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.     

STUDIES ON DANDELION YELLOW MOSAIC AND OTHER VIRUS DISEASES OF LETTUCE

The symptoms caused by dandelion yellow mosaic virus on cultivated lettuce, Lactuca serriola and L. virosa , are described and compared with those caused by lettuce mosaic virus. Lettuce is much more susceptible than dandelion to the yellow mosaic virus; no infections of dandelion were obtained by mechanical inoculation and only three by aphides, whereas infection of lettuce is regularly obtained by aphides and by inoculation provided an abrasive is used. Myzus ornatus, M. ascalonicus and Aulacorthum solani transmitted dandelion yellow mosaic virus but not lettuce mosaic virus, whereas Myzus persicae transmitted the latter but not the former. Nasonovia ribicola , the common lettuce aphis, transmitted neither. Aphides became infective only after feeding periods of some hours on the diseased plants and ceased to be infective within an hour of the infective feeding. Their efficiency as vectors was not increased by a preliminary starving period, as happens with Myzus persicae and lettuce mosaic virus. Lettuce mosaic virus was found in most samples of commercial seed, which explains its prevalence; no evidence was found for the seed-transmission of dandelion mosaic virus and it is doubtful if it occurs, for infected lettuce are so severely affected that they rarely set seed.
Cucumber mosaic virus was isolated from naturally infected lettuce.     

Over-wintering of aphids,especially Myzus persicae,Sulzer, in root clamps

Mangold clamps in many districts of the British Isles were found to provide overwintering sites for Myzus persicae (Sulz.), Hyperomyzns staphyleae (Koch) and Aulacorthum solani (Kalt.). After a severe winter, when other means of overwintering are few, clamps may be the most important source of Myzus persicae. Only Myzus ascalonicus Doncaster was found in swede clamps.
Factors affecting the infestation of clamped mangolds by M. persicae were the number of aphids on the crop when lifted, the methods of topping and clamping the roots, and the temperature in the clamp. M. persicae was introduced on the leaves, and close topping was often an efficient means of control. Close topping did not control Hyperomyzus staphyleae; normally, this aphid does not seem to be a root-feeding species, but with artificially colonized mangolds it fed on both exposed roots and foliage. It is not known how this species enters the clamps. The temperature in clamps was influenced by that of the outside air and the type of cover, but changes were long-term and did not reflect diurnal variations in external air temperature. Straw, covered with soil, was the best form of cover.
In addition to harbouring Myzus persicae , mangold clamps are also important sources of sugar-beet yellows virus.     

APHID BEHAVIOUR ON HEALTHY AND ON YELLOWS-VIRUS-INFECTED SUGAR BEET

Myzus persicae Sulz., M. ascalonicus Doncaster, Aphis fabae Scop, and Aulacorthum solani Kalt., when caged on sugar-beet leaves in the glasshouse preferred yellows-virus-infected leaves to healthy ones; they chose those with the most severe symptoms on which they bred more rapidly and lived longer than on green leaves. M. persicae behaved similarly on whole plants in the glasshouse. The previous host influenced the aphid's preference. Differences between the multiplication rates of aphids on healthy plants of inbred sugar-beet varieties were eliminated or reversed by infection with yellows virus.
Spraying healthy plants with either sugar solutions or hydrolysate of casein increased the multiplication rate of M. persicae , but much less than did virus infection.     

THE SPREAD OF BEET YELLOWS AND BEET MOSAIC VIRUSES IN THE SUGAR-BEET ROOT CROP I. FIELD OBSERVATIONS ON THE VIRUS DISEASES OF SUGAR BEET AND THEIR VECTORS MYZUS PERSICAE SULZ. AND APHIS FABAE KOCH

A survey of aphids and virus diseases of sugar-beet root crops in eastern England was made between 1940 and 1948. Prior to 1943 the observations were made on fertilizer experiments; from 1943 onwards they were made on commercial fields selected for position in relation to beet and mangold seed crops. The incidence of beet yellows increased with increasing numbers of Myzus persicae , but not of Aphis fabae. The relation with M. persicae was sufficiently close to suggest that it is the most important, possibly the only important, vector of beet yellows virus. Beet mosaic virus also increased with increasing numbers of M. persicae , but the relation was not close enough to exclude the possibility of other vectors.
Numbers of A. fabae on sugar beet were slightly, but consistently, depressed by the use of salt as a fertilizer. Other fertilizers had variable effects. Neither aphids nor virus are likely to be greatly affected by fertilizers.
Beet yellows is most prevalent in areas where seed crops are grown, but within these areas nearness to individual seed crops did not appear to increase its incidence. M. persicae were more numerous on sugar beet in seed-crop areas than elsewhere, and this alone might account for the prevalence of yellows. Beet mosaic virus is more closely associated with seed crops than is beet yellows. It is most prevalent near to seed crops within the seed-crop areas.     

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.     

