Transmission

Q&A

Description

Presumed virus particles mostly occur in parenchyma cells of the lesion in affected orange leaves, fruits or stems. Particles are short, bacilliform, 120-130 nm long (occasionally up to 300 nm) and 50-55 nm wide. They occur within the lumen of the endoplasmic reticulum (Kitajima et al., 1974, Colariccio et al., 1995). There is a report of similar but unenveloped particles in the nucleoplasm (Kitajima et al., 1972).;In addition to the presence of the rhabdovirus-like particles within the endoplasmic reticulum of tissues from the lesion, dense viroplasm-like material is commonly found in the cytoplasm, near the particles. Small vesicle-containing fibrillar materials are frequently present in the vacuole, associated with the tonoplast, next to the dense material (Kitajima et al., 1972, Colariccio et al., 1995).;Chloroplasts are usually affected with a disorganized hypertrophied lamella system (Kitajima et al., 1972, Rodrigues, 1995). There is a report in which rod-like particles, considered to be naked rhabdovirus particles accumulate in the nucleoplasm associated with the nuclear envelope (Kitajima et al., 1972).

Symptons

Round to elliptical local lesions are seen on fruits, leaves and twigs. The severity of the lesions varies with the type of citrus and the region of origin. Leaf symptoms are usually round with a dark-brown central spot about 2-3 mm diameter, surrounded by a chlorotic halo, in which 1-3 brownish rings frequently appear surrounding the central spot, the overall lesion size varies from 10 to 30 mm, though larger lesions may form by the fusion of 2 or more adjacent lesions.;On fruits, lesions are necrotic spots 10-20 mm in diameter, with a necrotic centre. Gum exudation is occasionally observed on the lesion. On green fruits, the lesions are initially yellowish, becoming more brown or black, sometimes depressed, and reducing the market value of the fruits.;On stems, lesions may be protuberant, cortical, grey or brown. Lesions may coalesce when present in large numbers, leading to the death of the twig. In extreme cases observed in different places (JCV Rodrigues, personal communication), as described initially in 'lepra explosiva' in Argentina, severe defoliation and fruit fall may occur (Frezzi, 1940, Bitancourt, 1955, Rossetti et al., 1969).;Citrus leprosis lesions are usually very characteristic, but may sometimes be mistaken for lesions of citrus canker caused by the bacterium Xanthomonas axonopodis pv. axonopodis, or zonate chlorosis (Rossetti, 1980). Zonate chlorosis, which is associated with infestation by the same mites, does not become necrotic. Symptoms are essentially concentric green and chlorotic rings (Bitancourt, 1934).;Other viral diseases are vectored by Brevipalpus phoenicis in Brazil. Coffee ringspot virus in Coffea arabica (Chagas, 1978), Ligustrum ringspot virus in Ligustrum lucidum (Rodrigues et al., 1995), and green spot of passion fruit in Passiflora edulis (Kitajima et al., 1997). In addition, Brevipalpus californicus is a vector of Orchid fleck virus in orchids (Maeda et al., 1998). However, cross-transmission was not described among these viruses and leprosis.


Source: cabi.org
Description

The distinct quasi-isometric, geminate (twin) particles of CMGs measure ca. 30 x 20 nm, and the coat protein has a molecular weight of ca. 30 kDa. The particles each contain one molecule of circular, single-stranded DNA (Mr ca. 0.92 x 10(sup)6), and the genome consists of two circular molecules of similar sizes. In leaf tissue, the virus particles accumulate mainly in the nuclei of phloem parenchyma and of cortical and epidermal cells (Bock and Harrison, 1985). Altogether, the two genomic components of CMGs (DNA-A and DNA-B) contain eight genes (open reading frames). The DNA-A harbours two overlapping virion-sense and four overlapping complementary-sense ORFs whereas the DNA-B encodes two oppositely oriented non-overlapping ORFs. Homologous ORFs are also found in other whitefly-transmitted geminiviruses and encode proteins of Mr 10 kDa. The DNA-A genes include those encoding the virus particle protein and others involved in DNA replication. Those in DNA-B influence virus cell-to-cell movement within the plant. The virus coat protein is implicated in vector transmission (Lazarowitz, 1992). Between the two genomes lies a shared ca. 200-nucleotide sequence referred to as 'common region' that also carries the replication initiation TAATATT ? AC nanonucleotide.


Source: cabi.org
Description

T. domingensis is a rhizomatous perennial emergent wetland macrophyte. Ramets (culms) range from 1-6 m tall (Denny, 1985b) and consist of numerous slender, linear, distichous leaves with a sheathing base that emerge vertically from a central meristem. Ramets often produce a single, erect, monoecious flowering stem consisting of a staminate spike above a pistillate spike. At maturity, ramets can collapse from wind, or under their own weight (S Hall, University of Wisconsin, USA, personal communication, 2008). Rhizomes often measure several centimeters in diameter and produce abundant adventitious roots. Smith (1967, 2000) distinguished T. domingensis from similar species primarily on the basis of pistillate spike characters. T. domingensis is characterized by: pistillate bracteoles pale to light brown, slightly exceeding pistil hairs in mature spikes;pistil hair apices colorless to orange;stigmas linear to lanceolate, slightly exceeding bracteoles in mature spikes;pistillate spikes at anthesis cinnamon to light-brown, darkening slightly at maturity;monad pollen;staminate bracteoles (scales) straw to orange-brown colored;mucilage glands present on the adaxial surface of leaf sheathes and adjacent blades. Leaves are 6-18 mm wide, mature pistillate spikes are 13-26 mm wide, and the pistillate and staminate spikes are separated by a gap of 0-8 cm. Some quantitative macroscopic characters including spike width, gap length between pistillate and staminate spikes, and leaf width are useful, but are too variable for conclusive identification, which depends on the above microscopic floral characteristics. Finlayson et al. (1985) combined measurements of the gap between male and female inflorescences with the length and diameter of the female inflorescences to distinguish T. domingensis from T. orientalis in Australia.

Impact

T. domingensis can spread prolifically by rhizomes after seedlings establish in disturbed vegetation, often forming monotypes that reduce wetland plant and animal diversity. The species thrives under eutrophic conditions and artificially stabilized hydroperiods, but in undisturbed, low-nutrient wetlands, T. domingensis often grows sparsely and does not appear to reduce diversity. T. domingensis is economically important in many regions as a weaving material, but when invasive, the species can replace other valuable plant commodities. Short-term Typha control is provided by cutting, burning, or grazing, each followed by flooding, or herbicide, but re-growth from rhizomes and a vast soil seed-bank complicate eradication.

