Studies on the seed transmission of cucumber mosaic virus in chickweed (Stellaria media) in relation to the ecology of the virus

Cucumber mosaic virus (CMV) was transmitted in the seed of infected Stellaria media plants. The rate of seed transmission varied both in manually infected plants (3–21%) and in plants grown from infected seed (21–40%). In naturally infected plants the rates of transmission found were 4–29%. Seeds recovered from field soil carried 4–5% infection and in infected seed placed in the soil the virus persisted for at least 5 months. Seed transmission of CMV also occurred in infected Lamium purpureum (4%), Cerastium holosteoides (2%) and Spergula arvensis (2%) but it could not be demonstrated in six other more common weed species in five botanical families. Seed transmission in Stellaria media occurred with a British (W) and an American (Y) strain of CMV. The virus was shown to occur in S. media pollen. The importance of CMV-infected S. media seed in the soil in relation to the epidemiology of the virus is discussed.     

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.     

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.     

Heat inactivation of cucumber mosaic virus in cultured tissues of Stellaria media

In vivo heat inactivation of cucumber mosaic virus (CMV) was studied in a low-temperature-tolerant species, Stellaria media. Infectivity remained high in infected 5. media cultures grown at 25 °C or less, but could not be detected in cultures incubated at 32 °C for 28 days. When infected tissues were grown at 12–32 °C for 24 days, virus was still detectable at a low level in cultures incubated at 30 °C. The highest level of infectivity was in cultures at 12 °C.     

Occurrence of latent virus infection in visually-rated cowpea (Vigna unguiculata L. Walp) seedlings

Abstract

The presence of latent infections was studied in five cowpeas varieties. Seeds of the varieties were planted and the seedlings inoculated with antigens from Cucumber mosaic virus (CMV) genus Cucumovirus, Bean common mosaic virus (BCMV) genus Potyvirus (Blackeye cowpea mosaic virus strain), Southern bean mosaic virus (SBMV) genus Sobemovirus and Cowpea mottle virus (CPMoV) genus Carmovirus seven days after planting. Seedlings expressing symptoms were rouged at two weeks after inoculation, while asymptomatic ones were subjected to serological indexing to detect the presence/absence of latent infection. Protein A-sandwich enzyme-linked immunosorbent assay (PAS ELISA) was employed for the serological detection of CMV, SBMV and CPMoV, while antigen-coated plate (ACP) ELISA was used to detect BCMV in the asymptomatic plants. Cowpea seedlings without virus symptoms but with positive serological reactions were considered as being latently infected. All of the inoculated TVu 1272 and SuVita-2 plants showed symptoms consistent with CMV and CPMoV infections, respectively. The rate of CMV latent infection was high in TVu 1179 (14.5%), low in SuVita-2 (1.3%) but not recorded in TVu 1272.     

Viruses infecting winter oilseed rape (Brassica napus ssp. oleifera)

Turnip mosaic virus (TuMV) and cauliflower mosaic virus (CaMV) have been found infecting field crops of winter oilseed rape (Brassica napus ssp. oleifera) in South Warwickshire. Other viruses found include broccoli necrotic yellows virus (BNYV) and a member of the beet western yellows virus group. Systemic leaf symptoms caused by TuMV varied within and between cultivars; the three predominant reaction types were classified as necrotic, mosaic and immune. Some recently introduced cultivars of oilseed rape were more severely affected by TuMV infection than older cultivars. Reactions to CaMV were less varied and immunity was not found. The seed yield from TuMV and CaMV-infected plants was less than that of healthy control plants. This effect was due to infected plants producing either fewer seeds, smaller seeds or both. Germination of seeds from infected plants was unaffected if sown soon after harvest. After storage for one year the germination of seed from a virus infected plant was significantly less than that of seed from a virus-free plant. All commercial cultivars tested were experimentally susceptible to turnip yellow mosaic virus (TYMV) and some American strains of cucumber mosaic virus (CMV).     

