Determining the effectiveness of resistance to subterranean clover mottle sobemovirus in different genotypes of subterranean clover in the field using the grazing animal as virus vector

The effectiveness of resistance to subterranean clover mottle sobemovirus (SCMoV) previously identified in different genotypes of subterranean clover (Trifolium subterraneum) inoculated with infective sap in the glasshouse, was tested in two field experiments which used the grazing animal as virus vector. Replicated plots each consisting of paired test rows of 20 different genotypes were used. Clover plants infected with SCMoV were transplanted in between the paired test rows and these acted as sources of the virus for spread by grazing sheep. Although used in different years at different sites with different virus isolates, the field exposure methodology employed produced consistent results. The genotypes each behaved similarly in both experiments as regards the relative extents of SCMoV infection that developed, levels ranging from 0–98%. The previously identified resistance in six ‘highly resistant’ and three ‘partially resistant’ cultivars was effective under field conditions. However, the ‘partial resistance’ in three others was overcome, cvs Green Range and Mt Barker developing levels of infection approaching those in ‘susceptible’ cultivars, while an intermediate infection level developed in cv. Karridale. The three cultivars in which partial resistance was not effective all belonged to ssp. subterraneum. In subterranean clover breeding programmes, field screening using the grazing animal as a vector is advisable to determine whether SCMoV resistance found by sap inoculation is still effective under field conditions.     

Determining the effectiveness of grazing and trampling by livestock in transmitting white clover mosaic and subterranean clover mottle viruses

Glasshouse and mini-sward experiments were done to determine the relative roles of grazing and trampling by livestock in transmitting white clover mosaic (WC1MV) and subterranean clover mottle (SCMoV) viruses between clover plants in pastures. Wounding due to grazing was simulated by repeatedly cutting plants with serrated scissors (glasshouse) or mowing (mini-swards), while wounding due to trampling was simulated by repeatedly bashing plants with the flat end of a wooden hammer handle (glasshouse) or rolling (mini-swards). In glasshouse experiments, cutting was more effective than bashing in transmitting WC1MV to white clover (Trifolium repens) plants but cutting and bashing transmitted it to subterranean clover (T. subterraneum) plants at similar rates. In an experiment with white clover mini-swards, mowing was more effective than rolling in transmitting WC1MV, and when both were combined, initially spread exceeded that obtained when the spread from mowing and rolling alone was added together. In glasshouse experiments, bashing was more effective than cutting in transmitting SCMoV to subterranean clover plants. In one experiment, neither mowing nor rolling spread SCMoV in mini-swards of subterranean clover. When transmission to subterranean clover cultivars which were ‘susceptible’ or ‘moderately susceptible’ to SCMoV was compared in glasshouse experiments, repeated bashing spread the virus more slowly to the ‘moderately susceptible’ cultivars. When mixed with ruminant saliva, infective sap containing WC1MV or SCMoV was still infective to clover plants after 4 wk storage at room temperature. When infective sap was allowed to dry naturally on a metal surface, SCMoV still infected clover plants when the dried sap was taken up in tap water after 4 but not 14 days, while WC1MV was infective after 24 h but not 4 days. These results suggest that grazing and mowing are more effective than trampling at transmitting WC1MV to white clover plants in pastures, while trampling is more effective at spreading SCMoV to subterranean clover. However, both transmitted WC1MV to subterranean clover at similar rates. Possible reasons for these differences are discussed in relation to differences in clover plant morphology and virus-specific factors.     

Seed-borne cucumber mosaic virus infection of subterranean clover in Western Australia