Hosts

T. domingensis can invade the margins of rice fields and lacustrine cornfields (Sykes 1981, cited in Finlayson et al., 1983;Hall, 2008).
Host Plants and Other Plants Affected
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Plant name|Family|Context
Oryza sativa|
Zea mays subsp. mays (sweetcorn)|Poaceae
Biology and Ecology
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Genetics
T. domingensis readily hybridizes with other sympatric species of Typha. T. domingensis x latifolia has mostly abortive pollen and low seed set, while T. angustifolia x domingensis (reported in France and California) is highly fertile and can form hybrid swarms (Geze, 1912, cited in Smith, 1987;Smith, 1967). T. domingensis, T. latifolia, and T. angustifolia share n=15 chromosomes (Smith, 1967). T. domingensis shows ecotypic variation for a number of traits, including salt tolerance, germination temperature, time of flowering, height, rhizome proliferation, and rhizome number (McNaughton, 1966). Because of the worldwide distribution of T. domingensis, quantitative data presented here will likely vary widely among regional ecotypes.
Reproductive Biology
T. domingensis is protogynous, self-compatible, and does not show apomixis (Smith, 1967). Pollen requires strong winds for dispersal, and T. latifolia pollen can travel distances of at least one km (Krattinger, 1975). Despite copious pollen production, self-pollination appears to exceed outcrossing even in dense stands of T. latifolia. Some populations of T. domingensis remain in anthesis for more than a month (McNaughton, 1966). Each inflorescence can produce 600,000 fruits, or 6-17 million seeds per m 2 depending on flowering ramet density, and plants established from seed can flower by the second year (Prunster, 1940, cited in Finlayson et al., 1983;Howard-Williams, 1975). Germination can occur year-round in many climates, given adequate moisture, although germination declines below 20 ° C (Finlayson et al., 1983). In the United States, southern populations germinated at a lower temperature (13 ° C) than their northern counterparts (McNaughton, 1966). Seeds germinate under moist or submerged conditions;in an extreme case, T. domingensis germinated under 80 cm of water and survived for 8 weeks (Nicol and Ganf, 2000). Salinity reduces germination, although limited germination can occur even at 20% salinity (Beare and Zedler, 1987). High salinity prevented T. domingensis from recruiting after a lake drawdown in Malawi (Howard-Williams, 1975). Exposure to light and hypoxia increase germination (Sifton, 1959), which is low under established vegetation (Finlayson et al., 1983). In natural areas not disturbed by humans, disturbance and herbivory by animals could facilitate seedling establishment of Typha seedlings (Svengsouk and Mitsch, 2001).
Lateral rhizomes can facilitate rapid vegetative expansion after seedling establishment. Individual T. latifolia clones can span 60 m (Krattinger, 1983), and T. domingensis can spread laterally at 3-10 m/year (Parsons and Cutherbertson, 1992;Fraga and Kvet, 1993). Rhizome production is stimulated by short days and cold temperatures (McNaughton, 1966).
Physiology and Phenology
In frost-free climates, T. domingensis can produce ramets (culms) year-round, although most emerge in summer and autumn, and do not survive longer than 10 months (Finlayson et al., 1983;Parsons and Cuthbertson, 1992). In a spring-fed wetland in central Mexico, T. domingensis growing in dense stands did not produce new ramets between May and October unless disturbed by leaf harvest (Hall et al., in press). Flowering ramets differentiate by spring, and become fertile by early summer. Grace and Harrison (1986) contend that high rhizome carbohydrate supplies promote Typha ramets to flower rather than to remain vegetative. Repetitive harvesting decreased rhizome starch reserves and flowering ramet density of T. domingensis, but drought stress could promote flowering (Hall, 2008). Carbohydrate dynamics have been studied for T. latifolia. Leaf biomass is at a maximum while rhizome biomass is minimized in late summer. By autumn, leaf carbohydrates have been translocated to rhizomes, biomass increases, and rhizome starch concentrations are maximized (Linde et al., 1976). For T. domingensis in Belize, leaf turnover averages 110 days (Rejmankova et al., 1996). Fraga and Kvet (1993) report that T. domingensis in Cuba had a net primary productivity of 1500 g/m 2 /year. Litterbag experiments showed only 50% decomposition after one year, and organic matter accumulated rapidly.
In flooded conditions, oxygen is conducted to Typha ’s underwater tissues via leaf aerenchyma cells (Sale and Wetzel, 1983), allowing T. domingensis to tolerate water 2 m deep (Finlayson et al., 1983). Flooded seedlings only produced additional ramets, however, when they reached the water surface (Nicol and Ganf 2000). T. domingensis is moderately salt-tolerant, and salinities of up to 5% should not impede vegetative growth or flowering. Salinity 5% prevents growth, and salinity 25% causes leaf mortality, although rhizomes re-sprout if salinity declines (Beare and Zedler, 1987). Freshwater inflows lasting 2 months allowed T. domingensis to invade California salt marshes. T. domingensis thrives in hot climates, and grows well in water at 30 ° C (Finlayson et al., 1983). Parsons and Cuthbertson (1992) reported maximum growth at 32 ° C, declining to 50% at 18 ° C. Typha spp. show a high tolerance for soil and water contaminated by heavy metals (McNaughton et al., 1974).
Nutrition
T. domingensis thrives under high nutrient loads and stable, prolonged, hydroperiods. In the Florida Everglades, T. domingensis invasion correlated with increased phosphorus and water levels, and muck-burning fires (Urban et al., 1993;Newman et al., 1998). Typha ’s limitation by phosphorus is supported by a comparison of soil and plant tissue samples from eutrophic and un-impacted areas of the Everglades (Koch and Reddy, 1992). T. domingensis also appeared limited by phosphorus in wetlands of Mexico’s Yucatan Peninsula and Belize (Rejmankova et al., 1996). In mesocosms, elevated nutrient levels and prolonged hydroperiods increased T. domingensis biomass and tissue phosphorus concentration relative to the co-occurring Cladium jamaicense (Newman et al., 1996). Substantial peat, nitrogen, and phosphorus accumulated where T. domingensis dominated nutrient-rich areas of the Everglades (Craft and Richardson, 1993). Seedlings produced more biomass, had a greater root/shoot ratio, and contained more phosphorous when grown in burned soil than in unburned or surface-burned soil in the Everglades, suggesting that soil-burning fires promote T. domingensis by releasing phosphorus (Smith and Newman, 2001). In low-nutrient areas of the Everglades, Typha is present but does not dominate (Davis, 1994).
Nitrogen and phosphorus appeared to co-limit the congener T. latifolia when it was grown in mesocosms, whereas in the field, T. latifolia increased along a gradient of increasing phosphorus (Svengsouk and Mitsch, 2001). T. x glauca required both nitrogen and phosphorus for growth in a greenhouse experiment, but adding a higher proportion of phosphorus stimulated growth regardless of nutrient concentration (Woo and Zedler, 2002).
Associations
In disturbed and eutrophic wetlands, T. domingensis tends to form monotypes. However, T. x glauca ’s invasive growth may be dependent on anthropogenic modifications (e.g. from dams, wastewater discharge, or irrigation canals). In little-disturbed wetlands where hydroperiods fluctuate seasonally, many genera co-occur with T. domingensis. In Australian wetlands, Baumea, Eleocharis, Gahnia, Melaleuca, Muehlenbeckia, and T. orientalis co-dominate with T. domingensis where water levels fluctuate (Finlayson et al., 1983;Nicol and Ganf, 2000). In Cuba, Bidens, Cyperus, Eleocharis, Hyparrenia, Panicum, and Sagittaria can co-occur with T. domingensis in shallow water, although T. domingensis often forms temporary monotypes in deeper water (Fraga and Kvet 1993). In this system, shrubs can replace Typha because of rapid organic matter accumulation;frequent fire might reduce litter and retard succession. In Africa’s Lake Victoria, T. domingensis is less abundant than the dominant Cyperus or Miscanthidium (Kansiime et al., 2007);in Lake Chad, Vossia, Cyperus, and Phragmites dominate, while T. domingensis is rare (Denny, 1985a). Thompson (1985) ranked T. domingensis as the third most-dominant African wetland plants, behind Phragmites australis and P. mauritianus. In Belize, T. domingensis normally dominates on clay soils with low salinity, while growing sparsely with dominant Eleocharis and Cladium on marl and sandy soil with higher salinity (Rejmankova et al., 1996). T. domingensis monotypes in this region may be relics of phosphorus-rich agricultural run-off. In Iran, T. domingensis and Schoenoplectus tabernaemontani co-dominate diverse wetlands (Karami et al., 2001). In a groundwater-fed wetland in central Mexico, harvesting T. domingensis increased species richness and the recruitment of uncommon species (Hall, 2008). Here, more than 40 species co-occurred with Typha and the co-dominant Schoenoplectus americanus.
Environmental Requirements
T. domingensis tolerates a broad climatic spectrum, growing between 40 ° latitude north and south under a variety of rainfall regimes (Smith, 2000). Although T. domingensis tolerates widely variable hydroperiods, it can decline during extended drawdowns, and grows best under flooded conditions (Rejmankova et al., 1996;Palma-Silva et al., 2005). Rainfall does not appear to limit wide-scale geographic distribution, because even in seasonally dry climates (e.g. central Mexico), T. domingensis can persist in isolated springs or on lakeshores. Seedlings can tolerate anaerobic conditions, but mature plants are intolerant of anaerobic conditions created when leaves are severed below water (Sale and Wetzel, 1983).
Climate
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Climate|Status|Description|Remark
Af - Tropical rainforest climate| Preferred
60mm precipitation per month
Am - Tropical monsoon climate| Preferred
Tropical monsoon climate (60mm precipitation driest month but (100 - [total annual precipitation(mm}/25]))
As - Tropical savanna climate with dry summer| Preferred
60mm precipitation driest month (in summer) and (100 - [total annual precipitation{mm}/25])
Aw - Tropical wet and dry savanna climate| Preferred
60mm precipitation driest month (in winter) and (100 - [total annual precipitation{mm}/25])
BS - Steppe climate| Preferred
430mm and 860mm annual precipitation
BW - Desert climate| Preferred
430mm annual precipitation
C - Temperate/Mesothermal climate| Preferred
Average temp. of coldest month 0°C and 18°C, mean warmest month 10°C
Cf - Warm temperate climate, wet all year| Preferred
Warm average temp. 10°C, Cold average temp. 0°C, wet all year
Cs - Warm temperate climate with dry summer| Preferred
Warm average temp. 10°C, Cold average temp. 0°C, dry summers
Cw - Warm temperate climate with dry winter| Preferred
Warm temperate climate with dry winter (Warm average temp. 10°C, Cold average temp. 0°C, dry winters)
Latitude/Altitude Ranges
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Latitude North (°N)|Latitude South (°S)|Altitude Lower (m)|Altitude Upper (m)
40
40
0
0
Soil Tolerances
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Soil drainage
impeded
seasonally waterlogged
Soil reaction
acid
alkaline
neutral
Soil texture
heavy
light
medium
Special soil tolerances
infertile
other
saline
shallow
sodic
Notes on Natural Enemies
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Herbivory is common but variable. In Australia, kangaroos, rodents, and water birds lightly graze T. domingensis, while water buffalo can cause heavy damage (Finlayson et al., 1983). In Africa, large herbivores do not extensively feed on T. domingensis, despite its abundance (Howard-Williams and Gaudet 1985). In Costa Rica and elsewhere throughout Latin America, cattle heavily graze T. domingensis (McCoy et al., 1994). Muskrats (Ondatra zibethicus) can eliminate entire stands of Typha spp. through herbivory, at least in temperate climates (Kadlec et al., 2007). Barreto et al. (2000) mention a variety of fungal pathogens, although none have been extensively studied in the field. A variety of insects feed on T. latifolia and T. angustifolia. Lepidopteran larvae often inhabit inflorescences, while noctuid caterpillars and coleoptera attack leaves, stalks, and sometimes rhizomes (Grace and Harrison, 1986).
Means of Movement and Dispersal
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Natural Dispersal (Non-Biotic) Typha ’s tiny seeds (1 - 2 mm long) are contained in achenes attached to pistil hairs, and are often dispersed by the wind. Spikes do not shed fruits until they have dried (Krattinger, 1975), often delaying dispersal until many months after seed maturation. The entire female spike sometimes collapses in place, providing a floating platform for germination (Hall, 2008). Masses of achenes and hairs, and rhizomes, can disperse by floating on currents of water (Grace and Harrison 1986;Parsons and Cutherbertson, 1992).
Vector Transmission (Biotic)
When achenes are moistened, seeds are released, which have a pointed end that can become embedded in fish scales (Krattinger, 1975). Also, pistil hairs (with attached acenes) adhere to the clothing of fieldworkers, and could attach to animals as well (S Hall, University of Wisconsin, USA, personal communication, 2008). Mud with embedded seeds readily sticks to humans, livestock, birds, and agricultural implements (Parsons and Cuthbertson, 1992).
Intentional Introduction
Indigenous people in the Northwestern United States propagated T. latifolia using rhizome fragments (Turner and Peacock, 2005). Similar propagation of T. domingensis has not been documented.
Pathway Causes
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Cause|Notes|Long Distance|Local|References
Crop production|Seeds attach to mud on agricultural implements.| Yes
Parsons and Cuthbertson,
1992
Disturbance|Seedlings establish in disturbed vegetation.| Yes
Finlayson et al.,
1983
Hitchhiker|Achenes with hairs attach to humans and animals.| Yes
Yes
Parsons and Cuthbertson,
1992
Interbasin transfers|Achenes and rhizomes disperse with water currents.| Yes
Grace and Harrison,
1986;Parsons and Cuthbertson,
1992
Interconnected waterways|Achenes and rhizomes disperse with water currents.| Yes
Grace and Harrison,
1986;Parsons and Cuthbertson,
1992
Self-propelled|Achenes with hairs are wind-dispersed.| Yes
Krattinger,
1975
Pathway Vectors
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Vector|Notes|Long Distance|Local|References
Clothing, footwear and possessions|Achenes with hairs.| Yes
Parsons and Cuthbertson,
1992
Host and vector organisms|Achenes adhere to fish scales.| Yes
Krattinger,
1975
Water|Achenes with hairs, rhizomes.| Yes
Grace and Harrison,
1986;Parsons and Cuthbertson,
1992
Wind|Achenes with hairs.| Yes
Yes
Krattinger,
1975
Impact Summary
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Category|Impact
Economic/livelihood
Positive and negative
Environment (generally)
Positive and negative
Economic Impact
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T. domingensis can interfere with agriculture in wet areas. With the adoption of year-round rice cropping in Australia, T. domingensis invaded fields and decreased yields by 5% (Sykes 1981, cited in Finlayson et al., 1983). In central Mexico’s Lake Pátzcuaro, T. domingensis can invade low-lying cornfields. This species also tends to replace the bulrush Schoenoplectus californicus, a valuable species traditionally used to weave mats (Hall, 2008). In southern Mexico, T. domingensis invades wetlands used for horse pasture, and replaces valuable fodder (S Hall, University of Wisconsin, USA, personal communication, 2009). In lacustrine wetlands, T. domingensis can interfere with fishing and water transportation (Mitchell, 1985).


