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.     

Incidence, field spread, seed transmission and effects of broad bean stain virus and Echtes Ackerbohnenmosaik-Virus in Vicia faba in eastern Scotland

In eastern Scotland seed-borne infection with broad bean stain virus (BBSV) and/or Echtes Ackerbohnenmosaik-Virus (EAMV) was detected in five of 39 seed lots of field bean in 1975 and in four of 21 commercial crops of field bean or broad bean sampled in 1975 or 1976. Tests failed to detect the main weevil vector of these viruses, Apion vorax, in 1975 and 1976 but Sitona weevils were found in most crops and were numerous in many, reaching maximum numbers in August. No spread of BBSV and EAMV was detected in commercial crops containing seed-borne infection. In experimental field bean crops containing plants manually inoculated with virus, no virus spread was detected in 1975, and only 0–015% uninoculated plants became infected with EAMV in 1976. Sitona, therefore, was an inefficient vector. The percentage of virus infection in seed harvested from field bean plants manually inoculated 3, 5, 7 and 11 wk after emergence in the field was 1–5, 2–7, 0–4 and 0–06 for BBSV and 0–5, 2-1, 0–6 and 0 for EAMV respectively. Seed harvested from unrogued and rogued plots of field bean grown from seed containing 3–4% seed-borne infection produced 0–05% and no infected plants, respectively. Yield losses in field bean plants manually inoculated with virus before flowering were up to 20% but were much greater in plants infected through the seed. Loss in yield was largely caused by a decrease in number of seeds per pod. The absence of A. vorax, the late arrival of Sitona weevils in the crop and their inefficiency as vectors, and the smaller effects of BBSV and EAMV on crop yield than in southern England appear to make eastern Scotland very suitable for the production of bean seed free from BBSV and EAMV.     

Transmission of broad bean stain virus and Echtes Ackerbohnenmosaik-Virus to field beans (Viciajaba) by weevils

Sitona lineatus and Apion vorax were the two most common species of weevil on field beans (Vicia faba minor) at Rothamsted between 1970 and 1974. In glasshouse tests, A. vorax was a much more efficient vector than 5. lineatus of broad bean stain virus (BBSV) and Echtes Ackerbohnenmosaik-Virus (EAMV), and both species transmitted EAMV more often than BBSV. Five other species of Apion transmitted the viruses infrequently or not at all. S. lineatus adults transmitted no more often after 8–16 days on infected plants than after 1–2 days. Some A. vorax adults transmitted EAMV, but not BBSV, after feeding on infected leaves for a few minutes. After 4 days on infected plants, A. vorax sometimes remained infective for the following 8 days. No A. vorax collected from woodland plants in spring was infective with BBSV or EAMV, but 4% from bean crops containing seed-borne infection carried BBSV and 17% carried EAMV. BBSV and EAMV were recovered from triturated weevils, but not from weevil haemolymph. Possibly the viruses are transmitted as contaminants of the mouthparts or by regurgitation during feeding, but A. vorax was observed to regurgitate only when anaesthetized. BBSV and EAMV were not transmitted by aphids (Aphis fabae and Acyrthosiphon pisum), nor by pollen beetles {Meligethes spp.). Field observations suggest that infected seed is the main source of BBSV and EAMV in spring-sown crops, and that crops grown from virus-free seed, and isolated from infected crops by 250–500 m, remain free of infection for most of the season.     

