The relationships of some viruses causing necrotic diseases of the potato

Potato virus B , and some other viruses with reactions in potato varieties different from any previously described, are strains of virus X . All produce intracellular inclusions which vary with different hosts and virus strains. Except with virus B, the inclusions are larger and more frequent in potato than in tobacco or tomato. All give systemic infection when inoculated to tobacco, tomato and potato varieties in which they are carried or cause mosaic symptoms; some give systemic infection when inoculated to varieties in which they cause top-necrosis, whereas others give only local lesions.
Potato virus C is a strain of virus Y: in tobacco and a few potato varieties both produce similar symptoms, but in those varieties in which Y causes leaf-drop streak, C causes top-necrosis. C causes systemic infection when inoculated to tobacco and to potato varieties in which it causes mosaic symptoms, but not when inoculated to potato varieties in which it causes top-necrosis. Virus C was not transmitted by M . persicae. Viruses C and Y produce a few small intracellular inclusions in potato and tobacco.
Virus A is not related to Y or X : no inclusions were found in plants infected with A alone.     

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 EFFECTS OF CHANGING TEMPERATURE AND OF VIRUS INHIBITORS ON INFECTION BY CUCUMBER MOSAIC VIRUS

Whereas the spinach strain of cucumber mosaic virus fails to multiply and cause symptoms in tobacco plants kept above 30° C., the yellow strain infects at 36° C. and causes more severe symptoms than at 20° C. Increasing the temperature up to 28° C. increases the initial rate at which the spinach strain multiplies, but the virus later reaches much higher concentrations in leaves at lower temperatures, presumably because it is rapidly inactivated at 28° C. Exposing inoculated plants to 36° C. for 6 hr. decreases the number of infections by the spinach strain when the exposure starts within 6 hr. of inoculation, but not afterwards.
Pancreatic ribonuclease inhibits infections by strains of cucumber mosaic virus; inhibition is greatest when the enzyme is present in the inoculum, and when applied to inoculated leaves its effect decreases rapidly with increasing time after inoculation.
Infection by and the multiplication of strains of cucumber mosaic virus in tobacco are only slightly affected by thiouracil and greatly by azaguanine, whereas strains of tobacco mosaic virus are inhibited much more by thiouracil than by azaguanine. Like tobacco mosaic virus, cucumber mosaic virus multiplies more when inoculated leaves are floated in nutrient solutions than in water, but unlike tobacco mosaic virus, its multiplication is not inhibited by thiouracil more in nutrient solutions than in water.     

THE INFLUENCE OF LIGHT INTENSITY ON THE SUSCEPTIBILITY OF PLANTS TO CERTAIN VIRUSES

Reducing the light intensity under which plants were grown in summer to one-third increased their susceptibility to infection with tobacco necrosis, tomato bushy stunt, tobacco mosaic and tomato aucuba mosaic viruses. With the first two viruses shading increased the average number of local lesions per leaf by more than ten times and by more than five times with the second two.
Reducing the light intensity increased the virus content of sap from leaves inoculated with Rothamsted tobacco necrosis virus by as much as twenty times. As it also reduced the total solid content of sap by about one-half, purification was greatly facilitated; crystalline preparations of the virus were readily made from shaded plants but not from unshaded controls.
Reducing the light intensity also increased the virus content of systemically infected leaves; the greatest effect was with tomato bushy stunt virus with which increases of up to ten times were obtained, but with tobacco mosaic and aucuba mosaic viruses there were also significant increases.
The importance of controlled illumination in raising plants for virus work and the possible mechanisms responsible for the variations in susceptibility are discussed.     

THE INFECTION OF PLANTS BY VIRUSES THROUGH ROOTS

Roots of young tomato plants became infected when inoculated with tomato bushy stunt, tobacco mosaic, and potato X viruses. Root infections also occurred when these viruses were added to soil or culture solutions in which plants were growing.
The viruses were sometimes localized around their initial entry points in roots; sometimes they invaded the root system but not the shoots, and sometimes they produced full systemic infection of roots and shoots. In some experiments, but not all, systemic infections were more frequent when the upper tap root or superficial roots were inoculated than when fibrous roots were inoculated.
In both tomato and potato, virus X spread from diseased to healthy plants sharing the same culture solution, if their roots were in contact, but not otherwise. Infection of the roots of potato plants by inoculation, produced only one plant with virus-infected haulms, although several had infected tubers.     