In Western Australia, infection with cucumber mosaic virus (CMV) was widespread in all three subspecies of subterranean clover (Trifolium subterraneum) growing in plots belonging to the Australian National Subterranean Clover Improvement Programme. Seed-borne CMV was detected in seed harvested in 1984–1986 of 18/25 cultivars from two collections of registered cultivars; seed transmission rates ranged up to 8.8%. Seed samples from CMV-inoculated plants of 11 cultivars transmitted the virus to 0.5–8.7% of seedlings. Seed transmission rates greater than 5% were obtained only with cvs Enfield, Green Range and Nangeela. CMV was not detected in seed harvested in 1975–1981 from one of the registered cultivar collections, in 17 commercial seed stocks from 1986 or in a survey of subterranean clover pastures.
Symptoms in subterranean clover naturally infected with CMV included mottle, leaflet downcurling and dwarfing but severity varied with cultivar and selection. CMV isolates from different sources varied in virulence when inoculated to subterranean clover; two (both from subterranean clover) were severe, two moderate and three (including one from subterranean clover) mild. In pot tests, CMV decreased herbage production and root growth (dry wts) of cv. Green Range by 49% and 59% respectively. In spaced-plants growing in plots, CMV decreased herbage production and root growth of cvs Green Range and Northam by 59–630 and seed production of cv. Green Range by 45%. In rows sown with infected seed, aphid spread increased infection levels to 75% in cv. Green Range and 44% in cv. Esperance and losses in herbage production of 42% and 29% respectively were recorded.
CMV isolated from subterranean clover included isolates from both serogroups.     

Factors affecting aphid acquisition and transmission of soybean mosaic virus

Myzus persicae transmitted soybean mosaic virus (SMV) most efficiently following 30 or 60 s acquisition probes on infected plants. There were no differences in susceptibility to SMV infection of soybean plants 1 to 12 wk old, but symptoms were more severe in plants inoculated when young than when old. Soybeans inoculated between developmental stages R3 and R6 only showed yellowish-brown blotching on one or more leaves. There were no observable differences in the time of appearance or type of symptoms shown by soybean seedlings inoculated either by sap or by aphids; infected plants became acquisition hosts for aphids 5–6 days after inoculation. There was no change in the efficiency with which M. persicae transmitted SMV from source plants up to 18 wk after inoculation. M. persicae transmitted SMV from leaves of field-grown soybeans when plants were inoculated at developmental stages V6, R2, and R3 and tested as sources 57–74 days after inoculation but not from plants inoculated at R5 and tested as sources 14 to 32 days after inoculation. M. persicae acquired SMV from soybean buds, flowers, green bean pods, and unifoliolate, trifoliolate, and senescent leaves. Middle-aged and deformed leaves were better sources of the virus than buds, unfolding and old symptomless leaves. The results are being incorporated into a computer model of SMV epidemiology.     

Effects of Temperature on Disease Severity in Plants of Subterranean Clover Infected Singly or in Mixed Infection with Bean yellow mosaic virus and Kabatiella caulivora

Many epidemics involve plants infected with more than one pathogen, but few experiments address climate change scenarios that influence mixed infections. This study addresses the interactive effects of co‐infection and temperature on disease development in plants of the annual pasture species subterranean clover (Trifolium subterraneum), which is widely sown in different world regions. Bean yellow mosaic virus (BYMV) and the fungus Kabatiella caulivora are two important pathogens causing considerable production losses in pastures containing this species. Both occur together in such pastures causing a severe necrotic disease when mixed infection occurs. Effects of temperature on symptom expression were investigated in subterranean clover plants infected singly or in mixed infection with these pathogens. Plants were maintained in controlled environment rooms at 18°C, 20°C or 22.5°C after sap inoculation with BYMV. K. caulivora conidia suspensions were inoculated to plants once systemic BYMV symptoms developed. Plants were assessed for three disease assessment parameters, dead petioles numbers, marginal leaflet necrosis and overall plant damage. In general, mixed infection caused most severe symptoms, K. caulivora least severe symptoms, and BYMV symptoms of intermediate severity. In single infections, effects of temperature on disease severity differed between pathogens: BYMV symptoms were most pronounced at 18°C, but K. caulivora induced more severe symptoms at 20°C and 22.5°C. In mixed infections, disease severity generally followed the pattern developed with BYMV alone as temperature increased. Also, synergistic increase in disease severity sometimes occurred at 18°C, but increases were only additive at 20°C and 22.5°C. These results reflected the greater BYMV multiplication detected in infected leaves at 18°C compared with 20°C or 22.5°C. Our findings indicate that in rainfed subterranean clover pastures, as global warming progresses disease severity from infection with BYMV and K. caulivora alone may decline or increase, respectively, and mixed infection with them may become less damaging.     