Tests for transmission of four potato viruses through potato true seed

The Andean potato calico strain of tobacco ringspot virus (TRSV-Ca) was detected in 2–9% of potato seedlings grown from true seed from plants of cv. Cara and clone G5998(6) infected with TRSV-Ca. Similarly, a potato isolate of the oca strain of arracacha virus B (AVB-O) was detected in 4–12% of progeny seedlings of cv. Cara and clone D42/8 infected with AVB-O. Potato virus T (PVT) passed through 33–59% of seed from PVT-infected cv. Cara, but only 0–2% infection was detected in seedlings from seed of PVT-infected clone D42/8. By contrast, no infection was detected in seedlings grown from seed from plants of G5998(6), D42/8 or cv. Cara infected with Andean potato latent virus strains Hu (APLV-Hu) or Caj (APLV-Caj), although both strains passed through seed of Nicotiana clevelandii. AVB-O, PVT and TRSV-Ca were detected in all tests of pollen from flowers of infected potato plants, but APLV-Hu and APLV-Caj were detected less frequently. AVB-O and PVT were transmitted through 2% and 8% respectively, of seed from healthy potato plants pollinated with pollen from infected plants. However, no transmission through seed was detected when pollen from TRSV-Ca infected plants was used. None of the four viruses were transmitted to healthy potato plants pollinated with pollen from infected plants. APLV-Hu caused exceptionally severe symptoms in the cv. Cara plants used for seed production, but the Bolivian strain of PVT induced only mild symptoms rather than the severe systemic necrosis previously reported for the type of strain of PVT in this cultivar. No symptoms developed in potato seedlings infected with TRSV-Ca, AVB-O or PVT through the seed.     

Necrosis in field beans (Vicia faba) induced by interactions between bean leaf roll, pea early-browning and pea enation mosaic viruses

Mixed infections with two or three viruses - bean leaf roll (BLRV), pea early-browning (PEBV) and pea enation mosaic (PEMV) - were detected in plants showing leaf curling, stunting and necrosis in a crop of field beans grown for seed in 1980. In glasshouse tests, field bean plants infected with any one of these viruses showed no necrosis, and plants infected with PEBV and PEMV together showed symptoms of PEMV only. However, mixed infection with BLRV and PEMV almost invariably induced severe stunting and leaf necrosis, and infection with BLRV and PEBV often induced both leaf and stem necrosis and sometimes caused early death. Thus it seems that the necrotic symptoms seen in the field were induced by interactions between BLRV and the other viruses. No transmission of PEBV was detected through seed harvested from the crop, but up to 5% transmission was detected through seed from experimentally-infected plants. The infected seedlings were symptomless.     

Seed-transmission in the ecology of nematode-borne viruses

Virus-free populations of vector nematodes can acquire tomato black ring (TBRV), raspberry ringspot (RRV) and arabis mosaic (AMV) viruses from weed seedlings grown from virus-carrying seed. When soils from fields where nematode-borne viruses occurred naturally were air-dried to kill vector nematodes and then moistened, TBRV and RRV occurred commonly in the weed seedlings that grew, but AMV occurred only rarely. Similar tests did not detect tobacco ringspot, grapevine fanleaf or tobacco rattle viruses in weed seeds in the single soil studied in each instance, although these three viruses are also seed-borne in some of their hosts. Many weed species, when infected experimentally, readily transmit TBRV and RRV to their seed, but the viruses were much commoner in naturally occurring seed of some of these species than of others. These discrepancies between the frequency of seed-transmission of viruses from experimentally infected plants and the extent of natural occurrence of infected seed seem largely to reflect the host preferences of the vectors. Infective Longidorus elongatus kept in fallow soil retained TBRV and RRV only up to 9 weeks. When weed seeds in the soil were then allowed to germinate, the nematodes reacquired virus from the infected seedlings. Some weed species were better than others as sources of virus. Persistence of these viruses in fields through periods of fallow or fasting of the vector therefore depends on a continuing supply of infected seedlings produced by virus-containing weed seeds. This is probably less true of viruses like AMV and grapevine fanleaf, which persist for 8 months or more in their vectors (Xiphinema spp.). A few seeds containing TBRV and RRV were found in soils free of vector nematodes, suggesting that the viruses are disseminated in weed seed. This probably explains how TBRV and RRV have reached a large proportion of L. elongatus populations in eastern Scotland.     