STUDIES ON THE HOST RANGE, PROPERTIES AND MODE OF TRANSMISSION OF BEET RINGSPOT VIRUS

An apparently undescribed mechanically transmissible virus has been named beet ringspot virus (BRV). It occurs naturally in Scotland in sugar-beet, turnip, swede, potato and many kinds of weed plants. BRV is readily distinguished from raspberry ringspot virus by the symptoms produced in Chenopodium amaranticolor , French bean, tobacco and Petunia hybrida plants. BRV lost infectivity when heated for 10 min. at 63°C. but not at 60°C.: at 20°C. its longevity in vitro was about 3 weeks. BRV was precipitated by ammonium sulphate, ethanol and acetone.
Protection experiments with tobacco plants, and serological tests, gave no evidence that BRV was related to tobacco ringspot, raspberry ringspot, potato bouquet or tobacco rattle viruses, but showed that viruses isolated from different host plants and from different localities were strains of BRV.
BRV is soil-borne: in glasshouse experiments sugar-beet, beetroot, potato, turnip, swede, French bean, Fragaria vesca , oat and wheat plants often became systemically infected when grown in soil from the site of a disease outbreak, but the virus was restricted to the roots of many infected plants. When sugar-beet seedlings were grown in virus-containing soil, BRV was first detected in their roots, where its concentration increased, before progressively increasing amounts of virus were found in the shoots.
Soils from five localities were found to contain BRV. BRV has been found only where the soil is light in texture, and often in fields where raspberry ringspot virus occurs.     

Systemic Acquired Resistance Induced in Tobacco Plants by Localized Virus Infection Does Not Operate Against Challenging Viruses That Infect Systemically

Localized infections produced by tobacco necrosis virus (TNV) or tomato mosaic virus (ToMV) in White Burley tobacco induced a systemic acquired resistance in upper, uninoculated leaves. This resistance was effective against challenge infection by TNV or ToMV but not by potato virus Y, necrotic strain (PVYⁿ), tobacco mosaic virus (TMV) or tobacco rattle virus (TRV), viruses giving systemic infections. Systemic acquired resistance against TNV or ToMV was expressed as a reduction in lesion size but not in viral antigen content of the resulting necrotic local lesions. The acquisition of resistance was concurrent with an increased capacity of the resistant leaves to convert 1-aminocyclopropane-1-carboxylic acid into ethylene. Systemic acquired resistance was ineffective to contrast or minimize in whatever way the systemic challenge infection produced by PVY^N, TMV or TRV. Severity of symptoms and virus multiplication did not differ in resistant leaves from controls. This result does not allow any optimistic promise on possible application of the systemic acquired resistance against severe viral diseases of crops.     

Study of the viral interference between infectious pancreatic necrosis virus (IPNV) and infectious haematopoietic necrosis virus (IHNV) in rainbow trout (Oncorhynchus mykiss)

The resistance of rainbow trout (Oncorhynchus mykiss) to an infectious haematopoietic necrosis virus (IHNV) challenge following a preceding non-lethal infection with infectious pancreatic necrosis virus (IPNV) was investigated through experimental dual infections. Trout initially infected with IPNV were inoculated 14 days later with IHNV. Single infections of trout with 1 of the 2 viruses or with cell culture supernatant were also carried out and constituted control groups. No mortality was noted in fish after a single infection with IPNV. This virus had no influence on the head kidney leucocyte phagocytic activity and plasma haemolytic complement activity. IHNV induced a high mortality (72%) and reduced the macrophage phagocytic activity and complement haemolytic activity. It also induced a late production of anti-IHNV antibodies which occurred after clearance of the virus in the fish. In trout co-infected with both viruses, a mortality rate of 2% occurred and the immune parameters were similar to those observed in the fish infected with IPNV only, demonstrating that in co-infected trout IPNV inhibits the effects of IHNV. The studied parameters did not allow us to define the mechanism of interference occurring between these 2 viruses, but some hypothesis are put forward to explain the interference between the 2 viruses.     