Source: cabi.org
Description


Eggs

Recognition


The majority of thrips species are so small and cryptic that, except when present in very large numbers, many inspectors and commercial operators may fail to see them. Adults and larvae are able to hide in concealed places on plants such as beneath plant hairs, within tight buds, enclosed in developing leaves, or underneath the calyx of fruits. Eggs are laid concealed within plant tissues. Casual inspection may thus not reveal the presence of thrips, and even insecticide treatment may be ineffective because the chemical fails to contact the hidden thrips. Effective detection methods have yet to be deployed by most quarantine inspection systems, reliance usually being placed on inspection for feeding damage and simple beating to reveal thrips. However, adult and larval thrips can be extracted from plant material within two or three minutes if a sample is placed in a small Tullgren Funnel using turpentine as an irritant rather than light;the living thrips then run down into a glass tube at the bottom of the funnel where they are readily observed and counted.
Infestation levels in glasshouse crops are usually monitored by means of blue or yellow sticky traps. One shade of blue is particularly attractive to flying adult thrips and is widely used for monitoring the species (Brødsgaard, 1989a). Pheromone lures that attract males and females are now available to increase the sensitivity of monitoring at low levels of infestation or in easily damaged crops (Hamilton et al., 2005). Thrips can also be monitored by extracting thrips from flowers and recording their numbers or the percentage occupancy of flowers (Navas et al., 1994;Steiner and Goodwin, 2005). Western flower thrips adults are easily carried into glasshouses by wind, as well as on the clothes or in the hair of working personnel, thus making re-infestation from surrounding weeds a constant probability. Indeed, weed control around a crop, whether inside a glasshouse or on surrounding land, is the first measure to be adopted in any control strategy. Thrips are also easily carried on equipment and containers that have not been properly cleaned, and infestations in sterile laboratories with filtered air are usually due to thrips being carried in on the clothes and hair of workers. Nationally and internationally, F. occidentalis is readily transported to new areas on all types of planting material as well as on cut flowers, both commercial and domestic (Vierbergen, 1995).