Seed-transmission of nematode-borne viruses

Transmission through seed of crop and weed plants seems to be characteristic of nematode-borne viruses. It occurred with tomato black ring virus (TBRV) in nineteen species (thirteen botanical families), with arabis mosaic virus (AMV) in thirteen species (eleven families), with raspberry ringspot virus (RRV) in six species (five families), and also, in more limited tests, with tomato ringspot, cherry leaf roll and tobacco rattle viruses. A remarkable feature was that infected seedlings, except those containing tobacco rattle virus, often appeared healthy. The occurrence and extent of seed-transmission depended on both the virus and the host plant. In many progenies more than 10%, and in some 100%, of seedlings were infected. The viruses were transmitted through at least two or three generations of seed of those host species tested. After 6 years' storage, TBRV- and RRV-containing seed of Capsella bursa-pastoris and Stellaria media germinated to give infected seedlings. In controlled crossing experiments with strawberry and raspberry, virus was transmitted to seed from both male and female parents but, at least in raspberry, the presence of competing virus-free pollen much decreased the ability of pollen from infected plants to set seed. There was no evidence that healthy mother plants became infected when their flowers were pollinated with infected pollen.     

Properties and relationships of broad bean stain virus and Echtes Ackerbohnenmosaik-Virus

The sedimentation coefficients (^s0₂₀, w) of the two sedimenting nucleoprotein components of broad bean stain virus (BBSV) were 92 S and 113 S, and of Echtes Ackerbohnenmosaik-Virus (EAMV) were 93 S and 114 S. Particles from each of these sedimenting components contained a single RNA species and two polypeptides. Estimates of the molecular weights of these constituents obtained by electrophoresis in polyacrylamide gels were: 42000 and 22200 (BBSV) and 41400 and 21800 (EAMV) for the polypeptides; and 2–64 and 1·62 × 10⁶ (BBSV) and 271 and 175 × 10⁶ (EAMV) for the RNAs. In mixtures, the protein and RNA components of BBSV and EAMV were indistinguishable from those obtained from particles of the yellow strain of cowpea mosaic virus. In freshly made virus preparations each of the sedimenting components of BBSV contained two, and those of EAMV contained three electrophoretic components. After storage for 7–10 days, BBSV preparations contained only the component migrating fastest towards the anode. Both BBSV and EAMV are distantly related serologically to cowpea mosaic but, whereas BBSV reacted only with antiserum to the severe strain, EAMV reacted only with antiserum to the yellow strain.     

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

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

Preliminary studies on sap-transmissible viruses of red clover (Trifolium pratense L.) in England and Wales

Red clover plants, collected from nine widely separated permanent pastures in England and Wales, were tested for sap-transmissible viruses. Viruses were identified by the symptoms they caused in test plants, by electron microscopy, and by serological tests. Of the 265 plants tested 14% were infected. Only pea mosaic virus was common and widespread; it was found in 8% of the plants, and in seven of the fields. Other viruses isolated were arabis mosaic, bean yellow mosaic, red clover mottle, and red clover vein mosaic; only red clover mottle virus produced diagnostic symptoms in red clover. No viruses were detected in seedlings grown from seed from eighty-nine commercial seed crops. Attempts to transmit red clover mottle virus by the Collembolan Sminthurus viridis L., which is common on red clover, failed.     

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.     

Effects of virus genotype and temperature on seed transmission of nepoviruses

Transmission of different nepoviruses through chickweed (Stellaria media) seed was differently affected by ambient temperature during seed production. Raspberry ringspot and tomato black ring (Scottish isolate) viruses were similarly and frequently transmitted at 14 , 18 and 22 ^oC, whereas arabis mosaic virus was transmitted most frequently at 14 ^oC, and strawberry latent ringspot and tomato black ring (German isolate) viruses at 22 ^oC. When infected by seed-borne nepoviruses, seedlings of S. media and other species were symptomless at 15–25 ^oC, and the viruses were therefore detected by inoculating sap to Chenopodium quinoa indicator plants. However, typical symptoms of arabis mosaic and tomato black ring viruses were induced by growing Nicotiana tabacum, N. clevelandii and C. quinoa seedlings infected with seed-borne virus at 33–37 ^oC during the third and fourth weeks after sowing, preceded and followed by periods at 15–25 ^oC. The proportion of N. tabacum seedlings developing symptoms was the same as that of untreated seedlings yielding sap-transmissible virus. Seed transmissibility of pseudo-recombinant isolates of raspberry ringspot and tomato black ring viruses, containing RNA-i from one virus strain and RNA-2 from another strain, depended greatly on the transmissibility of the strain contributing RNA-i. The source of RNA-2 had an additional but smaller influence. The satellite RNA (RNA-3) of tomato black ring virus was seed-transmitted in S. media and its occurrence in cultures did not affect the frequency of transmission of the virus. Results of testing the infectivity of extracts of seed from infected mother plants suggested that failure of seed transmission reflected failure to become established in the seed, not subsequent inactivation. Whereas seed transmissibility of raspberry ringspot virus is primarily dependent on information carried in RNA-i, transmissibility by nematode vectors, another property of major ecological importance, is determined by RNA-2. In the field, selection pressures presumably can act independently on the two parts of the genome but evidence was also obtained of selection for mutual compatibility of RNA-i and RNA-2.     

