peregrinus maidis (from Hawaii) with respective frequencies of 0,18, and 60% after a 1‐day acquisition access period on diseased plants, and with frequencies of 18, 71 and 93%, respectively, after a 7‐day access period. These isolates were transmitted transovarially to progeny planthoppers with respective frequencies of 21, 32, and 47%. The latent period in the vector, following oral acquisition of MSt V, was significantly longer in the US isolate than in the AF‐or CR isolates. ELISA tests of MSt V‐inoculative planthoppers indicated a significantly lower titre of MSt V‐US compared with MSt V‐CR or MSt V‐AF. These results suggest that, compared with the US isolate, the AF and CR isolates of MSt V multiply faster and reach higher levels in, and are transmitted more efficiently by, P. maidis from Hawaii.

Relationships of the Sardinian isolate of tomato yellow leaf curl geminivirus with its whitefly vectorBemisia tabaci Gen

Relationships of an Italian isolate of tomato yellow leaf curl geminivirus from Sardinia (TYLCV-S) with its whitefly vectorBemisia tabaci were studied by means of experimental transmissions from tomato to tomato plants. TYLCV-S was confirmed to be transmitted in a persistent, circulative manner. The minimum latent period in the vector was between 17 and 20 h from the beginning of the acquisition access period (AAP). The maximum retention of infectivity was 8 days from the end of the AAP. Both acquisition and inoculation feeding times influenced the detected proportion of infective insects, with patterns well described by an exponential model. Acquisition was more efficient than inoculation. Males were significantly less efficient vectors than females. Nymphs were as efficient as adults in acquiring the virus. The length of AAP influenced both the retention of infectivity, and the pattern of transmission in serial transfer transmission tests with individual females. No significant difference in transmission efficiency was detected between two colonies ofB. tabaci, one inducing typical silverleaf symptoms on squash, the other inducing only mild symptoms with more than 50 whiteflies per plant. The phenomenon of periodic acquisition was not unequivocally proved for TYLCV-S.

Transmission of Tomato Yellow Leaf Curl Geminivirns by Bemisia tabaci (Homoptera: Aleyrodidae)

The sweetpotato whitefly, Bemisia tabaci (Gennadius), is an economically important pest worldwide. A new biotype of sweetpotato whitefly, biotype B, causes extensive damage by direct feeding and by the transmission of plant viruses, such as geminiviruses. Because of the importance of whitefly-transmitted geminiviruses, studies were conducted to quantify the characteristics of tomato yellow leaf curl geminivirus (TYLCV) transmission by the vector B. tabaci. Transmission of an isolate of TYLCV from Egypt was achieved with one adult B. tabaci per plant, but the efficiency of transmission increased fourfold when the number of adults were increased to five per plant. B. tabaci transmitted TYLCV after a minimum acquisition-access period of 15 min, and rate of transmission increased as the acquisition-access period was lengthened and reached a maximum after 24 h. A minimum inoculation-access period of 15 min was observed with the rate of transmission increasing as the inoculation-access period was lengthened, reaching a maximum after a 12-h inoculation-access period. When B. tabaci were transferred serially after acquisition, adults were unable to transmit TILCV until 24 h after the initiation of the acquisition-access period regardless of the length of acquisition provided. The 24-h transmission threshold from initial vector access to a TYLCV-infected plant until transmission of the virus includes both the acquisition-access period and the latent period during which the virus circulates within the vector. Retention of TYLCV from the nymphal to adult stages of B. tabaci was supportive of a circulative mode of transmission. TILCV titer in B. tabaci after an acquisition-access period of 12 h continuously increased on a non-TYLCV host starting at 12 h, reaching a peak at 108 h and remaining stable from 132 to 180 h after the acquisition-access period. These data indicate multiplication of the virus in the vector as the most likely explanation for the continuous increase of TYLCV titer.