Symptons


The symptoms of infestation by F. occidentalis vary widely among the different plants that are attacked. On roses or gerberas with red flowers, or on dark Saintpaulia flowers, feeding damage is readily visible as white streaking. This type of damage is less apparent on white or yellow flowers, and these commonly tolerate very much higher thrips populations with no visible symptoms. Severe infestation leads to deformation of buds if the feeding occurs before these start opening. Capsicums and cucumbers that have been attacked whilst young, show serious distortions as they mature. Leaf damage is variable, but includes silvering due to necrotic plant cells that have been drained of their contents by thrips feeding, malformation due to uneven growth, and a range of spots and other feeding scars. Eggs laid in petal tissue cause a 'pimpling' effect in flowers such as orchids. Egg laying on sensitive fruits such as table grapes, tomatoes and apples leads to the spotting of the skin of the fruit, which reduces the aesthetic value of the fruit. It can also lead to splitting and subsequent entry of fungi. However, the most serious effect of thrips feeding is due to the transmission of tospoviruses into susceptible crops, such as tomatoes, capsicums, lettuce or Impatiens. At least five different tospoviruses are known to be transmitted by western flower thrips and more may well be discovered: Tomato spotted wilt virus (TSWV), Impatiens necrotic spot virus (INSV), Groundnut ringspot virus (GRSV), Chrysanthemum stem necrosis virus (CSNV) and Tomato chlorotic spot virus (TCSV) (Whitfield et al., 2005). These viruses are acquired by the first-instar or early second-instar larvae when feeding on an infected plant, and are then transmitted only later when these larvae develop into the mobile adults;it is not possible for an adult to acquire and then transmit any of these viruses (Moritz et al., 2004). Virus symptoms vary considerably among plants, ranging from the disastrous wilting and collapse of lettuce plants, through a range of leaf mottling and distortions, to ring-spotting on tomato and capsicum fruits. These virus attacks can lead to the total loss of certain crops (see reviews in Kuo, 1996). F. occidentalis also transmits a carmovirus (Pelargonium flower break virus, PFBV) and may transmit an ilarvirus (Tobacco streak virus, TSV) (Jones, 2005).

Impact


Since the 1970s Frankliniella occidentalis has successfully invaded many countries to become one of the most important agricultural pests of ornamental, vegetable and fruit crops globally. Its invasiveness is largely attributed to the international movement of plant material and insecticide resistance, both of which have combined to foster the rapid spread of the species throughout the world (Kirk and Terry, 2003). Individuals are very small and they reside in concealed places on plants;thus are easily hidden and hard to detect in transported plant material. They reproduce rapidly and are highly polyphagous, breeding on many horticultural crops that are transported around the world.

Hosts

F. occidentalis is a highly polyphagous species with at least 250 plant species from more than 65 families being listed as 'hosts'. Unfortunately, the term 'host plant' is poorly defined in the literature on thrips. Plant species have sometimes been listed as 'hosts' simply because adults have been collected from them. The concept of 'host plant' is best restricted to those plants on which an insect can breed, and for many of the 250 plants from which F. occidentalis has been recorded there is little or no evidence of successful breeding. However, the association of adults with various plants has economic importance when viruliferous adults feed on susceptible plants. In its native range of the western USA, this thrips species can be found in large numbers on a very wide range of native plants, from lowland herbs to alpine shrubs and forbs. As a pest it is found both outdoors and in glasshouses, and it attacks flowers, fruits and leaves of a wide range of cultivated plants. These include apples, apricots, peaches, nectarines and plums, roses, chrysanthemums, carnations, sweet peas, Gladiolus, Impatiens, Gerbera and Ranunculus, peas, tomatoes, capsicums, cucumbers, melons, strawberries, lucerne, grapes and cotton. In northern Europe it is found particularly on glasshouse crops, such as cucumbers, capsicums, chrysanthemums, Gerbera, roses, Saintpaulia and tomatoes. In southern Europe it is extremely damaging to many field crops, including capsicums, tomatoes, strawberries, table grapes and artichokes, and at least in southern Italy, it has become a dominant member of the thrips fauna in wild flowers. Similarly, in Kenya the species has become a dominant member of the wild thrips fauna near agricultural fields. In contrast, in Australia it has not been found breeding on any native plant species. A further complication in considering its pest status is that in some areas this thrips species is an important predator of plant-feeding mites, such as on cotton in California, and it is then regarded as a beneficial (Trichilo and Leigh, 1986).


Source: cabi.org
Description


Ultrastructural aspects of AY group phytoplasmas in sieve tube elements of diseased plants have been studied by several researchers using transmission and scanning electron microscope observations (Hirumi and Maramorosch, 1973;Haggis and Sinha, 1978;Marcone et al., 1995;Marcone and Ragozzino, 1996;Fránová and Šimková, 2009;Fránová et al., 2009). The phytoplasma bodies varied in size and shape. They showed a very high polymorphism, appearing in round, ovoid, encurved and elongated forms. Octopus-like structures, as well as budding, dimpled- and dumbbell-shaped forms were also observed. The size of spherical forms ranged from 100 to 800 nm and filamentous bodies were up to 2600 nm in length. However, the morphological variations observed most probably represented various developmental stages of phytoplasmas and they cannot be considered as distinctive characteristics.

Recognition


For reliable diagnosis, the identity of phytoplasmas occurring in plants characterized by the symptoms described (see Symptoms), should be determined by molecular techniques.

Symptons


AY group phytoplasmas affect plants by causing extensive abnormalities in plant growth and development, suggestive of profound disturbance in plant hormone balance. Symptoms typical on herbaceous plant hosts include yellowing of the leaves, stunting, proliferation of auxiliary shoots resulting in a witches'-broom appearance, bunchy appearance of growth at the ends of stems, virescence of flowers and sterility, phyllody, shortening of internodes, elongation and etiolation of internodes, small and deformed leaves. Yellowing, decline, sparse foliage and dieback are predominant in woody plant hosts. However, it is well-known that distantly related phytoplasmas can cause identical symptoms in a given host plant, whereas closely related phytoplasmas can cause distinctly different symptoms. Lee et al. (1992) determined that different symptoms could be induced in Catharanthus roseus (periwinkle) by closely related strains of the AY phytoplasma group.

Hosts


AY group phytoplasmas appear to have a wide host range. The vast majority of strains in the AY group infect herbaceous dicotyledonous plant hosts. However, a number of strains that belong to subgroups 16SrI-A, 16SrI-B and 16SrI-C are capable of infecting monocotyledonous plants (e.g., maize, onion, gladiolus, oat, wheat and grass). Some strains in subgroups 16SrI-A, 16SrI-B, 16SrI-D, 16SrI-E, 16SrI-F and 16SrI-Q can induce disease in woody plants (e.g., grey dogwood, sandalwood, blueberry, mulberry, peach, cherry, olive, grapevine and paulownia). For many of the plant hosts which have previously been reported to be affected by AY diseases on the basis of symptomatology and/or microscopic examinations (see McCoy et al., 1989), the identity of the infecting phytoplasmas has never been determined with molecular techniques, or proved to be different from that of other established AY phytoplasma strains (Schneider et al., 1997;Marcone et al., 2000).


