RESOLUTION OF STRAWBERRY VIRUS COMPLEXES

Strawberry virus 4 produces vein chlorosis and necrosis on strawberry (var. Royal Sovereign) and slight chlorotic spotting on wild strawberry ( Fragaria vesca L.). No vector is known. Virus 5 produces leaf curling and vein necrosis on Royal Sovereign and F. vesca. It is transmitted by strawberry aphids ( Pentatrichopus fragaefolii Cock.) which have fed on an infected plant for 1 hr. or more and persists for about 1 hr. in the vector.
The names strawberry mottle, mild yellow-edge, crinkle, vein chlorosis and leaf-curl virus are proposed for strawberry viruses 1, 2, 3, 4 and 5 respectively.     

Resolution of strawberry virus complexes; the isolation and some properties of virus 3

Aphids ( Capitophorus fragariae Theob.) allowed to feed for several days on a strawberry plant with severe crinkle transmitted two viruses. The isolation and properties of one (virus I) have already been described. The other (virus 3) was separated by transferring the aphids to fresh indicators after 24 hr.
Virus 3 was transmitted by aphids which had been allowed to feed on an infected plant for 6 days or more and persisted in the vector for several days. There was some evidence that the virus has a latent period in the vector. The symptoms produced by virus 3 on Fragaria vesca and Royal Sovereign strawberry are described.
On Royal Sovereign, viruses 1 and 3 together produced symptoms of severe crinkle and viruses z and 3 together produced yellow-edge. A form of severe crinkle is thus shown to be caused by a virus complex which can be resolved by means of the vector, and severe crinkle is shown to be etiologically distinct from mild crinkle.     

Resolution of Strawberry Virus Complexes

Aphids ( Capitophorus fragariae Theob.) allowed to feed for several days on a strawberry plant infected with yellow-edge transmitted two virus fractions. The isolation and properties of one (virus 1) have been described previously. The other (virus 2) was separated by transferring the aphids to fresh indicators after 24 hr.
Virus 2 was retransmitted after infection feeding periods of 24 hr. or more and persisted in the vector for several days. There is some evidence that it is itself a complex of viruses which can be separated further. On Fragaria vesca virus 2 produced chlorotic spotting, slight marginal chlorosis of the leaves and slight cupping of the leaflets. On Royal Sovereign strawberry it produced slight chlorosis of the young leaves.
On Royal Sovereign viruses 1 and 2 together produced symptoms of yellow-edge which is thus shown to be caused by a virus complex which can be resolved by means of the aphis vector.     

TRANSMISSION AND HOST-RANGE STUDIES OF STRAWBERRY GREEN-PETAL VIRUS

The virus causing phyllody (virescence) in clover flowers was transferred by Cuscuta subinclusa to Fragaria vesca and Duchesnia indica plants which then produced symptoms of strawberry green-petal disease.
The jassid Euscelis plebejus (Fall.) in several forms, including E. lineolatus Brullé, transmitted green-petal virus from clover to clover, to and from a wide range of other hosts, and from but not to strawberry. Two viruses (or strains) were distinguished, one causing phyllody and the other witches' broom on clover; both were retained for more than 2 months by the vector, in which both had a latent period of about 30 days. Macrosteles viridigriseus (Edwards) also transmitted both viruses.
Variation in symptoms on strawberry plants infected naturally, and experimentally through dodder, suggested that two diseases have previously been grouped under the name 'green petal". It is proposed to distinguish these as ( a ) green petal caused by the virus inducing phyllody in clover, and ( b ) bronze leaf wilt caused by the clover witches' broom virus.     

Irregular Occurrence of Rhabdoviruslike Particles in Tissues of Fragaria vesca Plants Infected with Strawberry Crinkle Virus