Characterization of an Isolate of Tobacco Necrosis Virus From Zucchini

Tobacco necrosis virus (TNV) was isolated from glasshouse zucchini plants in Liguria, northern Italy, showing yellow spots on young leaves and necrotic symptoms on older leaves, petioles and stems. No other viruses were present in these plants. Experimentally, the virus systemically infected zucchini seedlings grown in naturally infected soil but only local infections were induced when leaves and roots were mechanically inoculated. The virus was identified as serotype D, devoid of satellite particles. This appears to be the first report of TNV in zucchini. Three ELISA procedures for field detection of TNV using polyclonal antibodies for serotype D were compared. Antibodies were conjugated with I) alkaline phosphatase (AP-AB);2) N-Hydroxy-succinimid obiotin (BNHS-Ab); 3) N-biotinyl-6-aminocaproic hydrazide (BACH-Ab). The procedure using BNHS-Ab detected the lowest concentration of the homologous virus, followed by BACH-Ab and AP-Ab. Only with biotinylated antibodies. TNV serotype A was also detected.     

Prolonged replication in the mouse central nervous system of reoviruses isolated from persistently infected cell cultures.

We examined pathogenic characteristics of plaque-purified reoviruses isolated from persistently infected L-cell cultures (PI viruses) after intracranial inoculation into newborn mice. The PI viruses were isolated from independent cultures initiated with high-passage stocks of the wild-type (wt) strain, type 3 Dearing. The virulence of most PI viruses was equivalent to that of the wt strain. However, replication of PI viruses in the central nervous system of infected mice was prolonged to 25 (but not 50) days postinoculation. Thirty-eight percent (n = 186) of mice inoculated with the PI viruses had residual virus detectable in brain tissue 25 days after inoculation, in contrast to only 16% (n = 57) of mice inoculated with wt virus (P = 0.009). Mean residual brain titers were more than 20-fold higher in mice inoculated with PI viruses compared with wt virus (4.3 x 10(4) versus 2.1 x 10(3); P = 0.006). Tropism of PI virus within the brain resembled that of wt virus, and the distribution of PI virus antigen in the brain did not change over time. The extent of necrosis in the brains of mice harboring PI virus 25 days after inoculation was minimal, despite continued presence of high titers of infectious virus. The latter observation resembles the absence of cytopathicity seen in L-cell cultures persistently infected with reovirus. These observations suggest that the interaction of PI viruses with cells can be altered in vivo as well as in cell culture, but virus is eventually cleared from the infected animal.     

Photosynthesis and predisposition of plants to infection with certain viruses

The effects on susceptibility to infection with certain viruses of subjecting plants to various periods of darkness or reduced illumination before and after inoculation were tested. The viruses and hosts used were a tobacco necrosis virus in French bean and tobacco; tomato aucuba mosaic virus in tobacco; and tobacco mosaic and tomato bushy stunt viruses in Nicotiana glutinosa . All the virus-host combinations give necrotic local lesions, and susceptibility was measured by local lesion counts. Susceptibility was consistently increased by pre-inoculation treatments of host plants, whereas post-inoculation treatments had relatively little effect, but most often decreased susceptibility.
Short periods in the dark produced similar responses to longer periods in shade, but the different plants varied in their response to, and tolerance of, darkness. The maximum number of lesions was usually obtained with bean plants kept for only 24 hr. in the dark before inoculation, but with tobacco plants susceptibility increased with increasing time in the dark up to 5 days.
It is suggested that the successful establishment of infection occurs in two stages, the first of which is affected by. the accumulation of photosynthetic products. Whether these products confer resistance by increasing cell turgor or by reacting specifically with virus particles is unknown, but sap from plants in the light possesses no greater virus-inhibiting power than sap from plants kept in the dark.     