Source: cabi.org
Description


Phytoplasmas are cell wall-less prokaryotes, too small in size to be resolved adequately by light microscopy methods. By transmission electron microscopy of ultrathin sections, phytoplasmas appear to consist of rounded to filamentous bodies bounded by a trilaminar unit membrane. These bodies contain granules the size of ribosomes and strands of DNA that apparently condense during specimen preparation (Thomas, 1979;Thomas and Norris, 1980). In phloem sieve tube elements of coconut palms, cells of the LY phytoplasma are generally 142-295 nm in diameter and may vary from 1 to 16 µm in length (Waters and Hunt, 1980).

Recognition


Because of a protracted incubation phase in palms (Dabek, 1975), visual examination for LY symptoms is insufficient to conclusively determine the disease status of palms. To date, no biological or serological tests for detection of the LY phytoplasma have been successfully developed. PCR is the most sensitive test currently available for phytoplasma detection although this diagnostic method is complicated by the unusually low pathogen titres in palm tissues. When symptomless, pre-bearing coconut palms were evaluated for natural infection by LY, monthly assessment of spear leaf samples by LY-specific PCR in a year-long study revealed phytoplasma titres reached detectable levels in these palms between 47 and 57 days prior to the appearance of visible foliar symptoms (Harrison et al., 1994c).

Symptons


Palm lethal yellowing disease involves a prolonged latent (incubation), 'symptomless', phase. The time from primary infection to appearance of overt visible symptoms on young, non-bearing coconut palms has been estimated as between 112 and 262 days (Dabek, 1975). About 80 days prior to symptom appearance, growth of infected palms is stimulated. This is followed by a period of gradual decline and then complete growth inhibition about 1 month before the end of the incubation phase.
The early stages of LY on coconut palms are accompanied by numerous biochemical and physiological abnormalities in roots that include marked fluctuations in respiration, total sugars and reducing sugars (Oropeza et al., 1995;Islas-Flores et al., 1999;Martínez et al., 2000;Maust et al., 2003). Decreased respiration and increased root necrosis occur prior to the appearance of any visible symptoms in above-ground portions of palms (Eden-Green, 1976, 1982). The onset of symptoms also coincides with alterations in phloem flux rates (Eden-Green and Waters, 1982) and changes in water relations (McDonough and Zimmerman, 1979;Eskafi et al., 1986) due to irreversible suppression of leaf stomatal conductance (Oropeza et al., 1991;León et al., 1996). Reduction of photosynthetic capacity is marked by decreases in photosynthetic pigments, growth regulators and activity of enzymes of the carbon reduction cycle (Dabek and Hunt, 1976;León et al., 1996).
Visible symptoms on the highly susceptible Atlantic tall (also known as Jamaica tall) coconut ecotype chronologically include premature shedding of all fruit (nutfall) regardless of their developmental stage. Aborted nuts often develop a brown-black calyx-end rot reducing seed viability. Premature nutfall is accompanied or followed by inflorescence necrosis. This next symptom is most readily observed as newly mature inflorescences emerge from the ensheathing spathe. Normally light yellow to creamy white in colour, affected inflorescences are instead partially blackened (necrotic) usually at the tips of flower spikelets. As disease progresses, additional emergent or unemerged inflorescences show more extensive necrosis and may be totally discoloured. Such symptom intensification results in the death of most male flowers and an associated lack of fruit set.
Yellowing of the leaves usually starts once necrosis has developed on two or more inflorescences (Arellano and Oropeza, 1995) and discoloration is more rapid than that associated with normal leaf senescence. Beginning with the older (lowermost) leaves, yellowing progresses upward to involve the entire crown. Yellowed leaves turn brown, desiccate and die. In some cases, the advent of this symptom is seen as a single yellow leaf (flag leaf) in the mid-crown. Affected leaves often hang down forming a skirt around the trunk for several days before falling. A putrid basal soft rot of the newly emerged spear (youngest leaf) occurs once foliar yellowing is advanced. Spear leaf collapse and rot of the apical meristem invariably precedes death of the palm at which point the crown topples away leaving a bare trunk. Infected palms usually die within 3 to 6 months after the appearance of the first symptoms (McCoy et al., 1983).
LY symptomatology may be complicated by other factors. For example, non-bearing palms lack fruit and flower symptoms. Foliar discoloration also varies markedly among coconut ecotypes and hybrids. For most tall-type coconut palms, leaves turn a golden yellow before dying whereas on dwarf ecotypes leaves generally turn reddish to greyish brown.
Nutfall and inflorescence-necrosis are early stage symptoms common to all other palm species affected by LY disease. Differences may occur in the stage at which spear leaf necrosis appears. For edible date palm (Phoenix dactylifera), death of the spear leaf usually precedes foliar discoloration whereas for Adonidia and Veitchia species, the spear is usually unaffected until after all other leaves have died. Two patterns of leaf discoloration have been described. Leaves yellow before dying in species such as fishtail palms (Caryota sp.), round leaf palm (Chelyocarpus chuco), gebang palm (C. elata), fan palms (Livistona and Pritchardia sp.), princess palm (Dictyosperma album) and windmill palm (Trachycarpus fortunei). In most other susceptible species, leaves turn brown rather than yellow. Irrespective of species, however, foliar discoloration generally advances from the oldest to youngest leaves in the crown (McCoy et al., 1983).

Hosts


Plant host range for LY phytoplasma (16SrIV-A) includes: Aiphanes lindeniana (Ruffle palm), Allagoptera arenaria (Kutze seashore palm), Caryota mitis (Burmese or clustering fishtail palm), C. rumphiana (Giant fishtail palm), Chelyocarpus chuco (Round leaf palm), Copernicia alba (Caranday palm), Corypha taliera (Buri palm), Crysophila warsecewiczii (Rootspine palm), Cyphophoenix nucele (Lifou palm), Dypsis cabadae (Cabada palm), D. decaryi (Triangle palm), Gaussia attenuata (Puerto Rican Gaussia palm), Howea belmoreana (Belmore Sentry palm), H. forsteriana (Kentia or Sentry palm), Hyophorbe verschaffeltii (Spindle palm), Latania lontaroides (Latan palm), Livistona chinensis (Chinese fan palm), L. rotundifolia (Footstool palm), Nannorrhops ritchieana (Mazari palm), Phoenix canariensis (Canary Island date palm), P. dactylifera (Date palm), P. reclinata (Senegal date palm), P. rupicola (Cliff date palm), P. sylvestris (Silver date palm), Pritchardia maideniana (Kona palm), P. pacifica (Fiji Island fan palm), P. remota (Remota loulu palm), P. thurstonii (Thurston palm), Ravenea hildebrantii (Hildebrants palm), Syagrus schizophylla (Arikury palm), Veitchia arecina (Montgomerys palm) and V. merrillii (McCoy et al., 1983;Eden-Green, 1997;Harrison and Jones, 2004;Harrison and Oropeza, 2008).
The LY phytoplasma (16SrIV-A subgroup) has also been experimentally transmitted to the following palm species: C. nucifera, P. canariensis, P. pacifica, P. thurstonii, T. fortunei and V. merrillii. Replicated transmissions to these palm species were achieved using the vector planthopper Haplaxius (syn. Myndus) crudus field-collected from palms in areas of high disease incidence in Florida, USA (Howard and Thomas, 1980;Howard et al., 1983, 1984).
Current knowledge of symptomless palm hosts include Thrinax radiata (Florida thatch palm) and Coccothrinax readii (Mexican silver palm) (Narvaez et al., 2006).
Although restricted primarily to the Arecaceae, the host range of the LY phytoplasma (16SrIV-A) also includes at least one non-palm host, namely the arborescent monocot Pandanus utilis (screwpine) (Thomas and Donselman, 1979;Harrison and Oropeza, 1997).
The host range of other coconut lethal yellows group (16rIV), subgroup phytoplasmas are as follows:
16SrIV-B subgroup - Acrocomia aculeata (coyol palm), C. nucifera (Roca et al., 2006);
16SrIV-C subgroup - C. nucifera (Lee et al., 1998);
16SrIV-D subgroup - Caryota urens (jiggery palm), P. canariensis (Canary Island date palm), P. dactylifera (date palm), P. reclinata (Senegal date palm), P. roebelenii (pygmy data palm), P. sylvestris (silver date palm), Pseudophoenix sargentii (buccaneer palm), Roystonea sp., Sabal mexicana (Mexican palmetto), Sabal palmetto (sabal or cabbage palm), Syagrus romanzoffiana (queen palm), S. romanzoffiana x Butia capitata (mule palm), Washingtonia robusta (Mexican fan palm) (Harrison et al., 2008, 2009;Rodriguez et al., 2010;Vázquez-Euán et al., 2011);
The host range of 16SrIV-D subgroup phytoplasmas includes the non-palm host Carludovica palmata (Panama hat or jipi palm) (Wei et al., 2007);
16SrIV-E subgroup - C. nucifera (Martinez et al., 2008);
The host range of subgroup 16SrIV-E phytoplasmas includes the non-palm hosts Cleome rutidosperma (fringed spiderflower), Cyanthillium cinereum (little ironweed cited as Vernonia cinerea), Macroptilium lathyroides (wild bushbean), Stachytarpheta jamaicensis (light-blue snakeweed) (Brown et al., 2008;Brown and McLaughlin, 2011).
16SrIV-F subgroup - Washingtonia robusta (Mexican fan palm), P. dactylifera (date palm) (Harrison et al., 2008).