Alfalfa mosaic and cucumber mosaic virus infection in chickpea and lentil: incidence and seed transmission

Samples collected in 1994 and 1995 from commercial crops of chickpeas and lentils growing in the agricultural region of south-west Western Australia were tested for infection with alfalfa mosaic (AMV) and cucumber mosaic (CMV) viruses, and for members of the family Potyviridae using enzyme-linked immunosorbent assay (ELISA). In 1994 no virus was detected in the 21 chickpea crops tested but in 1995, out of 42 crops, AMV was found in two and CMV in seven. With lentils, AMV and/or CMV was found in three out of 14 crops in 1994 and 4 out of 13 in 1995, both viruses being detected in two crops in each year. Similar tests on samples from chickpea and lentil crops and plots growing at experimental sites, revealed more frequent infection with both viruses. No potyvirus infection was found in chickpeas or lentils in agricultural areas either in commercial crops or at experimental sites. However, bean yellow mosaic virus (BYMV) was detected along with AMV and CMV in irrigated plots of chickpeas and lentils at a site in Perth. When samples of seed from infected crops or plots of chickpeas and lentils were germinated and leaves or roots of seedlings tested for virus infection by ELISA, AMV and CMV were found to be seed-borne in both while BYMV was seed-borne in lentils. The rates of transmission found through seed of chickpea to seedlings were 0.1–1% with AMV and 0.1–2% with CMV. Seed transmission rates with lentil were 0.1–5% for AMV, 0.1–1% for CMV and 0.8% for BYMV. Individual seed samples of lentil and chickpea sometimes contained both AMV and CMV. With both species, infection with AMV and CMV was sometimes found in commercial seed stocks or seed stocks from multiplication crops of advanced selections nearing release as new cultivars. Seed-borne virus infection has important practical implications, as virus sources can be re-introduced every year to chickpea and lentil crops or plots through sowing infected seed stocks leading to spread of infection by aphid vectors, losses in grain yield and further contamination of seed stocks.     

Seed-borne cucumber mosaic virus infection of narrow-leafed lupin (Lupinus angustifolius) in Western Australia

In 1986 in Western Australia, cucumber mosaic virus (CMV) infection was widespread in breeders' selections of narrow-leafed lupin (Lupinus angustifolius), and in collections of lupin cvs and wild L. angustifolius lines. When seed of some of these selections and cvs was sown, seed-borne CMV was detected in seedlings. Infection of F₁ progenies was traced to use of infected parent plants. CMV was also widespread in 25 seed crops of the new lupin cv. Wandoo but not in 42 seed crops of the new cv. Danja. When samples of the seed sown in 1986 were tested, CMV was detected in 3 - 34% of seedlings of cv. Wandoo but in none of cv. Danja. Following intensive roguing of symptom-bearing plants in the 1986 seed crop of new lupin cv. Gungurru, the level of seedling infection with CMV in seed samples after harvest was 0·1-0·2%. CMV was detected in 6 - 8%, 0·6-5% and 0 - 18% of seedlings from seed samples of established lupin cvs Chittick, Yandee and Illyarrie respectively. Highest levels of seed transmission were in seed from crops grown in high rainfall areas. When a sample of cv. Wandoo seed was graded for size by sieving, CMV was detected in seedlings grown from seed in all grades, but the smallest grade contained the highest level of infection. When seed was collected from pods at different positions on plants in a CMV-infected crop of cv. Illyarrie, seed from primary pods transmitted the virus to seedlings at a 3% rate, seed from first order lateral pods at 8% while seed from second and third order lateral pods transmitted at 13%. Examination of CMV-infected lupin crops indicated that seed-infected plants competed poorly and tended to be shaded out in dense crops but to survive in sparse crops. In 1987 during drought conditions after seeding, plant mortality was greater with seed-infected seedlings than with healthy seedlings despite wide plant spacing. An isolate of CMV from subterranean clover (Trifolium subterraneum) induced severer symptoms in lupins than four isolates from lupin; only the subterranean clover isolate prevented seed production. In tests at one lupin breeding site, CMV was found in 15 species of weeds and volunteer legumes. Fumaria officinalis, Stachys arvensis and volunteer lupins were most frequently infected.     

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.     

