Localization and Distribution of Wheat dwarf virus in Its Vector Leafhopper, Psammotettix alienus

Numerous virus pathogens are transmitted by specific arthropod vectors. Understanding the mechanism of transmission is a critical step in the epidemiology of plant viruses and is crucial for the development of effective disease control strategies. In this study, we describe the localization and distribution of Wheat dwarf virus (WDV), an economically important and widespread single-stranded DNA virus, in its leafhopper vector, Psammotettix alienus. The results suggest that WDV not only can move to the salivary glands from the anterior and middle midgut via the hemocoel but also can pass directly through the sheath of the filter chamber and be readily transmitted to healthy wheat plants within 5 min of an acquisition access period on infected plants. When a bacterial-expressed recombinant capsid protein (CP) was incubated with the internal organs of leafhoppers, CP-immunoreactive antigens were found at the anterior and middle midgut. Furthermore, when leafhoppers were fed with an antiserum raised against the CP, the accumulation of WDV in the gut cells, hemocoel, and salivary glands was significantly reduced. These data provide evidence that transmission of WDV is determined by a CP-mediated virion–vector retention mechanism.

Transmission and Retention of Salmonella enterica by Phytophagous Hemipteran Insects

ABSTRACTSeveral pest insects of human and livestock habitations are known as vectors ofSalmonella enterica; however, the role of plant-feeding insects as vectors ofS. entericato agricultural crops remains unexamined. Using a hemipteran insect pest-lettuce system, we investigated the potential for transmission and retention ofS. enterica. Specifically,Macrosteles quadrilineatusandMyzus persicaeinsects were fedS. enterica-inoculated lettuce leaf discs or artificial liquid diets confined in Parafilm sachets to allow physical contact or exclusively oral ingestion of the pathogen, respectively. After a 24-h acquisition access period, insects were moved onto two consecutive noninoculated leaf discs or liquid diets and allowed a 24-h inoculation access period on each of the two discs or sachets. Similar proportions of individuals from both species ingestedS. entericaafter a 24-h acquisition access period from inoculated leaf discs, but a significantly higher proportion ofM. quadrilineatusretained the pathogen internally after a 48-h inoculation access period.S. entericawas also recovered from the honeydew of both species. After a 48-h inoculation access period, bacteria were recovered from a significantly higher proportion of honeydew samples fromM. quadrilineatusthan fromM. persicaeinsects. The recovery ofS. entericafrom leaf discs and liquid diets postfeeding demonstrated that both species of insects were capable of transmitting the bacteria in ways that are not limited to mechanical transmission. Overall, these results suggest that phytophagous insects may serve as potential vectors ofS. entericain association with plants.

Plant Host Range and Leafhopper Transmission of Maize fine streak virus

Maize fine streak virus (MFSV), an emerging Rhabdovirus sp. in the genus Nucleorhabdovirus, is persistently transmitted by the black-faced leafhopper, Graminella nigrifrons (Forbes). MFSV was transmitted to maize, wheat, oat, rye, barley, foxtail, annual ryegrass, and quackgrass by G. nigrifrons. Parameters affecting efficiency of MFSV acquisition (infection) and transmission (inoculation) to maize were evaluated using single-leafhopper inoculations and enzyme-linked immunosorbent assay. MFSV was detected in ≈20% of leafhoppers that fed on infected plants but <10% of insects transmitted the virus. Nymphs became infected earlier and supported higher viral titers than adults but developmental stage at aquisition did not affect the rate of MFSV transmission. Viral titer and transmission also increased with longer post-first access to diseased periods (PADPs) (the sum of the intervals from the beginning of the acquisition access period to the end of the inoculation access period). Length of the acquisition access period was more important for virus accumulation in adults, whereas length of the interval between acquisition access and inoculation access was more important in nymphs. A threshold viral titer was needed for transmission but no transmission occurred, irrespective of titer, with a PADP of <4 weeks. MFSV was first detected by immunofluorescence confocal laser scanning microscopy at 2-week PADPs in midgut cells, hemocytes, and neural tissues; 3-week PADPs in tracheal cells; and 4-week PADPs in salivary glands, coinciding with the time of transmission to plants.