Effects of Salicylate on Systemic Invasion of Tobacco Plants by Various Viruses

Salicylate watered onto soil in which White Burley tobacco plants were grown represents a reversible stress characterized by stomatal closure, slight slackening of plant growth and low chlorophyll loss. Salicylate affected viral pathogenesis in opposite ways. It had no effect against local and systemic infections by potato virus X (PVX), potato virus Y⁰ (PVY⁰) or tobacco mosaic virus (TMV), whereas it completely prevented systemic infection by alfalfa mosaic virus (AIMV) or tobacco, rattle virus (TRV) in a high proportion of treated plants. When infection moved from leaves inoculated with AIMV or TRV, the tendency to limit systemic spread was shown by the restriction of systemic infection to very limited areas erratically distributed in some uninoculated leaves. The salicylate-induced restriction of AIMV or TRV infectivity to inoculated leaves did not appear due to inhibition of virus multiplication because the inoculation of potentially resistant leaves of salicylate-reated plants resulted in virus antigen accumulation comparable to that of untreated controls. Salicylate may therefore inhibit some long distance virus transport function. Salicylate appears able to evoke true hypersensitivity only against systemic viruses able to induce local necrotic lesions, probably by activating some genetic information for resistance that is normally not expressed.     

Inoculation of Infectious Pancreatic Necrosis Virus Serotype Sp did not cause Pancreas Disease in Atlantic Salmon (Salmo salar L.)

Atlantic salmon were selected from a fish farm with no previous record of pancreas disease (PD) or infectious pancreatic necrosis virus (IPNV) infection. Groups of fish were inoculated with either IPNV (strain Sp) from cell culture, organ material from fish with PD or control material as phosphate-buffered saline (PBS). Virological, histological and immunohistochemical examinations were carried out throughout the experiment. None of the fish died or showed clinical symptoms of PD. Histological examination revealed no pathological changes, and immunohistochemical studies were negative. Virus was isolated only sporadically from the group inoculated with organ material, whereas it was isolated consistently from the group inoculated with virus propagated in cell culture, as well as from in-contact control fish after the first week. In a latent carrier test, changes were entirely lacking in the first mentioned group, and were only slight in the last mentioned group. The data suggest that PD is not a transmissible disease, and that IPNV isolated from a PD outbreak does not play any part in the etiology of this disease.     

SOME EFFECTS OF HIGH TEMPERATURE ON THE SUSCEPTIBILITY OF PLANTS TO INFECTION WITH VIRUSES

When plants were kept at 36°C. for some time before inoculation, their susceptibility to infection by five mechanically transmissible viruses was greatly increased. When kept at 36° after inoculation, fewer local lesions were produced than at lower temperatures, but the effects of the post-inoculation treatment differed with different viruses. Tomato spotted wilt and tobacco mosaic viruses multiply in plants at 36°, and the post-inoculation treatment reduced the local lesions they caused to numbers that varied between 10 and 90% of the control; these two viruses also have large thermal coefficients of heat inactivation. By contrast, tobacco necrosis, tomato bushy stunt and cucumber mosaic viruses, were much affected by post-inoculation treatment, lesion formation being completely prevented by exposure to 36° for a day or more. These three viruses appear not to multiply in plants at 36°, and although they have high thermal inactivation points, they have small temperature coefficients of thermal inactivation.
The extent to which lesion formation was affected by pre- or post-inoculation exposure of plants to 36° depended not only on the length of the treatment, but also on the physiological condition of the plants.
The symptoms of infected plants changed considerably if kept at 36°. At 36° Nicotiana glutinosa , inoculated with tobacco mosaic virus, gave chlorotic local lesions instead of necrotic ones, and became systemically infected. When systemically infected plants were brought to ordinary glasshouse temperature, the infected tissues all collapsed and died in a day.     

FACTORS AFFECTING THE PRODUCTION OF LOCAL LESIONS BY PLANT VIRUSES

Under uncontrolled glasshouse conditions the number of local lesions produced by a tobacco necrosis virus in beans depended on time of day at which plants were inoculated. Inoculations in the afternoon produced most lesions and inoculations near the end of the night produced least. Tobacco mosaic virus in Nicotiana glutinosa , and lucerne mosaic and turnip mosaic viruses in tobacco showed similar but smaller effects. The variations could affect the results of quantitative experiments with local-lesion counts.     

SEROLOGICALLY RELATED STRAINS OF POTATO VIRUS Y THAT ARE NOT MUTUALLY ANTAGONISTIC IN PLANTS

Tobacco veinal necrosis virus is serologically related to potato viruses Y and C. It does not protect tobacco, Nicotiana glutinosa , or potato plants from infection by them, and tobacco and N. glutinosa plants infected with either virus Y or C are still susceptible to it. There is some evidence that it does not multiply normally in potato plants infected with virus Y and that it is sometimes suppressed in such plants.
The possession of antigenic groups in common with viruses Y and C is considered to justify identifying tobacco veinal necrosis virus as a strain of virus Y , and to be of greater taxonomic significance than failure to protect plants against other strains. A further difference from other strains is that it is more virulent towards tobacco than towards potato.     