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.     

Properties of a resistance-breaking strain of potato virus X

During indexing of a potato germplasm collection from Bolivia, a strain of potato virus X (PVX), X_HB, which failed to cause local lesions in inoculated leaves of Gomphrena globosa was found in 7% of the clones. X_HB was transmitted by inoculation of sap to 56 species from 11 families out of 64 species from 12 families tested. It was best propagated in Nicotiana glutinosa or N. debneyi; Montia perfolia and Petunia hybrida were useful as local lesion hosts. Inoculated leaves of G. globosa plants kept at 10°, 14°, 18°, 22°, or 26 °C after inoculation were always infected symptomlessly. X_HB caused a mild mosaic, systemic chlorotic blotching or symptomless infection in 16 wild potato species and eight Andean potato cultivars, systemic necrotic symptoms in clone A6 and cultivar Mi Peru, and bright yellow leaf markings in cultivar Renacimiento. It caused necrotic local lesions in inoculated leaves of British potato cultivars with the PVX hypersensitivity gene Nb but then invaded the plants systemically without causing further necrosis; with gene Nx systemic invasion occurred but no necrotic symptoms developed. These reactions resemble those of PVX strain group four. X_HB differed from other known strains of PVX in readily infecting PVX-immune clones 44/1016/10, G. 4298.69 and USDA 41956, cultivars Saphir and Saco, and Solanum acaule PI 230554. X_HB had slightly flexuous filamentous particles with a normal length of 516 nm. It was transmitted readily by plant contact and it partially protected G. globosa leaves from infection with X_CP, a group two strain of PVX. Sap from infected N. glutinosa was infective after dilution to 10^--⁶ but not 10^--⁷ after 10 min at 75° but not 80 °C and after 1 yr at 20 °C. X_HB was readily purified from infected N. debneyi leaves by precipitation with polyethylene glycol followed by differential centrifugation. Microprecipitin tests showed that X_HB and X_CP are closely related serologically.     

Crimson clover latent virus - a newly recognised seed-borne virus infecting crimson clover (Trigolium incarnatum)

Crimson clover latent virus (CCLV) was detected in five seed lots of crimson clover (Trifolium incarnatum) from Europe and in one from the United States of America. Ninety-seven per cent of all crimson clover plants examined were found to be infected but were without symptoms. Keeping crimson clover plants at 32–38°C for 34 days failed to free them from CCLV. The virus was not transmitted by Myzus persicae, but was transmitted by inoculation of sap to Chenopodium album, C. amaranticolor and C. quinoa. Twenty-four other plant species from seven families were not infected. CCLV was best propagated in C. quinoa in which it caused stunting and systemic chlorosis. Sap from infected C. quinoa was infective after dilution to 10^-2 but not 10^-3, after 10 min at 60°C but not 65°C, and after 20 days at 20°C. In neutral phosphotungstate, CCLV had isometric particles c. 26 nm in diameter with a hexagonal profile. About 20 to 80 A_1cm,260 units of purified virus were obtained from 1 kg of infected C. quinoa or C. amaranticolor leaves by extraction in 0.5 M phosphate buffer, pH 7.5, containing 0.01 M ethylene diamine tetra-acetate and 0.4% 2–mercaptoethanol and clarification with chloroform-butanol followed by two precipitations with polyethylene glycol (mol. wt 6000) and several cycles of differential centrifugation. Purified virus sedimented as three components with sedimentation coefficients (s°_{20, w}) of 52S, 101S and 122S. The 101S and 122S components had buoyant densities in CsCl of 1.438 and 1.495 g/cm³ respectively. From these values the nucleic acid content of the 101S and 122S components was estimated to be 32–35% and 40–41% respectively. The virus contained a single protein with an estimated mol. wt of 52 000 and two single-stranded RNA species of estimated mol. wt 1.6 × 10⁶ and 2.2 × 10⁶. CCLV was serologically unrelated to 31 other morphologically similar viruses. Although its vector is unknown, CCLV seems to have affinities with nepoviruses. The cryptogram of CCLV is R/1:2.2/40–41 + 1.6132–35:S/S:S/*.     