Source: cabi.org
Transmission Aedes albopictus Long
Description

Adults are known as tiger mosquitoes due to their conspicuous patterns of very black bodies with white stripes. Also, there is a distinctive single white band (stripe) down the length of the back. The body length is about 3/16-inch long. Like all mosquitoes, Asian tiger mosquitoes are small, fragile insects with slender bodies, one pair of narrow wings, and three pairs of long, slender legs. They have an elongate proboscis with which the female bites and feeds on blood.

Impact

The Asian tiger mosquito is spread via the international tyre trade (due to the rainwater retained in the tyres when stored outside). In order to control its spread such trading routes must be highlighted for the introduction of sterilisation or quarantine measures. The tiger mosquito is associated with the transmission of many human diseases, including the viruses: Dengue, West Nile and Japanese Encephalitis.


Source: cabi.org
Description

Evidence for a Viral Agent
A virus, BBTV, is the causal agent of bunchy top disease of banana. Although unequivocal evidence by reproduction of the disease through inoculation of purified virions or cloned genomic components is lacking, definitive association of BBTV with bunchy top disease was demonstrated by insect vector-mediated transmission of BBTV from an infected banana to a healthy banana plant. The virions are intimately associated with the disease (Harding et al., 1991;Thomas and Dietzgen, 1991) and have been detected in all symptomatic plants tested (Dietzgen and Thomas, 1991;Thomas, 1991;Thomas and Dietzgen, 1991;Karan et al., 1994;Kumar et al., 2011). Dale et al. (1986) and an published study (ML Iskra-Caruana, Montpellier, France) isolated dsRNA, suggestive of luteovirus infection, from Cavendish cultivars and from bunchy top affected plant samples. However, neither these, nor any subsequent studies, have identified or established a role for any virus other than BBTV in banana bunchy top disease aetiology.
Particle and Genome Properties
The virions of BBTV are icosahedra, ca 18-20 nm in diameter, have a coat protein of ca 20,000 Mr, a sedimentation coefficient of ca 46S and a buoyant density of 1.29-1.30 g/cm³ in caesium sulphate (Wu and Su, 1990c;Dietzgen and Thomas, 1991;Harding et al., 1991;Thomas and Dietzgen, 1991). Purified preparations have an A 260/280 of 1.33 (Thomas and Dietzgen, 1991). The virus possesses a multi-component genome, consisting of at least six circular, single-stranded DNA (ssDNA) components each ca. 1000-1100 nucleotides long, previously referred to as DNA-1 to -6 (Wu et al., 1994;Yeh et al., 1994;Burns et al., 1995;Xie and Hu, 1995). However, they were renamed as DNA-R, -U3, -S, -M, -C and -N. The DNA-R component encodes two open reading frames and other components each encode one protein (Burns et al., 1995;Dale et al., 1986;Beetham et al., 1997). Two areas of the non-coding regions are highly conserved between the six components (Burns et al., 1995). The first is a stem-loop common region of up to 69 nucleotides. It contains a nonanucleotide loop sequence conserved amongst ssDNA plant viruses and which may be involved in rolling circle replication and initiation of viral strand DNA synthesis. The second, 5' to the stem-loop common region, is a major common region varying in size between components from 65 to 92 nucleotides and which may have a promoter function. The initiation factor for endogenous DNA primers is also located within the major common region (Hafner et al., 1997a). DNA-R (c omponent 1) encodes a putative replication initiation protein and contains a second functional open reading frame internal to this, referred as U5, the function of which is unknown;whilst DNA-S (component 3) codes for the coat protein (Harding et al., 1993;Dale et al., 1986;Hafner et al., 1997b, Wanitchakorn et al., 1997). DNA-U3 (component 2) codes a protein of unknown function, DNA-M (component 4) codes for movement protein, DNA-C (component 5) has been shown to produce a gene product containing an LXCXE motif and to have retinoblastoma protein (Rb)-binding activity, known to perform cell-cycle-link protein, and DNA-N (component 6) codes for a nuclear shuttle protein. The gene product may be produced very early in the infection cycle and be responsible for switching the first infected cells to S-phase in preparation for virus replication. Recent research indicates that component 1 is the minimal replicative unit of BBTV and encodes the 'master' viral Rep (Horser et al., 2001a). Additional Rep encoding circular ssDNA components were reported in a few BBTV isolates from East Asia and the South Pacific region (Horser et al., 2001b). They were named BBTV-S1 and BBTV-S2, of 1109 and 1095 nts, and encoded a protein similar to the DNA-R segment but the genomic organization differed from that of DNA-R. The –S1 and –S2 components lack internal ORF U5, and the stem loop sequence was not similar to the six genomic components. These sequences were not considered as an integral part of the BBTV genome but the precise function of these additional DNAs was not known.
Strains of BBTV
Most isolates of BBTV are associated with typical severe disease symptoms. However, mild and symptomless isolates have been reported from Taiwan (Su et al., 1993;Djailo et al., 2016). BBTV has been confirmed in specimens of mild and symptomless infections from Taiwan by both ELISA and PCR (HJ Su, JL Dale and JE Thomas, Brisbane, personal communication, 1996) and the isolates can be transmitted by Pentalonia nigronervosa (HJ Su, Taipei, personal communication, 1996). Genomic differences, which correlate with these biological variants, have not yet been determined.
Two broad groups of isolates have been identified on the basis of nucleotide sequence differences between some, possibly all, of the six recognized genome components (Karan et al., 1994;Hu et al., 2007;Kumar et al., 2015;Qazi, 2016). The 'South Pacific' group (also referred as Pacific Indian Ocean (PIO) group) comprises isolates from Australia, Bangladesh, India, Myanmar, Pakistan, Sri Lanka, Fiji, Western Samoa, Tonga, Hawaii (USA) and all the isolates identified, as of 2017, in Africa (Angola, Benin, Burundi, Cameroon, CAR, Congo, DRC, Egypt, Equatorial Guinea, Gabon, Malawi, Nigeria, Rwanda, South Africa and Zambia), whilst the 'Asian' group (also referred as Southeast Asian (SEA) group) comprises isolates from China, Indonesia, Japan, the Philippines, Taiwan, Thailand and Vietnam. These differences are present throughout the genomes of components 1 and 6, but are most striking in the untranslated major common region. No biological differences have been associated with these sequence differences.
Magee (1948) noted that certain plants of 'Veimama', a cultivar originally from Fiji and growing then in New South Wales, showed a 'partial recovery' from bunchy top symptoms and produced bunches. After an initial flush of typical severe symptoms in three or four leaves, subsequent leaves showed few, if any, dark-green flecks. Suckers derived from these partially recovered plants also displayed a flush of typical symptoms followed by partial recovery. The origin of the infection, whether from Australia or Fiji, was uncertain. This partial recovery was noted for some infected plants of 'Veimama' only, and in Fiji was noted for one sucker only on a single infected stool from among hundreds of infected stools of 'Veimama' observed. Magee was not able to transmit the virus from partially recovered plants and was only able to super-infect them, with difficulty, with high inoculum pressure. This may be an example of a mild strain of BBTV, possibly a non-aphid transmitted one, propagated vegetatively, reaching only a low titre and conferring a degree of cross-protection. Alternatively, 'Veimama' may not be uniform and individual plants with a degree of resistance may exist. The complete explanation for this phenomenon is unclear. Evidence from recent studies suggests the occurrence of Musa cultivars with variable response to BBTV infection, ranging from extreme to moderate susceptibility and recovery associated with reduced virus titre (Ngatat et al., 2017;PL Kumar, IITA, Nigeria, personal communication, 2017). It is likely that previous observations of mild symptoms and lack of aphid-transmission may be related to virus-host interaction rather than to mild strains.
The inability to transmit bunchy top from abacá to banana (Ocfemia and Buhay, 1934) was originally considered evidence that two distinct strains of the virus existed. However, recent studies have identified a new virus, Abaca bunchy top virus (ABTV) which also belongs to the genus Babuvirus, as the cause of bunchy top-like symptoms in abacá (Sharman et al., 2008). The possibility of co-infection or single infection of BBTV and ABTV in abacá cannot be ruled out in endemic regions.

