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.     

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.     

Transmission of pangola stunt virus by Sogatella kolophon

A fijivirus causing minor enations, stunting, leaf notching, seed head deformity and excess tillering of Digitaria spp. was transmitted from naturally infected Digitaria ciliaris to D. ciliaris, D. decumbens and Urochloa panicoides by the planthopper Sogatella kolophon; 40–70% of insects transmitted after an incubation period of 15–21 days, and continued to transmit for up to 30 more days until death. Symptoms developed in test plants 30–50 days after inoculation. Sogatella longifurcifera failed to transmit the virus under similar conditions. Virus particles were present in roots, stems and leaves of infected plants, and particles were found in regular arrays and random aggregates in fat body cells of transmitting insects. Viroplasm and tubular structures were associated with these particles. Extracts from infective insects contained 10-segment dsRNA when analysed by polyacrylamide gel electrophoresis. Virus survives over winter in planthoppers and D. ciliaris seedlings in frost-free areas of coastal Queensland, but infected plants have debilitated root systems and compete poorly with healthy plants.     

Infection of strawberry flowers by Botrytis cinerea and its relevance to grey mould development

Corn stunt spiroplasma (CSS) multiplied in all injected Dulbulus maidis, reaching titres of over 1 × 10⁶ colony forming units (cfu)/insect and 1 × 10⁴ cfu/salivary gland of each insect. Spiroplasmas could be isolated from the haemolymph and from the salivary glands 1 h after injection and at any time subsequently. Insect extract at a concentration greater than the equivalent of 0.1 insects/ml was inhibitory to the growth of CSS in cultures. Helices could be seen in the haemolymph at any time after injection. However, distorted or partially deformed cells and small aggregates were not present until 2–3 wk after injection. The salivary gland cells of injected insects contained membrane-bound ‘pockets’ or ‘colonies’ packed with pleomorphic organisms, which included some filamentous forms. Intracellular colonies were always on the periphery of cells and were easily detectable by fluorescent microscopy. Both pleomorphic and filamentous forms were also seen intercellularly in the salivary glands. Following injection, transmission of CSS to maize and to sterile feeding solution were compared using 1 day feeding periods. A proportion of injected leafhoppers began to transmit to maize by the third day following injection (5%) and reached a maximum of 72% by day 14. By day 9 , 82% of the population had transmitted at least once to plants and by day 12 , 100% had transmitted. Similar insects transmitted through membranes to sterile feeding solution on day 4 (3%) reaching a maximum of 62% by day 14.     

Investigation on maize (corn) mosaic virus (mosaico rayado del maiz) and its spread in cuba

Maize mosaic and stunt, occurring to a considerable degree under Cuban field conditions, was determined as a virus disease, transmissible by the corn planthopperPeregrinus maidis Ashm. Negative results were obtained in the experiments in which aphidsRhopalosiphum maidis Fitch, served as vectors or when transmission was realized mechanically using the sap of infected plants. The incubation period in the infected plants fluctuated between 15 and 27 days. Besides maize, the virus disease could be transmitted also to sorghum(Sorghum vulgare Pees.) and to weed plants of the grassRottboellia exaltata L. It follows from the results that the disease concerned is identical with virus stripe, described in 1927 in Cuba by Stahl as corn stripe (raya del maiz). Under field conditions the disease spread fluctuated in average from 3.4 to 46 per cent of inflected plants. Plant communities concentrated on larger areas which received good agro technique and chemical protection against pests exhibited lower infection for the most part The most suitable time for the determination of spread and infection intensity under field conditions is that shortly before and during flowering of maize.     