Characterisation of a labile RNA virus-like agent from white clover

A labile virus has been identified in white clover in New Zealand. The virus was mechanically transmitted to nine species of herbaceous test plants. No virus-like particles were identified by electron microscopy in ultrathin sections or in negatively stained sap extracts, although in infected Chenopodium quinoa there were prominent membraneous inclusion bodies in the cell cytoplasm and membrane-bound structures c. 50 nm in diameter associated with the tonoplast in cell vacuoles. Double-stranded RNA species of approximately 6800, 3500 and 3300 bp were isolated from infected tissues. DsRNA denatured by boiling was infectious to C. quinoa, but undenatured dsRNA was not infectious. Total nucleic acid preparations from infected leaves were highly infective without boiling, indicating that most of the infectivity was single-stranded RNA. Infectivity was recovered in the poly (A)^- faction following oligo (dT)-cellulose chromatography, indicating that the RNA probably lacks a 3′ tract of poly (A). The labile white clover virus is tentatively named white clover virus L (WCIVL).     

The occurrence and effects of red clover necrotic mosaic virus in red clover (Trffbliumpratense)

In 1976, red clover necrotic mosaic virus (RCNMV) was identified in red clover variety trials at the Scottish Colleges of Agriculture and at the trial centres of the National Institute of Agricultural Botany (NIAB) in Northumberland, Dyfed, Devon and Cambridge. In 1977, RCNMV was also found in two commercial crops of red clover in South Wales. The only previous finding of this virus in Britain was in 1971.
In red clover leaves RCNMV causes veinal chlorosis, often followed by severe necrosis and deformation; the plants become stunted. All cultivars tested were infected either in field or glasshouse experiments and three of the four most susceptible cultivars were tetraploids. Yield losses in cv. Hungaropoly averaged 57% over three cuts. RCNMV was transmitted manually but not through seed or by aphids {Acyrthosiphon pisum and Myzus persicae) or weevils (Apion spp. and Sitona lineatus). Seedlings became infected when grown in pots containing RCNMV-infected plants or soil from infected sites, and the roots of infected test seedlings contained an Olpidium sp. which may be the vector.
White clover mosaic virus (WCMV), also common in red clover at some sites, was less damaging than RCNMV and in a glasshouse experiment decreased yield by only 22%. An unidentified seed-borne virus with spherical particles c. 33 nm in diameter was the only virus detected in clover seedlings screened for RCNMV.     

Selection, biological properties and fitness of resistance-breaking strains of Tomato spotted wilt virus in pepper

In glasshouse tests, infective sap from plants infected with 17 different isolates of Tomato spotted wilt virus (TSWV) from four Australian states was inoculated to three Capsicum chinense accessions (PI 152225, PI 159236 and C00943) carrying single genes that confer hypersensitive resistance to TSWV. The normal response to inoculation was development of necrotic (hypersensitive) local lesions in inoculated leaves without systemic invasion, but 3/1386 infected plants also developed systemic susceptible reactions in addition to hypersensitive ones. Similarly when two isolates were inoculated to C. chinense backcross progeny plants, 1/72 developed systemic susceptible reactions in addition to localised hypersensitive ones. Using cultures from the four plants with susceptible reactions and following three to five further cycles of serial subculture in TSWV‐resistant C. chinense plants, four isolates were obtained that gave systemic susceptible type reactions in the three TSWV‐resistant accessions, and in TSWV‐resistant cultivated pepper (C. annuum). When three of these isolates were inoculated to tomato (Lycopersicon esculentum) breeding lines with single gene resistance to TSWV, resistance was not overcome. Similarly, none of the four isolates overcame partial resistance to TSWV in Lactuca virosa. When TSWV isolates were inoculated to tomato breeding lines carrying partial resistance from L. chilense, systemic infection developed which was sometimes followed by ‘recovery’. After four successive cycles of serial passage in susceptible cultivated pepper of a mixed culture of a resistance‐breaking isolate with the non resistance‐breaking isolate from which it came, the resistance‐breaking isolate remained competitive as both were still found. However, when the same resistance‐ breaking isolate was cultured alone, evidence of partial reversion to wild‐type behaviour was eventually obtained after five but not four cycles of long term serial subculture in susceptible pepper, as by then the culture had become a mixture of both types of strain. This work suggests that resistance‐breaking strains of TSWV that overcome single gene hypersensitive resistance in pepper are relatively stable. The findings have important implications for situations where resistant pepper cultivars are deployed widely in the field without taking other control measures as part of an integrated TSWV management strategy.     

Natural resistance to cucumber mosaic virus in lupin species

Ten species of lupins (Lupinus spp.) were tested for resistance to cucumber mosaic cucumovirus (CMV) in field experiments where inoculation was by naturally-occurring aphid vectors, and in the glasshouse by sap or graft-inoculation. L. albus and six species of ‘rough-seeded’ lupins did not become infected with CMV either under intense inoculum pressure in the field or when graft-inoculated. Two L. hispanicus, 17 L. luteus and four L. mutabilis genotypes became infected with CMV in the field, but no infection was detected in L. hispanicus P26858 or seven L. luteus genotypes. CMV was detected at seed transmission rates of 0.2–16% in seedlings of infected L. luteus, differences in levels of seed transmission between genotypes being significant and relatively stable from year to year. Graft-inoculation of CMV to plants of six genotypes of L. luteus in which no infection was found in the field induced a systemic necrotic reaction suggesting that the resistance they carry is due to hypersensitivity. In L. hispanicus accessions P26849, P26853 and P26858, CMV sub-group II isolate SN caused necrotic spots in inoculated leaves without systemic movement, while sub-group I isolate SL infected them systemically without necrosis. Another sub-group I and two other sub-group II isolates behaved like SL in P26849 and P26853 but infected only inoculated leaves of P26858. This suggests that two strain specific hypersensitive resistance specificities are operating against CMV in L. hispanicus. When plants of L. luteus genotypes that gave hypersensitive reactions on graft-inoculation were inoculated with infective sap containing two sub-group I and seven sub-group II isolates, they all responded like L. hispanicus P26858. A strain group concept is proposed for CMV in lupins based on the two hypersensitive specificities found: strain group 1 represented by isolate SN which induces hypersensitivity with both specificities, strain group 2 represented by the three isolates which induced hypersensitivity only with the specificity present in L. luteus and L. hispanicus P26858, strain group 3 by as yet hypothetical isolates that induce hypersensitivity only in presence of the specificity in L. hispanicus P26849 and P26853 that responded just to isolate SN, and strain group 4 by isolate SL which overcomes both specificities. When F₂ progeny plants from crosses between hypersensitive and susceptible L. luteus parents were inoculated with isolate SN, the resistance segregated with a 3:1 ratio (hypersensitive:susceptible), suggesting that a single dominant hypersensitivity gene, Ncm-1, is responsible. As gene Ncm-1 had broad specificity and was not overcome by any of the five CMV isolates from lupins tested, it is valuable for use in breeding CMV resistant L. luteus cultivars.     

Host range and some properties of potato mop-top virus

Potato mop-top virus (PMTV) was transmitted by inoculation of sap to twenty-six species in the Solanaceae or Chenopodiaceae and to Tetragonia expansa; species in eleven other plant families were not infected. The virus was cultured in inoculated leaves of Nicotiana tabacum cv. Xanthi-nc or in N. debneyi. Diagnostic local lesions were produced in Chenopodium amaranticolor. In winter, ten solanaceous species were slowly invaded systemically but the first leaves infected were those immediately above inoculated leaves. When transmitted to Arran Pilot potato by the vector Spongospora subterranea, PMTV induced all the main types of shoot and tuber symptoms found in naturally infected plants. Isolates of PMTV from different sources differed considerably in virulence. PMTV-containing tobacco sap lost infectivity when heated for 10 min at 80 °C, diluted to 10^-4, or stored at 20 °C for 14 weeks. Infectivity was partially stabilized by 0·02% sodium azide. When sap was centrifuged for 10 min at 8000 g, infectivity was mainly in the sediment. Infective sap contained straight rod-shaped particles about 20 nm wide, with lengths up to 900 nm and crossbands at intervals of 2·5 nm. Many of the particles were aggregated side-to-side, and the ends of most seemed damaged. The slight infectivity of phenol-treated leaf extracts was abolished by pancreatic ribonuclease. The present cryptogram of PMTV is R/*:*/*:E/E:S/Fu.     

Natural resistance to cucumber mosaic virus in lupin species

Ten species of lupins (Lupinus spp.) were tested for resistance to cucumber mosaic cucumovirus (CMV) in field experiments where inoculation was by naturally-occurring aphid vectors, and in the glasshouse by sap or graft-inoculation. L. albus and six species of ‘rough-seeded’ lupins did not become infected with CMV either under intense inoculum pressure in the field or when graft-inoculated. Two L. hispanicus, 17 L. luteus and four L. mutabilis genotypes became infected with CMV in the field, but no infection was detected in L. hispanicus P26858 or seven L. luteus genotypes. CMV was detected at seed transmission rates of 0.2–16% in seedlings of infected L. luteus, differences in levels of seed transmission between genotypes being significant and relatively stable from year to year. Graft-inoculation of CMV to plants of six genotypes of L. luteus in which no infection was found in the field induced a systemic necrotic reaction suggesting that the resistance they carry is due to hypersensitivity. In L. hispanicus accessions P26849, P26853 and P26858, CMV sub-group II isolate SN caused necrotic spots in inoculated leaves without systemic movement, while sub-group I isolate SL infected them systemically without necrosis. Another sub-group I and two other sub-group II isolates behaved like SL in P26849 and P26853 but infected only inoculated leaves of P26858. This suggests that two strain specific hypersensitive resistance specificities are operating against CMV in L. hispanicus. When plants of L. luteus genotypes that gave hypersensitive reactions on graft-inoculation were inoculated with infective sap containing two sub-group I and seven sub-group II isolates, they all responded like L. hispanicus P26858. A strain group concept is proposed for CMV in lupins based on the two hypersensitive specificities found: strain group 1 represented by isolate SN which induces hypersensitivity with both specificities, strain group 2 represented by the three isolates which induced hypersensitivity only with the specificity present in L. luteus and L. hispanicus P26858, strain group 3 by as yet hypothetical isolates that induce hypersensitivity only in presence of the specificity in L. hispanicus P26849 and P26853 that responded just to isolate SN, and strain group 4 by isolate SL which overcomes both specificities. When F₂ progeny plants from crosses between hypersensitive and susceptible L. luteus parents were inoculated with isolate SN, the resistance segregated with a 3:1 ratio (hypersensitive:susceptible), suggesting that a single dominant hypersensitivity gene, Ncm-1, is responsible. As gene Ncm-1 had broad specificity and was not overcome by any of the five CMV isolates from lupins tested, it is valuable for use in breeding CMV resistant L. luteus cultivars.     

Age-related Resistance in Bell Pepper to Cucumber mosaic virus

We demonstrated the occurrence of mature plant resistance in Capsicum annuum‘Early Calwonder’ to Cucumber mosaic virus (CMV) under greenhouse conditions. When Early Calwonder plants were sown at 10 day intervals and transplanted to 10‐cm square pots, three distinct plant sizes were identified that were designated small, medium and large. Trials conducted during each season showed that CMV accumulated in inoculated leaves of all plants of each size category. All small plants (with the exception of the winter trial) developed a systemic infection that included accumulation of CMV in uninoculated leaves and severe systemic symptoms. Medium plants had a range of responses that included no systemic infection to detection of CMV in uninoculated leaves with the systemically infected plants being either symptomless or expressing only mild symptoms. None of the large plants contained detectable amounts of CMV in uninoculated leaves or developed symptoms. When plants were challenged by inoculation of leaves positioned at different locations along the stem or different numbers of leaves were inoculated, large plants continued to accumulate CMV in inoculated leaves but no systemic infection was observed. When systemic infection of large plants did occur, e.g. when CMV‐infected pepper was used as a source of inoculum, virus accumulation in uninoculated leaves was relatively low and plants remained symptomless. A time‐course study of CMV accumulation in inoculated leaves revealed no difference between small and large plants. Analyses to examine movement of CMV into the petiole of inoculated leaves and throughout the stem showed a range in the extent of infection. While all large plants contained CMV in inoculated leaves, some had no detectable amounts of virus beyond the leaf blade, whereas others contained virus throughout the length of the stem but with limited accumulation relative to controls.     