HEAT-THERAPY OF VIRUS-INFECTED PLANTS

Virus-free plants were produced from parents systemically infected with the following five viruses: tomato bushy stunt, carnation ring spot, cucumber mosaic, tomato aspermy and Abutilon variegation. The leaves formed while the infected plants were kept at 36°C. were free from symptoms, and test plants inoculated from these remained uninfected. When cuttings were taken from the infected plants at the end of the treatment most grew into healthy plants. The treated plants themselves usually developed symptoms after varying lengths of time at 20°C, but some that before treatment were infected with tomato aspermy, cucumber mosaic or Abutilon variegation viruses, remained permanently healthy.
The same method failed to cure plants infected with tomato spotted wilt, potato virus X and tobacco mosaic virus, although it decreased their virus content. Heat-therapy seems not to be correlated with the thermal inactivation end point of the virus in vitro.     

Identification of soybean mosaic, southern bean mosaic and tobacco ringspot viruses from soybean in the People's Republic of China

Samples of soybean plants with virus-like symptoms were collected from several locations in the People's Republic of China in 1981. These samples were used to prepare inocula for mechanical inoculation to soybean. Twenty-one virus cultures were obtained, the identities of which were determined by serology, symptomatology and host range. Sixteen cultures contained only soybean mosaic virus, four of which were more pathogenic than any previously studied; one culture contained only tobacco ringspot virus, another only southern bean mosaic virus, and three other cultures mixed infections of soybean mosaic and southern bean mosaic viruses. This is the first report of the occurrence of tobacco ringspot virus and southern bean mosaic virus in soybean in the People's Republic of China.     

VIRUSES OF COWPEA, VIGNA UNGUICULATA L. (WALP.), IN NIGERIA

The cowpea strain of tobacco mosaic virus was isolated from a range of leguminous hosts at Ibadan, but was rare in cultivated crops. Systemic symptoms in species infected experimentally are described.
A new virus of cowpea was also found in Nigeria. The physical properties (thermal inactivation point 56° C., dilution end-point 1/50,000 and longevity in vitro 4 days at 25° C.) differ from those for cowpea viruses reported elsewhere and the name cowpea yellow mosaic virus is proposed. This virus produces local lesions in French bean ( Phaseolus vulgaris L.) and local and systemic lesions in Bengal bean ( Mucuna aterrima Holland), but does not infect other leguminous hosts. The virus was purified and an antiserum prepared against it.
Both viruses are transmitted by a beetle ( Ootheca mutabilis Sahlb.) which loses infectivity within 48 hr. of leaving plants infected with either or both viruses.     

Further studies on seed transmission of broad bean stain virus and Echtes Ackerbohnenmosaik -Virus in field beans (Vicia faba)

Broad bean stain virus (BBSV) and Echtes Ackerbohnenmosaik-Virus (EAMV) were detected in the seed coat and embryo sac fluid of immature seeds from infected field beans (Viciafaba minor) by inoculation to Phaseolus vulgaris; BBSV was also detected in immature embryos. The proportion of seeds infected with either virus decreased during maturation. The viruses were transmitted to seedlings as often through fully ripened seeds from which the seed coats had been removed as through intact seeds. Both viruses were detected in pollen from infected plants, but in glasshouse tests only BBSV was transmitted through pollen to seeds. Delaying fertilization in plants infected with BBSV or EAMV seemed not to affect seed transmission of either virus. In glasshouse tests BBSV was transmitted more often through seeds from plants that were inoculated before flowering than during flowering, and was not transmitted through seeds from plants inoculated after flowering; EAMV was transmitted only through seeds from plants inoculated before flowering. In tests on seed from naturally infected plants BBSV was transmitted more often through seeds from plants that developed symptoms before flowering than during flowering. Both viruses were seed-borne in all cultivars tested and there was no marked difference in the frequency of transmission of either virus among the spring-sown cultivars most common in Britain. Both viruses persisted in seed for more than 4 yr.