Symptons

The typical symptoms of bunchy top of banana are very distinctive and readily distinguished from those caused by other viruses of banana. Plants can become infected at any stage of growth and there are some initial differences between the symptoms produced in aphid-infected plants and those grown from infected planting material.
In aphid-inoculated plants, symptoms usually appear in the second leaf to emerge after inoculation and consist of a few dark-green streaks or dots on the minor veins on the lower portion of the lamina. The streaks form 'hooks' as they enter the midrib and are best seen from the underside of the leaf in transmitted light. The 'dot-dash' symptoms can sometimes also be seen on the petiole. The following leaf may display whitish streaks along the secondary veins when it is still rolled. These streaks become dark green as the leaf unfurls. Successive leaves become smaller, both in length and in width of the lamina, and often have chlorotic, upturned margins. The leaves become dry and brittle and stand more erect than normal giving the plant a rosetted and 'bunchy top' appearance.
Suckers from an infected stool can show severe symptoms in the first leaf to emerge. The leaves are rosetted and small with very chlorotic margins that tend to turn necrotic. Dark-green streaks are usually evident in the leaves.
Infected plants rarely produce a fruit bunch after infection and do not fruit in subsequent years. Plants infected late in the growing cycle may fruit once, but the bunch stalk and the fruit will be small and distorted. On plants infected very late, the only symptoms present may be a few dark green streaks on the tips of the flower bracts (Thomas et al., 1994).
Mild strains of BBTV, which induce only limited vein clearing and dark-green flecks, and symptomless strains have been reported in Cavendish plants from Taiwan (Su et al., 1993). Mild disease symptoms are expressed in some banana cultivars and Musa species. The dark-green leaf and petiole streaks, so diagnostic and characteristic of infection of cultivars in the Cavendish subgroup, can be rare or absent (Magee, 1953). Some plants of 'Veimama' (AAA, Cavendish subgroup), after initial severe symptoms, have been observed to recover and to display few if any symptoms.

Impact

BBTV is the most serious virus disease of bananas and plantains. It occurs in Africa, Asia, Australia and South Pacific islands. The virus is transmitted in a persistent, circulative, non-propagative manner by the banana aphid, Pentalonia nigronervosa, which has worldwide distribution. The virus is also spread through infected planting material. All banana cultivars are thought to be susceptible, with no known sources of resistance.

Hosts

In the Musaceae, BBTV is known to infect a range of Musa species, cultivars in the Eumusa (derived mainly from M. acuminata and M. acuminata x M. balbisiana) and Australimusa (derived mainly from M. maclayi, M. lolodensis and M. peekelii) series of edible banana and Ensete ventricosum (enset). Susceptible Musa species include M. balbisiana (Magee, 1948;Espino et al., 1993), M. acuminata ssp. banksii, M. textilis (abacá) (Magee, 1927), M. velutina (Thomas and Dietzgen, 1991), M. uranoscopos, M. jackeyi, M. ornata and M. acuminata ssp. zebrina (ADW Geering and JE Thomas, Brisbane, personal communication, 1998).
To date, there are no confirmed reports of immunity to BBTV in any Musa species or cultivar. However, differences in susceptibility between cultivars subject to either experimental or field infection have frequently been noted (Magee, 1948;Muharam, 1984;Espino et al., 1993;Ngatat et al., 2017).
Espino et al. (1993) evaluated a total of 57 banana cultivars for their reaction to bunchy top, both by experimental inoculation and field observations. All cultivars in the AA and AAA genomic groups were highly susceptible. However, low levels of infection (as assessed by symptom expression) or total absence of symptoms following aphid inoculation was noted in some cultivars containing the B genome. These included 'Radja' (AAB, syn. 'Pisang Raja' - 12.5% of inoculated plants with symptoms), 'Bungaoisan' (AAB, Plantain subgroup - 0%), 'Pelipia' (ABB, syn. Pelipita' - 10%), 'Pundol' (ABB - 0%), 'Katali' (ABB, syn. 'Pisang Awak' - 0%), 'Abuhon' (ABB - 0%) and 'Turangkog' (ABB - 0%).
These cultivars were not back-indexed by aphid transmission to a susceptible banana cultivar or tested biochemically (for example, by ELISA), so the presence of symptomless infection cannot be ruled out. Also, greater numbers of aphids than the 15 used here may have resulted in infection. Cultivars 'Abuhon' and 'Bungaoisan' are susceptible to BBTV by experimental aphid inoculation (ADW Geering and JE Thomas, Brisbane, personal communication, 1998). Nevertheless, it appears that real differences exist in cultivar reaction to bunchy top and the time taken before symptoms are expressed.
Evaluation of 16 Musa genotypes in Cameroon comprising plantain landraces, Cavendish bananas and synthetic hybrids revealed a high level of tolerance to BBTV in Gros Michel (AAA, Cavendish sub-group) and Fougamou (ABB cooking banana) (Ngatat et al., 2017). In another study of 40 Musa genotypes in Burundi, 8 genotypes (Musa balbisiana type Tani (BB), Kayinja (ABB), FHIA-03 (AABB), Prata (AAB), Gisandugu (ABB), Pisang Awak (ABB), Saba (ABB) and Highgate (AAA, Gros Michel subgroup)) were found to be asymptomatic, although Pisang Awak, Saba and Highgate tested positive to virus indicating tolerance to BBTV in some genotpyes (Niyongere et al., 2011).
Cultivars within the Cavendish subgroup form the basis of the international banana export trade and are generally highly susceptible to bunchy top. However, it appears that not all cultivars with an AAA genome are similarly susceptible. 'Gros Michel' exhibits resistance to the disease under both experimental inoculation and field conditions and Magee (1948) considered that the introduction of this cultivar to Fiji in the early 1900s contributed to partial rehabilitation of the bunchy top-devastated industry. Compared to 'Williams' (AAA, Cavendish subgroup), the concentration of virions of BBTV in infected plants of 'Gros Michel' and the proportion of plants infected by aphid inoculation is lower. Symptoms are also slower to develop and are less severe (Ngatat et al., 2017;ADW Geering and JE Thomas, Brisbane, Australia, unpublished, 1997). These factors may contribute to a reduced rate of aphid transmission and field spread in plantations of 'Gros Michel' (Ngatat et al., 2017).
There is no evidence for hosts outside the Musaceae, though reports have been conflicting. Su et al. (1993) obtained positive ELISA reactions from BBTV-inoculated Canna indica and Hedychium coronarium, and recovery of the virus to banana, though not reported here, was demonstrated (HJ Su, Taipei, personal communication, 1996). Ram and Summanwar (1984) reported Colocasia esculenta as a host of BBTV. However, Hu et al. (1996) were unable to demonstrate C. esculenta or Alpinia purpurata as experimental (E) or natural (N) hosts of BBTV in Hawaii. Geering and Thomas (1996) also found no evidence for the following species as hosts of BBTV in Australia: Strelitzia sp. (N), C. indica (E, N), C. x generalis (N), C. x orchiodes (N), H. coronarium (E), Helocania psittacorum (E), Alpinia coerulea (E, N), A. arundinelliana (E), A. zerumbet (E), Alocasia brisbaensis (E, N) or C. esculenta (E, N). Magee (1927) was unable to infect Strelitzia sp., Ravenala sp., Canna sp. (including C. edulis), Solanum tuberosum and Zea mays. Since the advent of improved and reliable diagnostics for BBTV, searches for alternative hosts outside the Musaceae, including those earlier suspects, have turned out to be negative.
Primary hosts are banana cultivars derived from M. acuminata and M. acuminata x M. balbisiana, and Musa textilis (abacá).