Detection of the luteoviruses, beet mild yellowing virus and beet western yellows virus, in aphids caught in sugar-beet and oilseed rape crops, 1990–1993

The incidence of beet mild yellowing luteovirus (BMYV) and non-beet-infecting strains of beet western yellows luteovirus (BWYV) in individual winged aphids, caught in yellow water-traps, in sugar beet during the spring and early summer, and in oilseed rape plots in the autumn, was monitored using monoclonal antibodies in ELISA tests from 1990 to 1993. Between 0% and 8% of the Myzus persicae trapped in sugar beet each year carried BMYV, whereas 0% to 4% caught in oilseed rape in the autumn contained this virus. In 1990, 6.5% of Macrosiphum euphorbiae trapped in sugar beet contained BMYV, but in subsequent years less than 1% were carrying virus. Much higher proportions (26–67%) of the M. persicae tested from sugar beet contained BWYV, and similar proportions tested from oilseed rape (24–45%) also carried this virus in the autumn. In contrast only 3–19% of the M. euphorbiae caught in sugar beet contained BWYV, and none in oilseed rape. In 1991 and 1992 large numbers of Breuicoryne brassicae were caught in the plot of oilseed rape, of which over 50% contained BWYV; none were carrying BMYV. In transmission studies between 1990 and 1992, 1% and 27% of M. persicae transmitted BMYV and BWYV respectively to indicator plants; subsequent ELISA tests on the same aphids showed that 3% and 33% respectively contained the two viruses. One percent of M. euphorbiae transmitted BMYV, but none were found to contain BMYV using ELISA; 15% transmitted BWYV whilst only 5% were found to carry the virus. In 1992 and 1993 the incidence of BMYV-infection in the sugar-beet fields in which aphids had been trapped ranged from 1.2%, in a field which had received granular pesticide (aldicarb) at drilling plus three foliar aphicidal sprays, to 39.5% in a field which had received only one foliar spray. In 1992 in a sugar-beet crop which had received no aphicidal treatments, and where 2.8% of immigrant M. persicae and 2.5% of M. euphorbiae contained BMYV, 11.6% of plants developed BMYV infection. Lowest levels of infection were associated with the use of granular pesticides at drilling. In 1990, 80% of oilseed rape plants in a field plot were infested with a mean of seven wingless M. persicae per plant by mid-December; 37% of these plants were infected with BWYV. The studies show that M. persicae is the principal vector of BWYV, and large proportions of winged M. persicae carry the virus, in contrast to BMYV, which is consistent with the common occurrence of BWYV in brassica crops such as oilseed rape.     

Some effects of spraying with thiabendazole on the susceptibility of sugar beet to yellowing viruses and on their vector, Myzus persicae (Sulz.)

In a field experiment fewer sugar-beet plants became infected with aphid-transmitted yellowing viruses in plots that had been sprayed with solutions of thiabendazole lactate than in water-sprayed plots, after exposure to natural infestation with aphids. Subsequent glasshouse tests showed that foliar sprays of o·o1 % thiabendazole lactate in water significantly reduced the proportion of inoculated sugar-beet plants which became infected with beet yellows virus (BYV) or beet mild yellowing virus (BMYV) after inoculation with viruliferous Myzus persicae (Sulz.). This effect on virus transmission was not apparently due to a direct insecticidal action of thiabendazole, because adult aphids usually survived equally well on sprayed and unsprayed plants. Treatment of test plants with thiabendazole did not affect the transmission of beet mosaic virus to them by M. persicae. The fecundity of M. persicae was greatly reduced by transferring them to plants which had been sprayed with thiabendazole or by spraying them with thiabendazole before transfer to unsprayed plants. The fertility of adult Aphis fabae Scop, was also reduced by spraying with thiabendazole. The mechanisms whereby thiabendazole affected fecundity of aphids and transmission of viruses are not understood.     

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.     

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.     

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.     

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.     

SOME PROPERTIES OF THREE VIRUSES ISOLATED FROM A DISEASED PLANT OF SOLANUM JASMINOIDES PAXT. FROM INDIA

Three mechanically transmissible viruses were isolated from a diseased Solanum jasminoides plant obtained from India. One is a strain of potato virus Y , which in some potato varieties produces symptoms resembling those caused by potato virus C , but unlike potato virus C it is readily transmitted by Myzus persicae. The second, named tobacco wilt virus, is also transmitted by M. persicae but much less readily, whereas the third, named datura necrosis virus, is not. All three have a wide host range, but neither tobacco wilt nor datura necrosis viruses infects potato plants. All three have long flexuous particles and similar general properties.
Simultaneous infection with datura necrosis virus usually decreases the concentration reached by potato virus Y in tobacco plants but not in Nicotiana glutinosa.     

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.     