Transmission of Pigeon pea sterility mosaic virus by the Eriophyid Mite, Aceria cajani (Acari: Arthropoda)

The transmission characteristics of Pigeon pea sterility mosaic virus (PPSMV) to pigeon pea (Cajanus cajan) by its eriophyid mite vector, Aceria cajani, were studied. Nonviruliferous A. cajani colonies were established on detached healthy leaflets of a PPSMV-immune pigeon pea cultivar floating on water. The transmission efficiency of single A. cajani was up to 53% but was 100% when >5 mites per plant were used. A. cajani acquired PPSMV after a minimum acquisition access period (AAP) of 15 min and inoculated virus after a minimum inoculation access period (IAP) of 90 min. No latent period was observed. Starvation of A. cajani prior to, or following, PPSMV acquisition reduced the minimum AAP and IAP periods to 10 min and 60 min, respectively, and mites retained virus for up to 13 h. None of the mites that developed from eggs taken from PPSMV-infected leaves transmitted the virus, indicating that it is not transmitted transovarially. Taken together, these data suggest a semipersistent mode of transmission of PPSMV by A. cajani.

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

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.

Conditions for the experimental transmission of cabbage black ringspot virus by Myzus persicae (Sulz.) to turnip (Brassica rapa)

Experiments in a glasshouse have shown that the most favourable conditions for transmission of cabbage black ringspot virus by Myzus persicae (Sulz.) to turnip plants with two foliage leaves were as follows: an acquisition access period of five minutes; a test-feeding period of not less than two hours; the use of nine viruliferous aphids per test plant; the use of aphids from uncrowded colonies; the use of the third leaf from the base of an infected turnip (5-leaf stage) as virus source; and inoculation of the younget leaf of a test plant. Darkening plants for 24 or 48 h did not increase susceptibility to aphid inoculation, but increased it to mechanical inoculation in the summer. When the youngest leaf was inoculated, the age of the test plants (10-25 days after germination) did not influence transmission by aphid or mechanical inoculation.

TEMPERATURE IN RELATION TO THE TRANSMISSION AND PATHOGENICITY OF WHEAT STRIATE MOSAIC VIRUS

Endria inimica Say acquired the North American type of wheat striate mosaic virus during periods of 15 minutes or longer on diseased plants held at five constant temperatures ranging from 10 to 33 °C. When infective insects were given inoculation access periods varying from 1 to 4 days at different temperatures, the percentage of test plants infected increased with temperature from 12.5% at 10° to 81.4% at 33 °C. After an acquisition access period of 2 days at 24 °C, insects kept at 8 or 10 °C did not transmit virus, but the percentage of others that transmitted at successively higher temperatures increased from 3.3% at 16 °C to 73.3% at 33 °C. The preinfective period was more than 29 days for insects kept at 16 °C and only 5 days for some kept at 27, 30, and 33 °C. The average preinfective period was 11 days at 20 °C, but decreased to 6.4 days as temperature increased to 33 °C. The percentage of test plants that became infected increased from 0.1% at 16 °C to 44.3%, at 33 °C. Stewart and Ramsey wheat seedlings exposed to infective E. inimica for 2 days did not develop symptoms during a subsequent 60 day period at 10 °C. After the same plants were placed in a greenhouse at 20–25 °C, 26% and 27%, respectively, developed symptoms. The incubation period for symptoms in plants ranged from 17 to more than 62 days at 16 °C. It decreased as temperature increased but varied from 6 to 25 days at 30 °C. Forty-two and 48% of Stewart and Ramsey wheat plants respectively, developed symptoms at 16 °C, and increased to almost 100% for both varieties at 30 and 33 °C. The above results indicate that high temperatures during early summer are prerequisite for severe epidemics of wheat striate mosaic in spring wheat.