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.     

THE TRANSMISSION OF PLANT VIRUSES BY BITING INSECTS, WITH PARTICULAR REFERENCE TO COWPEA MOSAIC

Experiments on the virus-vector relationship of the Trinidad cowpea mosaic virus, transmitted by Ceratoma ruficornis , gave the following results: ability to infect decreased with increasing time after ceasing to feed on infected plants, but vectors remained infective for 14 days (much longer than the longevity in vitro of the virus at glasshouse shade temperatures of 23–31°C.); the beetles transmitted more consistently after longer feeding on infected plants, though feeds of under 5 min. made them efficient vectors; the proportion of plants infected increased with the amount of feeding damage on them; fasting the vectors before feeding on infected plants increased voracity but had no effect on their ability to transmit; beetles became infective immediately after feeding on infected plants. Cowpeas were infected by inoculation with macerated infective vectors or with juice regurgitated by vectors. There is no evidence that aphids or other sucking insects can transmit the virus. It seems similar to squash mosaic and turnip yellow mosaic, for vectors of all three viruses probably transmit by regurgitating infective juice during feeding.     

Resistance to six viruses in the legume goat's rue {Galega orientalis Lam.)

Plants from 2nd to 6th year leys of the legume goat's rue (Galega orientalis Lam.) were tested for infection with bean yellow mosaic (BYMV), bean common mosaic (BCMV), alfalfa mosaic (AMV), broad bean stain (BBSV), red clover mottle (RCMV) and cucumber mosaic (CMV) viruses by enzyme-linked immunosorbent assay (ELISA), electron microscopy, and by sap-inoculation to various test plant species. No virus infections were observed in goat's rue in the field. Glasshouse-grown seedlings of goat's rue were inoculated with the above viruses. No virus was detected in the inoculated plants. The results suggest that goat's rue is extremely resistant to the above six viruses which are important in other forage legumes.     

Host range, purification and properties of potato virus T

Potato virus T (PVT) infected nine species of tuber-bearing Solanum, most of them symptomlessly, and as a rule was transmitted through the tubers to progeny plants: two genotypes of S. tuberosum ssp. andigena were not infected. The virus was also transmitted by inoculation with sap to 37 other species in eight plant families. Chenopodium amaranticolor is useful as an indicator host, C quinoa as a source of virus for purification, and Phaseolus vulgaris as a local-lesion assay host; the systemic symptoms in Datura stramonium, Nicotiana debneyi and in these three species are useful for diagnosis. Attempts to transmit PVT by aphids failed, but the virus was transmitted through seed to progeny seedlings of four solanaceous species, and from pollen to seed of S. demissum. PVT was purified by clarifying sap with n-butanol or bentonite, followed by precipitation with polyethylene glycol, differential centrifugation and sedimentation in a sucrose density gradient. Purified preparations had an E260/E280 ratio of 1.18 and contained a single infective component with a sedimentation coefficient of 99 S. This component consisted of flexuous filamentous particles of about 640 times 12 nm that showed a characteristic substructure when stained with uranyl acetate. The virus particles contained a single species of infective single-stranded RNA, of molecular weight 2–2 times 106 daltons, and a single species of polypeptide of molecular weight about 27 000 daltons. PVT is serologically related to apple stem grooving virus but not to four other common potato viruses with flexuous filamentous particles. Apple stem grooving virus and PVT cause similar symptoms in several hosts, but also differ somewhat in host range and symptomatology. Apple stem grooving virus did not infect potato, caused additional symptoms in C. quinoa also infected with PVT, and its particles did not show the structural features specific to PVT. The two viruses are considered to be distinct. The cryptogram of PVT is R/1:2–2/(5): E/E: S/C.     