Mechanical Transmission, Purification and Properties of an Isolate of Maize Stripe Virus from Venezuela

A Venezuelan isolate of maize stripe virus (MStpV) was successfully transmitted mechanically and by the leafhopper Peregrinus maidis from field infected plants to sweet cv. Iochief. After purification of maize infected with MStpV, fine spiral filamentous particles about 4 nm in diameter and with variable lengths were consistently associated with a nucleoprotein band present in CsCl or Cs₂SO₄ isopycnic gradients. Purified preparations exhibited a typical nucleoprotein absorption spectrum with a maximum at 260–263 nm and a minimum at 240–243 nm and a 260–280 ratio of 1.38. The density of the nucleoprotein in CsCl gradients was estimated at 1.29 g/ml. The sedimentation coefficient was calculated at 62 S. The nucleoprotein consisted of 5 % single stranded RNA and a capsid protein of molecular weight 33.500 daltons. Large quantities of non-capsid proteins were isolated from infected tissue with a molecular weight of 17.500 and 16.500 daltons. Peregrinus maidis, injected with purified MStpV preparation failed to transmit the disease to healthy plants. However, they were infectious when injected with clarified infected plant sap. Antisera against capsid and non-capsid proteins from MStpV-Florida strain reacted positively with the Venezuelan antigens.     

Comparative transmission of bean leaf roll and pea enation mosaic viruses by aphids

Acyrthosiphon pisum was a more efficient vector than Myzus persicae of bean leaf roll virus (BLRV), but the two species transmitted pea enation mosaic virus (PEMV) equally well and much more often than Megoura viciae. M. viciae did not transmit BLRV, and Aphis fabae did not transmit BLRV or PEMV. BLRV and PEMV were transmitted more often by nymphs of A. pisum than by adult apterae or alatae that fed on infected plants only as adults, but both viruses were readily transmitted by adults that had developed on infected plants. The shortest time in which nymphs acquired BLRV was 2 h, and 50 % transmitted after an acquisition period of 4 days. Some nymphs acquired PEMV in 30 min and 50% in 8 h. The shortest time for inoculation of BLRV by adults was 15 min, but some transmitted PEMV in probes lasting less than 1 min. The median latent periods of BLRV and PEMV in aphids fed for 12 h on infected plants were, respectively, 105 and 44 h. Clones of A. pisum differed in their ability to transmit BLRV and PEMV, and efficiency in transmitting the two viruses seemed to be unrelated. Some aphids that fed successively on plants infected with each virus transmitted both viruses, and infectivity with one virus did not seem to affect transmission of the other.     

Survival strategies of Dalbulus maidis during maize off‐season in Brazil

Despite the importance of Dalbulus maidis (DeLong & Wolcott) (Hemiptera: Cicadellidae) as a vector of maize‐stunting pathogens, it is not understood how this leafhopper survives the maize off‐season in regions where overwintering hosts do not occur. We investigated migration and the use of alternate hosts as possible survival mechanisms for D. maidis during maize off‐season in Brazil. Dalbulus maidis populations were monitored with yellow sticky cards for 16–29 months in Anastácio (Mato Grosso do Sul State), in two farms with perennial pastures (Pasture1 and Pasture2), where maize had not been planted for >5 years, in a subsistence farm >20 km distant, where maize was annually planted (spring) (Maize1), and in Piracicaba (São Paulo State), where maize was grown year round (Maize2). RAPD‐PCR analysis of leafhoppers sampled on maize in two plots (Maize1 and Pasture1) at 15–20 and 110–120 days after germination was performed. Dalbulus maidis was trapped in the maize plots of all areas, but not in weedy or woody vegetation adjacent to the plots. Higher numbers were trapped throughout the year in Piracicaba, where maize was continuously grown under irrigation, and in the subsistence farm of Anastácio, where volunteer maize plants were available for long periods in the maize off‐season. In Anastácio farms, some population peaks were recorded in the absence of maize from midwinter to early spring, especially after soil plowing. RAPD‐PCR analysis showed that D. maidis populations sampled were genetically similar. Our data suggest that D. maidis uses a mixed strategy to survive the over‐season period in Brazil, in which part of the population overwinters locally on volunteer maize plants or nearby irrigated maize crops, whereas the other individuals migrate to colonize new maize crops in distant areas or regions. We hypothesize that immigrant D. maidis uses the contrast between plowed and vegetated soil as a visual cue for locating new maize crops.     