Isolation of an Arabidopsis thaliana mutant in which accumulation of cucumber mosaic virus coat protein is delayed

Cucumber mosaic virus (CMV) is known to systemically infect Arabidopsis thaliana ecotype Columbia plants. In order to identify the host factors involved in the multiplication of CMV, we isolated an A. thaliana mutant in which the accumulation of the coat protein (CP) of CMV in upper uninoculated leaves was delayed. Genetic analyses suggested that the phenotype of delayed accumulation of CMV CP in the mutant plants was caused by a single, nuclear and recessive mutation designated cum1-1, which was located on chromosome IV. The cum1-1 mutation did not affect the multiplication of tobacco mosaic virus, turnip crinkle virus or turnip yellow mosaic virus, which belong to different taxonomic groups from CMV. Accumulation of CMV CP in the inoculated leaves of cum1-1 plants was also delayed either when CMV virion or CMV virion RNA was inoculated. On the other hand, when cum1-1 and the wild-type Col-0 protoplasts were inoculated with CMV virion RNA by electroporation, the accumulations of CMV-related RNAs and the coat protein were similar. These results suggest that the cum1-1 mutation did not affect the uncoating of CMV virion and subsequent replication in an initially infected cell but affected the spreading of CMV within an infected leaf, possibly the cell-to-cell movement of CMV in a virus-specific manner.     

The Ability of Aphis craccivora, A. gossypii and A. citricola to Transmit Single and Mixed Viruses to Cowpeas

The ability of populations of adult apterous Aphis craccivora, A. gossypii and A. citricola to transmit the cowpea aphid-borne virus (CAMV) and cucumber mosaic virus (CMV) to cowpea was investigated. CMV was more readily transmitted than CAMV by all three aphid species, but was most readily transmitted by A. craccivora and A. citricola, with transmissions ranging from 64–71 %. With CAMV, the infection level with A. gossypii was higher than with A. craccivora and both were more efficient than A. citricola, although the differences were not significant. With mixed infections of CAMV and CMV, there was a higher level of CAMV transmission using A. craccivora from sequential feeding when CMV-infected plants were fed on first followed by CAMV-infected plants, suggesting that cowpea plants were more prone to infection by CAMV when already infected by CMV.     

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.     

First Report of Cucumber mosaic virus Infecting Chinese Mallow in China

The virus in naturally infected, stunted Chinese mallow plants and mosaic leaves was identified as Cucumber mosaic virus (CMV). Six symptomatic plants and one symptomless plant were collected in Chongqing, China. DAS‐ELISA suggested CMV was likely associated with the diseased Chinese mallow. Double‐stranded RNA was extracted from the samples, analysed by RT‐PCR, and the coding sequences of their coat proteins (CPs) were sequenced. The results further confirmed CMV was the pathogen causing Chinese mallow stunted, mosaic disease. The isolate was named CMV‐DXC. The full sequence of CMV‐DXC CP was determined, and it had the highest nucleotide identity (99.4%) of those of CMV‐lily, CMV‐WSJ and CMV‐Hnt, respectively. Phylogenetic analysis shows that CMV‐DXC belongs to CMV subgroup II. To our knowledge, this is the first report of CMV infecting Chinese mallow in China.     

Resistance to cucumber mosaic virus in potato

Reactions to two subgroup I isolates (Fny-CMV and Pf-CMV) and two subgroup II isolates (A9-CMV and LS-CMV) of cucumber mosaic virus (CMV) were studied in three non tuber-bearing wild potato species (Solanum spp.) of the series Etuberosa, and in two tuber-bearing interspecific potato hybrids and four potato cultivars using graft-inoculation. Three classes of phenotypic reactions (susceptible, hypersensitive, extreme resistance) were observed in the tuber-bearing genotypes. Susceptible genotypes developed mosaic or severe mosaic with leaf malformation and had high CMV titres. Hypersensitive genotypes developed either top necrosis or vein necrosis and/or necrotic spots on apical leaves, and had low CMV titres. Extremely resistant genotypes had no symptoms and no CMV was detected. The hybrid 87HW13.7 (S. tuberosum×S. multidissectum) developed top necrosis specific to infection with Fny-CMV. The hybrid ‘A6’ (S. demissum×S. tuberosum cv. Aquila) was hypersensitive to all CMV isolates tested. Extreme resistance was not functional against all CMV isolates. Neither hypersensitivity nor extreme resistance were related to the CMV subgroup.     