STUDIES ON THE IMPORTANCE OF WILD BEET AS A SOURCE OF PATHOGENS FOR THE SUGAR-BEET CROP

Beet yellows virus, beet mosaic virus, rust ( Uromyces betae (Pers) Lév.), and downy mildew ( Peronospora schachtii Fuckel.) were found to be common in wild beet ( Beta vulgaris s.-sp. maritima L.) growing on the foreshores of south Wales and southern England. The virus diseases were more prevalent in southeast England than in the west, rust more in the west than in the east, and downy mildew is equally prevalent in all regions.
Beet yellows is the most commercially important disease and is more common in sugar-beet crops in East Anglia than elsewhere in Great Britain. There was no evidence that beet yellows spread in East Anglia from wild beet to nearby sugar-beet crops during the springs of 1958 or 1959, and Myzus persicae Sulz., the principal vector of yellows, was rarely found on wild beet growing on the foreshore.
In glasshouse experiments aphids colonized sugar-beet plants watered with tap water in preference to those watered with sea water. Daily watering with sea water made plants unpalatable to aphids within 14 days. Aphids also preferred leaves sprayed with distilled water to those that had been sprayed with sea water. Salt solutions gave results similar to those obtained with sea water.     

Biological, Serological and Coat Protein Properties of a Strain of Turnip Mosaic Virus Causing a Mosaic Disease of Brassica campestris and B. juncea in India

Biological, serological and coat protein properties of a potyvirus (Poty-Rape) causing a mosaic disease of Brassica campestris and B. juncea in India were investigated. The virus readily infected 4 of the 5 plant species in the family Brassicaceae in which it induced severe systemic mosaic symptoms; it also induced chlorotic and necrotic local lesions in Chenopodium amaranticolor , but failed to infect 4 other species of Chenopodiaceae or 20 species of Amaranthaceae, Apiaceae, Canabinaceae, Compositae, Cucurbitaceae, Euphorbiaceae, Leguminosae and Solanaceae. The virus was transmitted in a non-persistant manner by Myzus persicae, Brevicoryne brassicae and Aphis gossypii. The Average size, of the virus particles in a purified preparation was 740 nm × 12 nm. SDS-PAGE analysis of the viral coat protein showed two major bands of approximately 37 kDa and 31 kDa, a pattern very similar to that of a reference isolate of turnip mosaic virus (TuMV) from the U.S. In Western-blot immunoassay, an antiserum to TuMV reacted with both the coat protein bands of the Poty-Rape islate and the reference TuMV, but not with the coat proteins of four other potyviruses. The high performance liquid chromatographic profile of tryptic peptides from the coat protein of Poty-Rape was found to be very similar to that of the reference TuMV, but differed substantially from those of four other potyviruses. The Poty-Rape isolate is considered to be a distinct strain of, TuMV.     

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.     

THE CORRELATION OF APHID NUMBERS WITH THE SPREAD OF LEAF ROLL AND RUGOSE MOSAIC IN POTATO CROPS

An analysis of the results of experiments in different parts of England and Wales from 1941 to 1947 on the spread of potato leaf roll and rugose mosaic showed that leaf roll spread was correlated with the number of alate Myzus persicae (Sulzer) caught on sticky traps throughout the potato-growing season; there was some correlation with the maximum count of M. persicae per 100 leaves, but this possibly results from the correlation between trapped aphids and the number per 100 leaves. Spread of rugose mosaic (potato virus Y) was correlated to a lesser degree with number of M, persicae , perhaps because other aphid species are often vectors. With both diseases higher correlations were obtained when the infected plants were dispersed among the healthy crop than when they were placed together in a row. It is concluded that it is possible to predict the average health of potato stocks in the following year from average trap data; further work may enable the health of individual stocks to be predicted.     

Aspermy,a new virus disease of the tomato

'Aspermy' is suggested for the name of a virus disease of tomato apparently distinct from any previously described. The virus was transmitted by Myzus persicae from infected to healthy tobacco plants, but not from or to other hosts. Some properties of the virus are described together with the symptoms it causes in various hosts.     

Calanthe Mild Mosaic Virus, a New Potyvirus Causing a Mild Mosaic Disease of Calanthe Orchid in Japan

Calanthe mild mosaic potyvirus (CalMMV), a previously undescribed virus found in several locations in Japan, causes mild leaf mosaic and flower colour breaking of Calanthe plants. CalMMV was mechanically transmitted only to Calanthe sp., Phalaenopsis sp. and Tetragonia expansa of 50 plant species tested and was transmitted by the aphid Myzus persicae in a nonpersistent manner. The virus has flexuotis particles about 764 nm long and induced the formation of intracellular cytoplasmic cylindrical inclusions. The virus particles contain a single poly-peptide of 32.0 kDa and a single RNA of mol. weight 3.1 × 10⁶. As determined by immuno-electron microscopy, CalMMV is distantly related to the Japanese isolate of dendrobium mosaic potyvirus (DeMV-J), but it showed no serological relationship to any of seven other potyviruses. The sequence of the 3'-terminal 1306 nucleo-tides of the viral genome was determined. The coat protein (CP) coding sequence is predicted to be 804 nucleotides in length, encoding a protein of 268 amino acids with a calculated mol. weight of 30 389. The 3' noncoding region is 169 nucleotides long and is followed by a polyadenylate tract. The amino acid sequence of the CP of CalMMV was 73% homologous to that of DeMV-J, but less than 66% to other potyviruses.