Transmission by Laodelphax striatellus Fallen of Rice black‐streaked dwarf virus from Frozen Infected Rice Leaves to Healthy Plants of Rice and Maize

Rice black‐streaked dwarf virus (RBSDV) is transmitted naturally to important crops such as rice, maize, barley and wheat in a persistent manner by the planthoppers, Laodelphax striatellus, Unkanodes sapporona and Unkanodes albifascia. Insect vector transmission tests are the basis for identifying viral incidence, evaluating the resistance of varieties and selecting resistance sources for rice and maize breeding. A simple, rapid and reliable method is described by which virus‐free small brown planthoppers (L. striatellus) acquired RBSDV from frozen infected rice leaves and transmitted it to healthy rice and maize plants. After feeding on frozen infected rice leaves, the planthoppers were tested by RT‐PCR for the presence of virus after 10, 15, and 22 days, respectively. The percentages of RBSDV‐containing insects were 0, 25 and 71.43% of L. striatellus fed on frozen infected rice leaves compared to 0, 28.25 and 71.43% of L. striatellus fed on fresh infected rice leaves, respectively. In transmission tests, three of eight rice seedlings (37.5%) and four of eight maize seedlings (50%) were inoculated by the planthoppers that had fed previously on frozen leaves and had allowed a 22 days latent period and showed typical disease symptoms. As a positive control, four of eight rice seedlings (50%) and four of six maize seedlings (66.67%) became infected. All rice and maize plants expressing disease symptoms were identified as virus‐positive by RT‐PCR. These results indicated that the planthoppers acquired RBSDV from frozen infected leaves and transmitted the virus to healthy plants.     

Aphid-injection experiments with carrot mottle virus and its helper virus, carrot red leaf

Carrot mottle virus (CMotV) and its helper virus, carrot red leaf (CRLV), were not transmitted by aphids (Cavariella aegopodii) that had fed through membranes on, or had been injected with, sap from mixedly infected chervil plants or partially purified preparations of CMotV. However, the viruses were transmitted by recipient aphids injected with haemolymph from donor aphids that had fed on mixedly infected plants but not by a second series of recipients injected with haemolymph from the first series. Some of the first series of recipients transmitted both viruses for up to 11 days but others transmitted erratically and many lost ability to transmit after a few days. The results confirm that both viruses are circulative but provide no evidence for multiplication in the vector. Non-viruliferous aphids, or aphids that had acquired CRLV by feeding, did not transmit CMotV when they were injected with haemolymph from aphids that had fed on a source of CMotV alone, confirming that they can only transmit CMotV when they acquire it from a mixedly infected plant. When extracts from donor aphids were treated with ether before injection, recipient aphids transmitted both CRLV and CMotV, although the infectivity of CMotV grown in Nicotiana clevelandii in the absence of CRLV is destroyed by ether treatment. CMotV particles acquired by aphids from mixedly infected plants therefore differed in some way from those in singly infected plants. A plausible explanation of these results, and of the dependence of CMotV on CRLV for aphid transmission, is that doubly infected plants contain some particles that consist of CMotV nucleic acid coated with CRLV protein.     

Host plant dispersion, leaf hopper movement and disease transmission

Abstract.

• 1 The plant-to-plant movement of the corn leafhopper, Dalbulus maidis Delong & Wolcott, and the spread of the leafhopper-borne maize rayado fino virus were investigated in four patterns of maize (Zea mays) dispersion.
• 2 D. maidis was less abundant and the spread of the virus was slower in dense stands of maize than in sparse stands.
• 3 When plant density was held constant, leafhoppers were more abundant in maize stands with relatively equidistant plant spacing (uniform dispersion) than in stands with densely-sown rows (linear dispersion) or double-sown hills (clumped dispersion), but there was no difference in virus incidence among these plant dispersion patterns.
• 4 Leafhoppers were less likely to move to adjacent plants in uniform plant dispersion patterns than in either linear or clumped dispersion patterns. This result may explain the lack of higher virus incidence in uniform stands, despite higher leafhopper abundance.
• 5 Leafhopper movement was consistent with a simple rule: the shorter the distance to the next adjacent plant, the more likely a leafhopper is to move between plants.
• 6 These results demonstrate that host plant dispersion can affect the abundance and behaviour of highly mobile herbivorous insects even when plant density is constant.

     

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.     

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.     