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.     

Arabis mosaic and Prunus necrotic ringspot viruses in hop (Humulus lupulus L.)

Purified virus preparations made from nettlehead-diseased hop plants, or from Chenopodium quinoa, to which the virus was transmitted by inoculation of sap, contained polyhedral virus particles of 30 mμ diameter which were identified serologically as arabis mosaic virus (AMV). There were serological differences between AMV isolates from hop and from strawberry, and also differences in host range and in symptoms caused in C. quinoa and C. amaranticolor. AMV was always associated with nettlehead disease. The nematode Xiphinema diversicaudatum occurred in small numbers in most hop gardens, but was numerous where nettlehead disease was spreading rapidly. Preparations from nettlehead-affected hops also contained a second virus, serologically related to Prunus necrotic ringspot virus (NRSV), in mild and virulent forms which infected the same range of test plants but showed some serological differences. Mild isolates did not protect C. quinoa plants against infection by virulent isolates. Hop seedlings inoculated with virulent isolates of NRSV developed symptoms indistinguishable from those of split leaf blotch disease. Latent infection with NRSV was prevalent in symptomless hop plants. Nettlehead disease is apparently associated with dual infection of AMV and virulent isolates of NRSV. An unnamed virus with rod-shaped particles 650 mμ long was common in hop and was transmitted by inoculation of sap to herbaceous plants. Cucumber mosaic virus was obtained from a single plant of Humulus scandens Merr.     

Symptom development and distribution of Tomato spotted wilt virus in flue-cured tobacco

Tomato spotted wilt virus (TSWV) is an economically important viral pathogen of flue‐cured tobacco, Nicotiana tabacum. Disease development and in planta distribution of TSWV were studied following mechanical inoculation of cv. K326 at various stages of growth. The effect of plant age on the disease development, distribution of symptoms and TSWV were studied by inoculating plants in five age groups, 40, 60, 75, 95 and 100 days after sowing (DAS). The plant age at the time of infection had no significant influence on the incidence of localised infection; however, it had a significant effect on the development of systemic symptoms and distribution of TSWV in the plant. In a higher proportion of plants (89.2%), no systemic symptoms developed when plants were inoculated at 60–100 DAS. However, 90% of plants became systemically infected when plants were inoculated at 40 DAS. The systemic symptom expression was severe and distributed in all the leaves in 40‐DAS plants, whereas in 60‐ to 100‐DAS plants, it was erratic and restricted only to a few upper leaves. Results show that plant age is an important factor for TSWV infection of tobacco and mature tobacco plants significantly reduced the systemic development of the disease.     

Infective behaviour and aphid-transmissibility of Italian isolates of potato virus Y in tobacco and peppers

When mechanically inoculated to susceptible tobacco (Nicotiana tabacum L.) cultivars, nine isolates of PVY from Umbria (Central Italy) and two from Southern Latium gave rise to rapid systemic infection which developed within 6–8 days after inoculation. Systemic spread of the same isolates was slower, or much slower, in infected pepper (Capsicum annuum L.) cultivars, 8–14 days for Southern Latium isolates and 20 - 35 days for Umbrian ones. Aphid (Myzus persicae)-moculation of pepper and tobacco plants with two of the Umbrian and one of the Southern Latium isolates confirmed the results from sap-transmission and showed that fewer inoculated pepper plants become infected, especially with Umbrian isolates. In agreement with the data on systemic spread, aphid-acquisition trials indicated that tobacco plants became efficient PVY sources for vectors 6–8 days after inoculation with either group of isolates. Peppers became efficient acquisition hosts 8–15 days after inoculation with Southern Latium isolates but not until 22–45 days after inoculation with Umbrian ones. Southern Latium isolates induced more severe symptoms in pepper cultivars than Umbrian isolates did. One of the Southern Latium isolates was able to systemically infect the resistant pepper cv. Yolo Y, which was never infected by the Umbrian isolates. The Umbrian isolates tested seem to be better adapted to tobacco than peppers, while Southern Latium ones are well adapted to both.     

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.