Source: cabi.org
Description


Flexuous, filamentous virus particles typical of the Closteroviridae have been found in infected plants. The length distribution of CYSDV particles has shown two peaks at 825-850 nm and 875-900 nm (Célix et al., 1996). Using an improved method for particle measurement, Liu et al. (2000) have recorded lengths of 800-850 nm for CYSDV. Its bipartite positive sense single-stranded RNA genome has been completely sequenced;RNA-1 contains 9126 nucleotides and RNA-2 7281 nucleotides (Livieratos and Coutts, 2002;Aguilar et al., 2003;Coutts and Livieratos, 2003). The coat protein gene contains 756 nucleotides and encodes the coat protein of 28.5 kDa (Livieratos et al., 1999). CYSDV was first classified with the Closterovirus species with bipartite genomes exemplified by Lettuce infectious yellows virus (Célix et al., 1996), as then named. These have now been transferred to a new genus Crinivirus.

Symptons


Cucumbers and melons infected by CYSDV show severe yellowing symptoms that start as an interveinal mottle on the older leaves and intensify as leaves age (Abou-Jawdah et al., 2000). Chlorotic mottling, yellowing and stunting occur on cucumber (Louro et al., 2000) and yellowing and severe stunting on melon (Kao et al., 2000). No description of symptoms on courgette has been provided by the authors reporting the natural infection (Berdiales et al., 1999). Symptoms on cucurbit crops are said to be indistinguishable from those caused by Beet pseudoyellows virus (BPYV;Wisler et al., 1998).
In experimental transmission experiments, chlorotic spots along the leaf veins of the melon cv. 'Piel de Sapo' were noticed after 14-20 days. Sometimes, initial symptoms also consisted of prominent yellowing sectors of a leaf. Symptoms evolved later to complete yellowing of the leaf lamina, except the veins, and rolling and brittleness of the leaves (Célix et al., 1996).

Hosts


The natural hosts of CYSDV are restricted to the Cucurbitaceae: watermelon, melon, cucumber and courgette. In addition, the following experimental host plants have been identified: Cucurbita maxima and Lactuca sativa. For further details, see Célix et al. (1996), Wisler et al. (1998), Berdiales et al. (1999), Abou-Jawdah et al. (2000), Desbiez et al. (2000), Kao et al. (2000) and Louro et al. (2000).


Source: cabi.org
Description

EACMV-type viruses possess bipartite, circular, ss(+)DNA genomes that are encapsidated in twin (geminate), small, quasi-isometric particles measuring 20 x 30 nm. Both the DNA-A and DNA-B genome components are needed for efficient transmission of the virus to healthy cassava plants (Liu et al., 1997). Whereas this description is used sensu lato, there is considerable variability among EACMV-type viruses at the molecular level. Each species is named after its country of first discovery/characterization although the use of country names in geminivirus nomenclature is now being discouraged (Brown et al., 2015).

Recognition

Symptoms caused by EACMV-like viruses are not distinguishable from those caused by other CMGs by visual inspection. However, mosaic patterns on cassava leaves indicate the presence of one or more of the CMD viruses, which can be discriminated using various diagnostic tools.

Symptons

Cassava plants infected by EACMV and other CMGs display diverse foliar symptoms, the type and severity of which are determined by a number of factors. Symptoms include yellow or green mosaic, mottling, and misshapen and twisted leaves that may be reduced in size. Although these symptoms are characteristic of all CMGs, they differ in distribution in fields, from plant to plant, and even on the same plant. In some cases, two branches emerging from the same cassava plant may show varying phenotypes, with one branch being symptomless and the other exhibiting typical CMD symptoms. Symptom severity also varies with variety, environment and infection type. Plants that are infected by mixed CMGs typically express more severe symptoms than those with single infections. For example, plants that are co-infected with ACMV and EACMV-UG show severe foliar symptoms, as observed in the pandemic movement of a severe form of cassava mosaic disease in East Africa (Zhou et al., 1997). In addition, so-called ‘sequences enhancing geminivirus symptoms (SEGS)’ can enhance cassava mosaic symptoms and break CMD resistance when they interact synergistically with CMGs in cassava plants (Ndunguru et al., 2016). Symptoms-based field diagnosis of EACMV and other CMGs is impracticable due to similarities of induced symptoms in infected plants regardless of the causative CMG. Consequently, it is imperative to confirm virus presence using PCR and/or ELISA methods with species-specific oligonucleotides and discriminating antibodies, respectively. PCR diagnosis is the method of choice for confirmation due to the high serological relationship among EACMV-type viruses and the cross reactivity of their antibodies.

Impact

Like other CMGs, cassava is the primary host of East African cassava mosaic virus (EACMV) and related viruses, although the virus has been detected in other plant species (Ogbe et al., 2006;Alabi et al. 2015). Analysis of the genomes of different isolates of EACMV-type viruses show considerable genetic variability and genome plasticity relative to ACMV isolates. The primary means of virus spread is via movement of contaminated vegetative cassava cuttings and secondary spread occurs via the whitefly vector, Bemisia tabaci. Perhaps the most notable documentation of invasiveness of EACMV-type viruses is the regional pandemic of a severe CMD in East Africa caused by EACMV-UG which began in Uganda in the early to mid-1990s (Gibson et al., 1996;Otim-Nape et al., 1997) on popular and widely cultivated cassava varieties and soon spread to other countries in East Africa, including Kenya and Tanzania (Otim-Nape et al., 1997;Legg, 1999). The pandemic resulted in famine-related deaths (Otim-Nape et al., 1998) due to complete devastation of affected cassava farms in the region. EACMV is not on the IUCN or ISSG alert list.

Hosts

EACMV, like the other CMGs, is primarily borne in cassava vegetative cuttings. Emerging leaves from such cuttings may manifest CMD symptoms and serve as sources of virus inoculum for secondary spread within and across fields by the whitefly vector. True cassava seeds are not known to carry the virus (Dubern, 1994). Depending on the mode of infection, symptoms appear in the first emerging leaves for cutting and 12-20 days after inoculation by viruliferous whiteflies (Storey and Nichols, 1938) and are usually determined by varietal characteristics.


Source: cabi.org
From Wikipedia:

Transmission may refer to: