The shallot aphis,Myzus ascalonicus Doncaster,and its behaviour as a vector of plant viruses

A new species of aphis, Myzus ascalonicus Doncaster, is briefly described, and compared with Myzus persicae Sulz., which it resembles superficially. It has been found on shallots in storage and on onions and other species of plants both in glasshouses and in the open between October and June. Its summer habits and hosts are unknown. In comparative virus transmission tests with M. persicae it was found that Myzus ascalonicus transmits dandelion yellow mosaic virus, which is not transmitted by Myzus persicae ; and also cucumber virus I, Hyoscyamus virus III and sugar-beet yellows virus, all of which are transmitted by M. persicae. Myzus ascalonicus does not transmit the viruses of potato Y severe etch, lettuce mosaic and sugar-beet mosaic which are transmitted by Myzus persicae.     

STUDIES ON TWO APHID-TRANSMITTED VIRUSES OF LEGUMINOUS CROPS

Pea mosaic virus was transmitted by Myzus persicae Sulz., Macrosiphum pisi Kalt., M. solanifolii Ash. and Aphis fabae Scop., but not by Hyperomyzus staphyleae Koch. It is a 'non-persistent' virus (Watson & Roberts, 1939), and is most readily transmitted when vectors are fasted and then given a short infection feeding. Vector efficiency was not increased by increases in preliminary fasting beyond 15 min. or with increasing infection feeding beyond 1 hr. Most aphids became non-infective within 15 min. when feeding, but fasting aphids remained infective for 3 hr. Species that fed readily on the infected plants were less efficient vectors than those which did not. Seed set by infected plants produced healthy seedlings.
Pea enation mosaic virus persisted in Myzus persicae and Macrosiphum pisi for more than 140 hr.; its transmission was unaffected by preliminary treatments of aphids. No transmission was obtained until at least 4 hr. after aphids had left infected plants; usually the 'latent' period exceeded 1 day and its duration was apparently unaffected by the length of the infection feeding.     

STUDIES ON THE APHID TRANSMISSION OF A STRAIN OF HENBANE MOSAIC VIRUS

A virus causing a wilt of Datura stramonium was identified as a strain of henbane mosaic virus. It causes necrotic local lesions in Nicotiana rustica , and local lesions are demonstrable in tobacco by staining with iodine. Some of the factors affecting its transmission by Myzus persicae (Sulz.) were studied quantitatively using these lesions.
Infective aphids differed little in their ability to cause infection, and usually produced two or three lesions. The duration of the feeding puncture did not affect the number of infections and had little effect on the percentage of aphids becoming infective. Transmissible virus did not seem to be continually imbibed while aphids fed on infected plants, and there were indications that it was acquired immediately before aphids withdrew their stylets from the leaf. Aphids became infective when allowed to make feeding punctures into epidermis stripped from infected leaves.
M. persicae transmitted during feeding punctures as brief as 5–10 sec; the probability of single feeding punctures resulting in infection reached a maximum with those lasting from 20 to 30 sec, during which the stylets did not penetrate as far as the centre of the epidermal cell and little or no saliva appeared to be ejected. M. persicae did not transmit the virus when its stylets were artificially wetted with infective sap.
Periods of darkness before inoculation with datura wilt virus increased the susceptibility of Nicotiana rustica to infection by rubbing, but not to infection by aphids.     

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.     

CHANGES IN THE TRANSMISSIBILITY BY APHIDS OF A STRAIN OF CUCUMBER MOSAIC VIRUS

A strain of cucumber mosaic virus isolated from a spinach plant in 1946 was readily transmitted by Myzus persicae until 1955 when it lost this property, although it was still being propagated in conditions in which other strains remained transmissible. M. circumflexus also transmitted other strains but not this one. It was transmitted as readily as other strains by Aphis gossypii and Myzus ascalonicus. M. ascalonicus transmitted less frequently than Aphis gossypii. Transmission of the spinach strain by other aphids did not make it transmissible by Myzus persicae ; nor did propagation in different plant species or several passages through spinach. In 1955 the spinach strain was occasionally transmitted by M. persicae , but the cultures isolated in this way were no more readily transmissible by the aphid than was the bulk culture maintained by manual inoculation of sap, and after a few weeks all cultures ceased to be transmitted by M. persicae.     

The behaviour of some naturally occurring strains of potato virus Y

Potato virus Y was obtained from field crops of potatoes in many strains which differed widely in virulence and caused diseases in the variety Majestic ranging from severe leaf-drop streak to mild mosaic. The symptoms caused by these strains in seven potato varieties and tobacco are described and compared with those caused by the serologically related potato virus C. No changes were noted in the behaviour of any of the strains over three years, during which they were transmitted to many different plants.
Potato virus C was not transmitted by Myzus persicae , the most efficient vector of other strains of virus Y. Nor was virus C transmitted by eleven other species of aphides, eight of which transmitted virus Y. The efficiency with which different species acted as vectors of virus Y varied greatly, and it is suggested that in some species only occasional individuals can transmit.
Possible mechanisms for the evolution of viruses C and Y are indicated, and the effects of changes in virus, vector, and host on the prevalence of insect-transmitted viruses are discussed.     

Transmission characteristics and some other properties of bean yellow vein-banding virus, and its association with pea enation

Bean yellow vein-banding virus (BYVBV) has been found occasionally in mixed infection with pea enation mosaic virus (PEMV) in spring-sown field beans (Vicia faba minor) in southern England. Glasshouse tests confirmed that, like PEMV, BYVBV is transmissible by manual inoculation and by aphids in the persistent manner. However, BYVBV can be transmitted by aphids only from plants that are also infected with a helper virus, usually PEMV. Thus after separation from PEMV by passage through Phaseolus vulgaris it was no longer aphid-transmissible. It became aphid-transmissible again only after re-mixing in plants with PEMV or with a substitute helper, bean leaf roll virus (BLRV). It was not transmitted by aphids that fed sequentially on plants singly infected with PEMV and BYVBV. Thus the interaction between BYVBV and PEMV (or BLRV) that enables BYVBV to be transmitted by aphids seems to occur only in doubly infected plants. However, it was not transmitted by aphids from plants doubly infected with BYVBV and broad bean wilt virus (BBWV). BYVBV and PEMV were transmitted more readily by Acyrthosiphon pisum than by Myzus persicae; neither virus was transmitted by Aphis fabae. Phenol extracts of BYVBV-infected leaves were more infective than phosphate buffer or bentonite-clarified extracts and were sometimes infective when diluted to 1/1000. The infectivity of BYVBV in phosphate buffer extracts of leaves singly infected with BYVBV, unlike that in extracts of leaves doubly infected with BYVBV and PEMV (or BLRV), was destroyed by treatment with organic solvents. BYVBV infected 11 of 28 plant species that were inoculated with phenol extracts; seven of the infected species were legumes. No transmission of BYVBV was detected through seed harvested from infected field bean plants. Isometric particles c. 30 nm in diameter were seen in extracts of plants doubly infected with BYVBV and PEMV but not in extracts of plants infected with BYVBV alone. Leaves of plants infected with BYVBV, alone or with PEMV, contained membrane-bound structures c. 50–90 nm in diameter associated with the tonoplast in cell vacuoles. These structures were not found in healthy leaves. BYVBV has several properties in common with other known aphid-borne viruses that are helper-dependent and transmitted in a persistent manner. Possibly, as suggested for some of them, aphid transmission of BYVBV depends on the coating of its nucleic acid with helper virus coat protein.     

Pepino mosaic virus, a new potexvirus from pepino (Solanum muricatum)

Pepino mosaic virus (PepMV), a previously undescribed virus, was found in fields of pepino (Solanum muricatum) in the Canete valley in coastal Peru. PepMV was transmitted by inoculation of sap to 32 species from three families out of 47 species from nine families tested. It caused a yellow mosaic in young leaves of pepino and either a mild mosaic or symptomless infection in 12 wild potato species, five potato cultivars and potato clone USDA 41956 but S. stoloniferum and potato cultivars Merpata and Revolucion reacted with severe systemic necrotic symptoms. The virus was transmitted by plant contact but not by Myzus persicae. It was best propagated and assayed in Nicotiana glutinosa. Sap from infected N. glutinosa was infective after dilution to 10^-1 but not 10^-6, after 10 min at 65°C but not 70°C and after 3 months at 20°C. PepMV had filamentous particles with a normal length of 508 nm; the ends of some seemed damaged. Ultra-thin sections of infected leaves of N. glutinosa revealed many inclusions containing arrays of virus-like particles some of which were banded or whorled; small aggregates of virus-like particles were also common. The virus was purified by extracting sap from infected leaves in a solution containing 0·065 M disodium tetraborate, 0·435 M boric acid, 0·2% ascorbic acid and 0·2% sodium sulphite at pH 7·8, adding silver nitrate solution to the extract, and precipitating the virus with polyethylene glycol followed by two cycles of differential centrifugation. Particles of PepMV normally yielded two proteins with molecular weights of 26 600 and 23 200, but virus obtained from infective sap aged overnight yielded only the smaller protein suggesting that it was a product of degradation of the larger one. The virus is serologically related to two potexviruses, narcissus mosaic and cactus X and its properties are typical of the potexvirus group.     

Some hosts and properties of bulbous iris mosaic virus

Iris mosaic virus (IMV) was the only virus isolated from forty-six bulbous iris plants of twenty-two cultivars tested; it was common also in Iris danfordiae and I. reticulata but was not detected in any of fifty-two rhizomatous iris plants with mosaic symptoms. IMV was transmitted to healthy irises with difficulty by mechanical inoculation but was transmitted efficiently by Myzus persicae. IMV infected eight of forty-six plant species inoculated mechanically with partially purified virus preparations. Characteristic local lesions without subsequent systemic infection were produced in Amaranthus caudatus, six Chenopodium spp., and Tetragonia expansa; of these, C. quinoa and T. expansa were the best indicator and assay hosts. The virus was moderately stable in vitro and, unlike some similar filamentous viruses, was best purified by differentially centrifuging infective sap clarified with n-butanol. Partially purified preparations from several hosts were infective, produced one specific light-scattering zone after centrifuga-tion in sucrose density-gradient columns, were antigenic and contained particles of 760 mμ model length. IMV was not serologically related to any of nine similar aphid-transmitted, filamentous viruses.     

Czech and scandinavian isolates resembling dandelion yellow mosaic virus

Virus isolates resembling the dandelion yellow mosaic virus (synonym: lettuce necrosis virus) were obtained from dandelion plants in twenty five localities of Bohemia and also of Norway, Sweden and Finland. All isolates were sap transmissible merely to lettuce, but some of them also toChenopodium quinoa; other test plants could not be infected. Attempted serological and biological identification of the isolates with some viruses presumed to be able to infect spontaneously dandelion plants have failed.     

INFECTIVITY OF APHIDS BRED ON VIRUS-INFECTED CAULIFLOWER PLANTS

Caged cauliflower plants infected with either cabbage black ring spot virus (CBRSV) or cauliflower mosaic virus (CIMV) were colonized with Myzus persicae or Brevicoryne brassicae. Winged and wingless aphids that voluntarily flew or walked from these plants were transferred singly to healthy cauliflower or other brassica seedlings to compare their feeding behaviour and ability to transmit the viruses. Wingless aphids settled to probe more readily than winged, and B. brassicae was initially more restless than M. persicae. CIMV was more readily transmitted than CBRSV by both species, and B. brassicae rarely transmitted CBRSV. Wingless aphids transmitted less often than winged ones, and no wingless B. brassicae transmitted CBRSV, although they did CIMV. Fewer aphids transmitted CBRSV from old plants than from young ones, but plant age had little effect on CIMV transmission.     

SOME FACTORS AFFECTING THE TRANSMISSION OF LEAF-ROLL VIRUS BY APHIDS

Datura tatula is a more suitable host than potato for studying the factors influencing the transmission of potato leaf-roll virus by Myzus persicae ; it is more easily infected, provides a better source of virus for feeding aphids, produces symptoms more quickly and over a longer period of the year.
Loughnane's (1943) claim that leaf-roll virus is transmitted by starved aphids that feed for only 5 min. on infected potato plants was not confirmed. The shortest infection-feeding time in which M. persicae aphids became infective was 2 hr.; such aphids did not infect healthy plants in the first 2 days and, when transferred to a series of healthy plants at intervals, infected only few. The ability to cause infections was increased by increasing the length of infection feeding. Aphids fed for many days on infected plants could infect healthy plants in the first 15 min. of test feeding, and they continued to cause infections for long periods.
Aphids became infective more readily when feeding on recently infected Datura tatula , showing only slight symptoms, than on older plants with pronounced chlorosis; similarly, young potato sprouts showing no symptoms were better sources of virus for aphids than older plants showing severe leaf roll.
The differences in severity of symptoms shown by potato plants with leaf roll in the field mainly occur because of differences in virulence of accompanying strains of potato virus X , but isolates of leaf-roll virus were found that also varied in virulence.     

The isolation and identification of rhubarb viruses occurring in Britain

Virus-like symptoms were common in British crops of rhubarb. All plants tested of the three main varieties, ‘Timperley Early’, ‘Prince Albert’ and ‘Victoria’, were virus-infected. Turnip mosaic virus and a severe isolate of arabis mosaic virus (AMV) were obtained from ‘Timperley Early’; and ‘Prince Albert’ contained turnip mosaic virus, cherry leaf roll virus (CLRV), a mild isolate of AMV and, infrequently, cucumber mosaic virus (CMV). The main commercial variety ‘Victoria’ contained turnip mosaic virus, CLRV, a mild isolate of AMV and, infrequently, strawberry latent ringspot virus (SLRV). All the viruses were identified serologically. The rhubarb isolates did not differ markedly from other isolates of these viruses in herbaceous host reactions, properties in vitro or particle size and shape. A rhubarb isolate of CLRV was distinguished serologically from a cherry isolate of the virus. Turnip mosaic virus, CLRV and SLRV, were transmitted with difficulty, but AMV isolates were readily transmitted by mechanical inoculation. Turnip mosaic virus was also transmitted to rhubarb by Myzus persicae and Aphis fabae. CLRV was transmitted in 6–8% of the seed of infected ‘Prince Albert’ and ‘Victoria’ rhubarb and in 72% of the seed of infected Chenopodium amaranticolor. Mild isolates of AMV were also transmitted in 10–24% of the seed of infected ‘Prince Albert’ and ‘Victoria’ plants.     

PROPERTIES AND HOST RANGE OF TURNIP CRINKLE, ROSETTE AND YELLOW MOSAIC VIRUSES

Three isolates of turnip yellow mosaic virus and two other flea-beetle transmitted viruses, turnip crinkle and turnip rosette, have many similar properties: thermal inactivation end-point between 80 and 90° C.; dilution end-point greater than 10^-4; longevity in vitro at about 20° C. at least 30 days. All were transmitted by mechanical inoculation to a wide range of cruciferous host plants, including many weeds. Turnip yellow mosaic virus infected only Reseda odorata outside the Cruciferae , whereas rosette virus infected a few and crinkle virus many non-cruciferous hosts.     

SOME PROPERTIES OF FOUR STRAINS OF CUCUMBER MOSAIC VIRUS

Different strains of cucumber mosaic virus differ in their host range, symptoms caused, virulence towards different plants, transmissibility by aphids, dilution end-point and thermal inactivation point.
There are seasonal variations in the susceptibility of some host species; French bean is apparently immune during the summer but during the winter produces countable local lesions suitable for quantitative assays.
Different host species differ in the ease with which cucumber mosaic virus is transmitted to and from them; systemic infection in beet rarely occurs unless the virus is introduced into young tissues. Inhibitors of infectivity in sap of sugar beet and Phytolacca sp. make mechanical transmission from these to other hosts difficult; the inhibitors interfere less with the infection of hosts in which they occur than with the infection of tobacco.
Cucumber mosaic virus has a low temperature coefficient of thermal inactivation and much infectivity is destroyed by heating at temperatures below the thermal inactivation point.
Myzus persicae (Sulz.) is a more efficient vector than M. ornatus Laing which is more efficient than Macrosiphum euphorbiae (Thomas); although individual aphids can cause more than one infection, most cease to be infective in feeding periods of from one to five minutes.     

Some Properties of Singapore Ageratum Yellow Vein Virus (SAYVV)

A whitefly-transmitted geminivirus was found in a local weed, Ageratum conyzoides L. It caused leaf vein yellowing which developed into a bright yellow mosaic at an advanced stage of infection. The virus could not be transmitted by inoculation with sap, by seed or by dodder. By whitefly transmission, the virus was transmitted to seven species belonging to the families of Compositae, Oxalidaceae and Solanaceae. Generally, symptoms observed were either vein yellowing or leaf curling. Cytopathological studies showed the reduction of chromatin to a few periphery clumps within infected nuclei and the appearance of electron-dense fibrillar rings within the sieve elements. Both large and small fibrillar rings which are considered as diagnostic features for geminivirus infection were observed.     

Lettuce mosaic virus: from pathogen diversity to host interactors

TAXONOMY: Lettuce mosaic virus (LMV) belongs to the genus Potyvirus (type species Potato virus Y) in the family Potyviridae. PHYSICAL PROPERTIES: The virion is filamentous, flexuous with a length of 750 nm and a width of 15 nm. The particles are made of a genomic RNA of 10 080 nucleotides, covalently linked to a viral-encoded protein (the VPg) at the 5' end and with a 3' poly A tail, and encapsidated in a single type of capsid protein. The molecular weight of the capsid protein subunit has been estimated electrophoretically to be 34 kDa and estimated from the amino acid sequence to be 31 kDa. GENOME ORGANIZATION: The genome is expressed as a polyprotein of 3255 amino-acid residues, processed by three virus-specific proteinases into ten mature proteins. HOSTS: LMV has a worldwide distribution and a relatively broad host range among several families. Weeds and ornamentals can act as local reservoirs for lettuce crops. In particular, many species within the family Asteraceae are susceptible to LMV, including cultivated and ornamental species such as common (Lactuca sativa), prickly (L. serriola) or wild (L. virosa) lettuce, endive/escarole (Cichorium endiva), safflower (Carthamus tinctorius), starthistle (Centaurea solstitialis), Cape daisy (Osteospermum spp.) and gazania (Gazania rigens). In addition, several species within the families Brassicaceae, Cucurbitaceae, Fabaceae, Solanaceae and Chenopodiaceae are natural or experimental hosts of LMV. Genetic control of resistance to LMV: The only resistance genes currently used to protect lettuce crops worldwide are the recessive genes mo1(1) and mo1(2) corresponding to mutant alleles of the gene encoding the translation initiation factor eIF4E in lettuce. It is believed that at least one intact copy of eIF4E must be present to ensure virus accumulation. TRANSMISSION: LMV is transmitted in a non-persistent manner by a high number of aphid species. Myzus persicae and Macrosiphum euphorbiae are particularly active in disseminating this virus in the fields. LMV is also seedborne in lettuce. The effectiveness of LMV transmission depends on the cultivar and the age of the seed carrier at the inoculation time. SYMPTOMS: The characteristic symptoms on susceptible lettuce cultivars are dwarfism, mosaic, distortion and yellowing of the leaves with sometimes a much reduced heart of lettuce (failure to form heads). The differences in virus strains, cultivars and the physiological stage of the host at the moment of the attack cause different symptom severity: from a very slight discoloration of the veins to severe necrosis leading to the death of the plant.     

THE PROPERTIES OF TOMATO ASPERMY VIRUS AND ITS RELATIONSHIP WITH CUCUMBER MOSAIC VIRUS*

Chrysanthemum plants infected with tomato aspermy virus (TAV) produce severely distorted and discoloured flowers but show only slight leaf mottle.
TAV infected twenty-five of forty-five species (belonging to seventeen genera) tested and was transmitted by the aphid species Aulacorthum solarti, Macrosiphoniella sanborni and Myzus persicae .
Sap from infected tobacco leaves lost infectivity when diluted more than 1 in 10,000, when heated for 10 min. at above 65°C. and when stored for more than 42 hr. at 16–18°C.
Partial protection was obtained between TAV and two strains of cucumber mosaic virus. Evidence was obtained that this was true protection between related viruses and serological tests confirmed the view that TAV is a strain of cucumber mosaic virus. Evidence was obtained that this was true protection between related viruses and serological tests confirmed the view that TAV is a strain of cucumber mosaic virus.     

Investigations of carnation viruses

Carnation vein mottle virus (CarVMV) is rare in glasshouse carnations in Britain, although locally common in Dianthus barbatus in private gardens. In Sim carnations free from other viruses, CarVMV caused slight diffuse chlorotic mottling in the younger leaves, decreased flower yield by c. 22%, and caused flower breaking in cvs William Sim and Dusty. In non-Sim cultivars Pink Shibiuya, Orchid Beauty and Vesta, leaf symptoms and flower breaking were more pronounced. In mixed infections with carnation mottle virus, symptoms were much more severe. CarVMV was not eliminated from carnation or D. barbatus plants grown for 4 wk at 37^oC, and only rarely from cuttings then taken from them, but it was readily eliminated by meristem-tip culture. Myzus persicae adults or nymphs acquired and transmitted the virus within a total time of 4 min, and remained infective for 30–60 min if feeding, or for 75 min if starved. The carnation aphid, M. persicae f. dianthi, transmitted the virus much less efficiently. The virus was not transmitted by dodder (Cuscuta campestris), or through seed of D. barbatus or Chenopodium quinoa. The maximum infective dilution in sap of D. barbatus, carnation and C. quinoa ranged from 10^-2 to 10^-5. The virus withstood 10 min at 60 but not 65^oC, up to 9 days at c. 18^oC or 3–4 wk at c. 2^oC. CarVMV infected twenty-two of 107 plant species in six of thirty-seven families; suscepts were confined to the Chenopodiaceae, Caryophyllaceae and closely allied families. C. quinoa was the best local lesion assay host. Seedling clones of D. barbatus, selected as resistant to carnation mottle virus, proved the best indicator and propagation species. Up to 50 mg virus/kg tissue were obtained by butanol clarification followed by differential and density gradient centrifugation. The preparations contained a single sedimenting component, s_20w= 144S, and had flexuous filamentous particles, c. 790 times 12 run; the particles contained a single polypeptide, mol. wt 34800, and 5% of a single-stranded ribonucleic acid (RNA) with nucleotide base ratios of G21: A25: C25: U29. Serologically CarVMV was related distantly to turnip mosaic (cabbage black ring strain), pea mosaic, watermelon mosaic (Strain 2) and bean yellow mosaic viruses, more closely to pepper veinal mottle virus, but unrelated to twelve other potyviruses. CarVMV is not at present a danger to carnation crops in Britain, but the recent trend of sending carnation plants to overwinter outdoors in warmer countries involves potential risks of more rapid spread by effective vector races of M. persicae.     

Aphid-transmitted viruses from lilies in Britain

Three aphid-transmitted viruses, tulip breaking, lily symptomless and cucumber mosaic, were obtained from lilies in Britain. Tulip breaking virus was detected by the leaf mottle produced in Lilium formosanum, cucumber mosaic virus by inoculation of sap to Nicotiana clevelandii and Chenopodium quinoa, and lily symptomless virus by electron microscopy of crude leaf extracts from symptomless L. formosanum. Liiy symptomless virus was transmitted by Myzus persicae, Macrosiphum euphorbiae, Aulacorthum solani and Aphis fabae. M. persicae, which in a small experiment appeared a more efficient vector than A. fabae, transmitted the virus in a non-persistent manner. This conflicts with the original report of transmission in a persistent manner by A. gossypii but no transmission by M. persicae. The possibility that there are two distinct viruses with similar pathological effects is discussed. Tulip breaking and lily symptomless viruses spread to bait plants of L. formosanum within a field planting of lilies in Scotland especially during July to September; lily symptomless virus was the more prevalent. No spread of cucumber mosaic virus was detected.