Interactions between a seed-borne strain of cucumber mosaic cucumovirus and its lupin host

The relationship between time of inoculation with cucumber mosaic cucumovirus (CMV) and the growth, seed production and rate of seed transmission of virus in lupin (Lupinus angustifolius cv. Illyarrie) was studied in field-grown plants. Plants inoculated at the seedling stage (2 days post-emergence) showed 45% mortality. Plants infected through the seed were more stunted than plants inoculated at the seedling stage. Plants inoculated up to the mid-vegetative growth stage (58 days post-emergence) yielded ≤ 27% of the dry matter and ≤ 9% of the seed of healthy plants. Late inoculation (114 days post-emergence) did not affect dry matter yield, but reduced seed yield to 75% of that of healthy plants. Rate of seed transmission depended on the time of inoculation of plants. The maximum rate was 24.5% for plants that were inoculated at the mid-vegetative growth stage (58 days post-emergence). However, early inoculation caused a large reduction in seed yield, and it was shown that plants inoculated at the beginning of flowering (94 days post-emergence) produced greater numbers of infected progeny than plants inoculated at earlier or later times. No relationship was observed between seed weight and transmission of CMV. Infectious CMV was recovered from the embryo, but not from the testa. A simple seed transmission model was used to evaluate several hypothetical epidemics and to determine the time of inoculation which results in greatest rates of seed transmission of CMV. For example, when fewer than 73% of plants in a crop become infected with CMV, then the rate of transmission of virus in crop seeds will be greatest when inoculations are at the beginning of flowering.     

Comparative transmission of bean leaf roll and pea enation mosaic viruses by aphids

Acyrthosiphon pisum was a more efficient vector than Myzus persicae of bean leaf roll virus (BLRV), but the two species transmitted pea enation mosaic virus (PEMV) equally well and much more often than Megoura viciae. M. viciae did not transmit BLRV, and Aphis fabae did not transmit BLRV or PEMV. BLRV and PEMV were transmitted more often by nymphs of A. pisum than by adult apterae or alatae that fed on infected plants only as adults, but both viruses were readily transmitted by adults that had developed on infected plants. The shortest time in which nymphs acquired BLRV was 2 h, and 50 % transmitted after an acquisition period of 4 days. Some nymphs acquired PEMV in 30 min and 50% in 8 h. The shortest time for inoculation of BLRV by adults was 15 min, but some transmitted PEMV in probes lasting less than 1 min. The median latent periods of BLRV and PEMV in aphids fed for 12 h on infected plants were, respectively, 105 and 44 h. Clones of A. pisum differed in their ability to transmit BLRV and PEMV, and efficiency in transmitting the two viruses seemed to be unrelated. Some aphids that fed successively on plants infected with each virus transmitted both viruses, and infectivity with one virus did not seem to affect transmission of the other.     

Further studies on seed transmission in the ecology of some aphid-transmitted viruses

A small proportion (1–4%) of the seeds of Stellaria media extracted from fallow soil from three widely separated areas contained cucumber mosaic virus (CMV). S. media seeds buried for 21 months produced 5 % infected seedlings. S. media plants from Britain, N. America and Australia were least severely affected by the CMV strain obtained from their country of origin and showed more severe reactions when infected with two alien strains. Several weed species were experimentally infected with lettuce mosaic, turnip mosaic and cauliflower mosaic viruses but, although virus was detected in the seeds of some species, it was not transmitted to any of their seedlings.     

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.