When evaluated by electron microscopy, rhabdoviruslike particles (RLP) were found most frequently in petioles, less frequently in midribs, and not at all in roots or petals of Alpine strawberry plants leaflet graft-inoculated with strawberry crinkle virus (SCV). In graft-inoculated plants RLP were found only in mesophyll and phloem parenchyma cells and in sieve tubes. Numbers of RLP detected were highest within one week of the onset of symptoms, decreased with time, and were extremely variable, both among identically treated Alpine seedlings that showed similar crinkle disease symptoms and within Alpine clones. Through electron microscopy we were not able to predict if a given petiole sample from a graft-inoculated Alpine plant showing typical symptoms of crinkle disease would have no, moderate, or many RLP. The percentage of Alpine plants developing crinkle symptoms after leaflet graft inoculation with symptom-bearing leaves from a crinkle source was higher in summer than in winter. It was not increased after the source plants had broken dormancy in the spring, compared to plants kept vegetative all winter, and was not dependent on the length of time a symptom-bearing source plant had been infected. When symptomless leaves from SCV-infected Alpine plants were grafted to healthy Alpine indicator plants, no symptoms developed on the latter.     

Different variants of the satellite RNA of groundnut rosette virus are responsible for the chlorotic and green forms of groundnut rosette disease*

Groundnut plants with symptoms of rosette disease contain groundnut rosette virus (GRV), but GRV is transmitted by Aphis craccivora only from plants that also contain groundnut rosette assistor virus (GRAV). Two main forms of rosette disease are recognised, ‘chlorotic rosette’ and ‘green rosette’. GRV cultures invariably possess a satellite RNA and this is the major cause of rosette symptoms: satellite-free isolates derived from GRV cultures from Nigerian plants with chlorotic or green rosette, or from Malawian plants with chlorotic rosette, induced no symptoms, or only transient mild mottle or interveinal yellowing, in groundnut. When the satellite RNA species from GRV cultures from Nigerian green or Malawian chlorotic rosette were reintroduced into the three satellite-free isolates in homologous and heterologous combinations, the ability to induce rosette symptoms was restored and the type of rosette induced was that of the cultures from which the satellite RNA was derived. Thus different forms of the satellite are responsible for the different forms of rosette disease. Other forms of the satellite induce only mild chlorosis or mottle symptoms in groundnut. Individual plants may contain more than one form of the satellite, and variations in their relative predominance are suggested to account for the variable symptoms (ranging from overall yellowing to mosaic) seen in some plants graft-inoculated with chlorotic rosette.     

STUDIES IN RUBUS VIRUS DISEASES

A chlorotic veinbanding disease of raspberries is shown to be due to a virus transmissible by the aphid Amphorophora rubi Kalt. after infection feeding periods of 18 hr. or more.
This virus causes leaf symptoms, masked in hot weather, on a wide range of European and North American varieties. Symptoms on the latter are analogous to those of the red raspberry mosaic of American authors.
The names veinbanding disease and raspberry veinbanding virus are proposed.     

VIRUSES CAUSING ROSETTE AND OTHER DISEASES IN GROUNDNUTS

From plants with a form of groundnut rosette disease, characterized by discrete areas of green and chlorotic tissue on the leaflets and here designated 'mosaic rosette', a virus was separated that produced only a mild mottle or sometimes a mottle with rare chlorotic flecks. It was separated by leaf grafts, by mechanical inoculation and by Aphis craccivora .
Plants inoculated simultaneously with the mottle virus and normal rosette virus usually developed the mosaic-rosette symptoms. When the mottle virus was introduced 14–35 days before the rosette virus, the plants failed to develop the severe chlorosis of rosette; the mottle virus thus protected the plant from rosette, and this was true whether the rosette virus was inoculated by aphids or by grafting.
Plants showing two other forms of mild mottle were collected in the field; viruses from them were readily transmitted by grafting or by mechanical inoculation, but not by A. craccivara . In plant-protection tests with one of these, it failed to protect plants from developing chlorotic symptoms when later inoculated with the rosette virus, although a form of interaction was evident and the doubly-infected plant was less severely chlorotic and less stunted than one infected with the rosette virus alone.     

STUDIES ON THE TRANSMISSION OF CASSAVA MOSAIC VIRUS BY BEMISIA SPP. (ALEYRODIDAE)

Whiteflies, which had originated from a mixed culture of Bemisia spp. collected from cassava ( Manihot utilissima Pohl.) in the field, needed to feed for at least 4 hr. on the young leaves of a cassava plant with mosaic before they acquired the virus. Whiteflies that acquired virus in 4–6 hr. required another 4 hr. to become viruliferous. Once viruliferous they could infect healthy plants in a feeding period of 15 min., but longer periods gave more infections. Adult whiteflies remained infective for more than 48 hr. after ending their infection feed. Cassava fed upon by only one viruliferous fly sometimes became infected.
The virus-vector relationships of cassava mosaic virus resemble those of cotton leaf-curl virus but the first could not be transmitted to cotton or the second to cassava.     