Characterization of Maize Iranian Mosaic Virus and Comparison with Hawaiian and Other Isolates of Maize Mosaic Virus (Rhabdoviridae)

Maize Iranian mosaic virus (MIMV) was characterized and compared with isolates of Maize mosaic virus (MMV, genus Nucleorhabdovirus, family Rhabdoviridae) in insect transmission, cytopathology and ultrastructure of infected maize cells, virion proteins and serologically. MIMV is naturally transmitted by Ribautodelphax notabilis, a delphacid planthopper, in Iran. In this study, another planthopper, Peregrinus maidis, vector of MMV, transmitted MIMV with an estimated efficiency of 0.4–1.6% following feeding on MIMV‐infected maize plants and 64% following injection of MIMV into the hemolymph, suggesting that P. maidis gut tissues largely blocked MIMV transmission. MIMV and MMV‐HI (Hawaii) induced similar cytopathologies in cells of infected maize leaves, with virions budding through inner nuclear and endoplasmic reticulum membranes. In thin sections, virions of MIMV were significantly shorter than those of MMV‐HI. Sodium dodecyl sulphate polyacrylamide gel electrophoresis analysis of virions of MIMV, MMV‐HI, MMV‐CR (Costa Rica) and MMV‐FL (Florida) yielded six proteins of which four were identified as the putative G, N, P and M proteins of plant rhabdoviruses. The N, P and M proteins of MIMV migrated faster in gels than those of the MMV isolates indicating a lower molecular weight, whereas the bands corresponding to the G proteins migrated similarly for both viruses. Polyclonal antibodies to MMV‐HI failed to react with virions of MIMV in enzyme‐linked immunosorbent assay (ELISA) and with MIMV proteins in Western blots. In contrast, these antibodies reacted strongly with MMV‐HI and MMV‐FL virions in ELISA and with MMV‐HI, MMV‐CR and MMV‐FL proteins in Western blots. Further, in ELISA, polyclonal antibodies to MMV‐MR (Mauritius) reacted weakly with MIMV virions but strongly with MMV‐HI and MMV‐FL virions. Thus, it is concluded that MIMV is a new virus of the Nucleorhabdovirus genus that may be distantly related to MMV.     

Vector and Host-Plant Relationships of the Leafhopper-Borne Maize Yellow Stripe Virus

Maize yellow stripe virus (MYSV), associated with tenuivirus-like filaments, is transmitted in a persistent manner by the leafhopper Cicadulina chinai. In this vector, MYSV acquisition and inoculation threshold times were 30 min each, latent period ranged from 4.5 to 8 days depending on temperature (14-25 °C), and retention periods were as long as 27 days. Up to 26 % of C, chinai collected from maize fields in Giza, Egypt, during September and October 1985 were naturally infective with MYSV. Two symptom-types (fine and coarse stripe) appeared on experimentally infected plants, usually on separate leaves of the same plant. However, these two symptom-types could not be isolated on separate plants through transmission by single C. chinai leafhoppers. MYSV was transmitted by nymphs and adults of C. chinai from maize to maize, wheat and barley, and from wheat to maize plants. Up to 6 % of the wheat plants examined in Naga Hamadi (Southern Egypt) in February 1986, were naturally infected. It is suggested that wheat, barley and possibly graminaceous weeds may serve as winter hosts or reservoirs for MYSV and its leafhopper vector in Egypt.     

THE TRANSMISSION OF PLANT VIRUSES BY BITING INSECTS, WITH PARTICULAR REFERENCE TO COWPEA MOSAIC

Experiments on the virus-vector relationship of the Trinidad cowpea mosaic virus, transmitted by Ceratoma ruficornis , gave the following results: ability to infect decreased with increasing time after ceasing to feed on infected plants, but vectors remained infective for 14 days (much longer than the longevity in vitro of the virus at glasshouse shade temperatures of 23–31°C.); the beetles transmitted more consistently after longer feeding on infected plants, though feeds of under 5 min. made them efficient vectors; the proportion of plants infected increased with the amount of feeding damage on them; fasting the vectors before feeding on infected plants increased voracity but had no effect on their ability to transmit; beetles became infective immediately after feeding on infected plants. Cowpeas were infected by inoculation with macerated infective vectors or with juice regurgitated by vectors. There is no evidence that aphids or other sucking insects can transmit the virus. It seems similar to squash mosaic and turnip yellow mosaic, for vectors of all three viruses probably transmit by regurgitating infective juice during feeding.     