Symptom attenuation by a normally virulent satellite RNA of turnip crinkle virus is associated with the coat protein open reading frame.

Many satellite RNAs (sat-RNAs) can attenuate or intensify the symptoms produced by their helper virus. Sat-RNA C, associated with turnip crinkle virus (TCV), was previously found to intensify the symptoms of TCV on all plants in which TCV produced visible symptoms. However, when the coat protein open reading frame (ORF) of TCV was precisely exchanged with that of cardamine chlorotic fleck virus, sat-RNA C attenuated the moderate symptoms of the chimeric virus when Arabidopsis plants were coinoculated with the chimeric virus. Symptom attenuation was correlated with a reduction in viral RNA levels in inoculated and uninoculated leaves. In protoplasts, the presence of sat-RNA C resulted in a reduction of approximately 70% in the chimeric viral genomic RNA at 44 hr postinoculation, whereas the sat-RNA wa consistently amplified to higher levels by the chimeric virus than by wild-type TCV. TCV with a deletion of the coat protein ORF also resulted in a similar increase in sat-RNA C levels in protoplasts, indicating that the TVC coat protein, or its ORF, downregulates the synthesis of sat-RNA C. These results suggest that the coat protein or its ORF is a viral determinant for symptom modulation by sat-RNA C, and symptom attenuation is at least partly due to inhibition of virus accumulation.     

A VIRUS DISEASE OF RED CURRANT (RIBES RUBRUM L.)

A virus causing ringspot patterns and vein clearing of red currant leaves is reported. It is inactivated at 66–68°C., tolerates a dilution of 1:2000 but not 1:5000, loses infectivity after 7–9 days in vitro and has been transmitted only by sap inoculation.
The virus infects plants in a wide range of families, resulting in chlorotic and necrotic symptoms. On Nicotiana tabacum it causes local and systemic symptoms of the ringspot type. Its host range and physical properties differ from other viruses causing ringspot symptoms, and the name red currant ringspot virus is therefore suggested.     

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 TRANSMISSION BY MITES, HOST-RANGE AND PROPERTIES OF RYEGRASS MOSAIC VIRUS

A virus that causes chlorotic streaks on ryegrass leaves was transmitted by the eriophyid mite Abacarus hystrix (Nalepa). Virus-free mites acquired the virus in 2 hr. feeding on infected ryegrass and the proportion that became infective increased with increased feeding time up to 12 hr.; vectors lost infectivity within 24 hr. of leaving the infected leaves. All instars of A. hystrix transmitted the virus.
The virus was transmitted by manual inoculation of sap to other species of Gramineae, including oats, rice, cocksfoot and meadow fescue, but none of these hosts seemed to contain as much virus as ryegrass; their saps did not precipitate specifically with antiserum prepared against the virus in ryegrass, whereas sap from infected ryegrass precipitated up to a dilution of 1/32. Infective sap of S22 Italian ryegrass contained flexuous rod-shaped particles; the dilution end-point of the virus was about 1 in 1000; the virus was inactivated when held for 10 min. at 60°C. and most of its infectivity was lost after 24 hr. at room temperature.     

Comparison of some apple latent viruses

Apple latent viruses were eliminated from, the tips of apple shoots by exposure to a temperature of 36 °C for various periods. The length of treatment needed to eliminate a particular virus differed from plant to plant, but viruses were always inactivated in the same order: first chlorotic leaf spot, followed by stem pitting and finally Spy decline. Quince plants developed sooty ring-spot and bark necrosis when inoculated with buds from some heat-treated apple clones infected with Spy decline virus. Only chlorotic leaf spot virus was transmitted to herbaceous hosts by sap extracts from apple leaves, petals and fruits, and returned from herbaceous plants to apple. This virus, isolated from either apple or cherry, caused a dark green mottle on peach leaves.     

Reactions in Maize Infected with Swedish Isolates of Barley Yellow Dwarf (BYDV)

Nine cultivars of maize (Zea mays L.) were tested for susceptibility to BYDV under three temperature ranges in the greenhouse. Three Swedish isolates of BYDV were used, two specifically transmitted by Rhopalosiphum padi (L.) (39/78) and by Sitobion avenae (Fabr.) (27/77), respectively, and the third by both species (70). The virus isolates were transmitted successfully from different grasses to maize and from infected maize to the susceptible oat cultivar "Sol II" by the respective aphid species. S. avenae showed very high ability to transmit the S. avenae specific isolate to and from maize plants.
The main symptoms that developed on maize were fine chlorotic irregular spots, reddish purple discoloration and malformed leaves.
The relationship between maize cultivar, temperature and percent of infection is discussed. Enzyme-linked Immunosorbent Assay (ELISA) was used with success to detect the virus isolate 27/77 in susceptible and symptomless maize plants.
Electron microscopy of maize (cv. LG ll) infected with the 27/77 isolate of BYDV revealed virus-like particles, about 22 nm in diameter, in the nuclei of companion cells, in the plasmodesmata connecting companion cells with mature sieve tubes, in the lumen of mature sieve tubes and in xylem tracheal elements.     

ANEMONE MOSAIC–A VIRUS DISEASE

The name anemone mosaic is proposed for a previously unrecorded virus disease of Anemone coronaria L.; infected plants have mottled leaves, and broken and distorted flowers. This virus can cause winter browning, and can contribute to crinkle in anemones.
The virus infected forty-seven out of ninety plant species tested; it was transmitted by mechanical inoculation, and by four of the six aphid species tested. Most aphids ceased to be infective within 30 min. when continuing to feed after leaving an infected plant.
Properties in vitro varied according to conditions of the tests; the thermal inactivation point was always below 62°C., the dilution end-point did not exceed 1/2500, and the virus inactivated at 18°C., the fewer than 72 hr.
Intracellular inclusion bodies were produced in all hosts examined.
Anemone mosaic virus is very similar to viruses placed in the turnip virus 1 group of Hoggan & Johnson, and is serologically related to cabbage black ringspot virus, although AMV infection did not protect plants against infection with cabbage black ring-spot virus.
Weeds naturally infected with AMV were found in anemone plantations, and this virus was detected, together with cucumber mosaic and tobacco necrosis viruses, in corms imported into this country.     

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.     

Infectibility and sensitivity of UK raspberry, blackberry and hybrid berry cultivars to Rubus viruses

Six blackberry or hybrid berry cultivars and 19 raspberry cultivars were assessed for their infectibility with, and sensitivity to, graft inoculation with 10 distinct viruses found infecting Rubus in the UK. Cultivars were grafted with each of, two isolates of the pollen borne raspberry bushy dwarf virus (RBDV), five aphid borne viruses: black raspberry necrosis, raspberry leaf mottle (RLMV), raspberry leaf spot (RLSV), rubus yellow net and raspberry vein chlorosis (RVCV); and isolates of the nematode transmitted nepoviruses, arabis mosaic, raspberry ringspot, strawberry latent ringspot and tomato black ring. All tested cultivars were infectible with a resistance breaking isolate of RBDV but only about half of that number with the Scottish type isolate of the virus. The raspberry cvs Autumn Bliss, and occasionally Glen Garry and Glen Prosen, developed leaf yellowing symptoms following infection with RBDV, but none of the other infected cultivars showed obvious leaf symptoms when kept in a heated glasshouse during the growing season. All tested cultivars were infectible with each of the four viruses transmitted in nature by the aphid, Amphorophora idaei. Most were infected symptomlessly, but seven cultivars developed severe leaf spotting symptoms due to infection with RLMV or RLSV. All but one of the raspberry cultivars were infectible with RVCV, which is transmitted in nature by the aphid Aphis idaei, and almost all infected plants developed leaf symptoms; only one of the hybrid berry or blackberry cultivars tested was infected with RVCV. In tests with the four nepoviruses, all tested cultivars, except Tummelberry, were infectible with at least one or more of these viruses. However, cultivars responded differently to challenge inoculation with different isolates of individual nepoviruses. Several cultivars developed chlorotic leaf mottling following infection with some nepovirus isolates. The implications of these results for virus control are discussed in the light of the changing pattern of virus and virus vector incidence in the UK.