Biological characteristics of grass geminiviruses from eastern Australia

Two serotypes of chloris striate mosaic virus (CSMV), paspalum striate mosaic virus (PSMV) and geminiviruses infecting Bromus catharticus and Digitaria didactyla were investigated. Their field occurrence and experimental hosts are listed. Serial transmission data for CSMV by single Nesoclutha pallida show a minimum latent period of 12–14 h, and regular transmission with occasional breaks for up to 50 days. Cicadulina bimaculata did not transmit any isolates after plant feeding acquisition, but transmitted CSMV inefficiently after insect injection. The vector of PSMV was found to be a specific biotype of N. pallida which bred only on Paspalum spp. The rate of transmission of CSMV with the Chloris biotype of N. pallida and of PSMV with the Paspalum biotype reached c. 50% with single insects, but only when freshly-infected source plants were used. Geminate particles were found in thin sections of leaf tissue infected with all four viruses, and partially purified preparations were made of three of these. In gel diffusion tests, the virus from Microlaena stipoides produced a spur reaction with CSMV, when reacted with CSMV antiserum. The B. catharticus and D. didactyla isolates failed to react serologically with CSMV, maize streak or Vanuatu digitaria streak viruses.     

A freeze/thaw technique for estimation of nitrogenase activity in detached nodules of Vigna mungo

The Ivory Coast and Nigerian strains of okra mosaic virus (OMV) were transmitted by the flea-beetle Podagrica decolorata, a serious pest of okra in the southern Ivory Coast. The Ivory Coast strain was also transmitted by the orthopteran, Zonocerus variegatus. The Ivory Coast strain was acquired faster than it was inoculated by P. decolorata. When groups of five beetles were given acquisition and inoculation access periods of 24 and 48 h, respectively, 60% of the okra test plants were infected. OMV-carrying P. decolorata remained infective for up to 6 days. The virus was readily detected in extracts of crushed beetles that had fed on infected plants for 20 h. The beetle was also able to transmit to and from plants of Hibiscus sabdariffa and Corchorus olitorius; as a food source it preferred C. olitorius to okra or H. sabdariffa. The beetle is active throughout the year, and presumably can spread OMV at any time between plants of these species. A considerable and unexplained decrease in frequency of transmission was observed in experiments done in the rainy season. The revised cryptogram of OMV is R/l:*/32:S/S:S/Ve/Cl.     

Comparative Studies on Aphid Transmission of the Sudanese Strain of Peanut Stunt Virus

The Sudanese strain of peanut stunt virus was transmitted by Aphis craccivora, Aphis solanella, Myzus persicae and Liaphis erysimi. Of these, A. solanella and L. erysimi were reported for the first time as vectors of peanut stunt virus. A. gossypii, A. solani and Rhopalosiphum maidis failed to transmit the virus. Viruliferous aphids retained the virus for 30 minutes but post-acquisition starvation beyond 30 minutes resulted in almost complete loss of the virus. A. craccivora transmitted the virus to four consecutive test plants and A. solanella, L. erysimi and M. persicae transmitted the virus to three consecutive test plants. It has been suggested that future research programmes should, include resistance to the virus in the major legume crops and that a crop like lucerne (Medicago sativa) which harbours both the virus and its vectors, should not be introduced in the major peanut-growing areas of the Sudan.     

Cucumber Vein Yellowing Virus; Host Range and Virus Vector Relationships

Cucumber vein yellowing virus (CVYV) is transmitted by Bemisia tabaci, it has a narrow host range restricted to some cucurbitaceous plants including Cucumis sativus, C. melo, C. melo var. flexousus, Cucurbita pepo, C. foesti, Citrullus vulgaris, C. colocynthis and Lagenaria siceraria. Although a single whitefly can transmit the disease, the efficiency of transmission was low. At least 15–20 insects per plant were required to cause an infection of 55 % of inoculated plants. The minimum acquisition and inoculation feeding periods were 30 and 15 min, respectively. The latent period in the vector is about 75 min and the whitefly was infectious for not more than 5 h.