The incidence and distribution of viruses infecting lettuce, cultivated Brassica and associated natural vegetation in Spain

A virus survey was conducted during the spring and autumn of 2001 and 2002 to determine the presence, prevalence and distribution in Spain of the viruses that are most commonly found infecting lettuce and Brassica worldwide. Crop plants showing virus symptoms from the principal lettuce and Brassica-growing regions of Spain, and some samples of the annual and perennial flora nearby, were tested by enzyme-linked immunosorbent assays using specific commercial antibodies against the following viruses: Alfalfa mosaic virus (AMV), Broad bean wilt virus 1 (BBWV-1), Beet western yellows virus (BWYV), Cauliflower mosaic virus (CaMV), Cucumber mosaic virus (CMV), Lettuce mosaic virus (LMV), Pea seed-borne mosaic virus (PSbMV), Turnip mosaic virus (TuMV) and Tomato spotted wilt virus (TSWV). Samples were also tested with a Potyvirus genus antibody. Virus incidence was much lower in spring than in autumn, especially in 2001. In spring 2002, CMV and LMV were the most prevalent viruses in lettuce, while CaMV was the most important virus present in Brassica crops grown in Navarra, followed by CMV and BWYV. In the autumn, the spectrum of viruses was different; potyviruses were widespread in lettuce grown in Madrid, but TSWV and BWYV were predominant in the Murcia region. The prevalent Potyvirus detected in lettuce fields was LMV, but none of the samples collected were positive for PSbMV or TuMV. In Brassica crops, TSWV was the most abundant in autumn-sown crops, especially in the Navarra region. All of the viruses present in lettuce and Brassica were also frequently detected in their associated natural vegetation at the same time, suggesting that they probably play an important role as virus reservoirs. Sonchus spp. were particularly common and were frequently infected with CMV, LMV and BWYV. Another common species, Chenopodium album, was often infected with TSWV and BWYV. Multiple infections were common, especially in non-crop plants, and the most common combination was BWYV and TSWV. The role of weeds in the epidemiology of viruses that infect lettuce and Brassica crops in Spain is discussed.     

Further studies on cucumber mosaic virus infection of narrow-leafed lupin (Lupinus angustifolius): seed-borne infection, aphid transmission, spread and effects on grain yield

Four field trials were done with narrow-leafed lupins (Lupinus angustifolius) in 1988 - 1989, to examine the effect of sowing seed with 5% and 0.5% cucumber mosaic virus (CMV) infection on subsequent virus spread, grain yield and percentage of infection in harvested seed. A proportion of the CM V-infected seed failed to produce established plants and thus, plots sown with 5% and 0.5% infected seed contained 1.5-2.9% and 0.2-0.3% of seed-infected plants respectively. The rate of virus spread by aphids was faster and resulted in more extensive infection at maturity in plots sown with 5% infected seed than with 0.5% infected seed. In three trials, sowing 5% infected seed resulted in yield losses of 34 - 53% and CMV infection in the seed harvested of 6 - 13%. The spread of CMV infection resulting from sowing 0.5% infected seed did not significantly decrease yield. However, late CMV spread in these plots caused > 1% seed infection. In the fourth trial, which was badly affected by drought, CMV spread only slowly, there was no significant effect of CMV on grain yield and the percentage of infected seed harvested was 3–5 times less than that in the seed sown. When CMV-infected seed was sown at different depths, target depths of 8 and 11 cm decreased the incidence of seed-infected plants by c. 15% and c. 50% respectively compared with sowing at 5 cm. However, in glasshouse tests, treatment with the pre-emergence herbicide simazine failed to selectively cull out seed-infected plants. The field trials were colonised by green peach (Myzus persicae), blue-green (Acyrthosiphon kondoi) and cowpea (Aphis craccivora) aphids. When the abilities of these aphid species and of the turnip aphid (Lipaphis erysimi) in transmitting CMV from lupins to lupins were examined in glasshouse tests, short acquisition access times favoured transmission. With 5–10 min acquisition access times, overall transmission efficiencies were 10.8%, 9.4%, 6.1% and 3.9% for the green peach, cowpea, blue-green and turnip aphids respectively.     

Effect of temperature on virus eradication and growth of infected tissue cultures

Cucumber mosaic (CMV) and alfalfa mosaic (AlfMV) viruses were eradicated or their concentrations greatly diminished when infected meristem tips were grown in static or shake culture at 32 or 34 ^oC. Similar cultures grown at 22 ^oC remained infected. Tobacco mosaic virus (TMV) was not eradicated from shake cultures at 32 ^oC and the concentration of TMV in such cultures at 32 ^oC was sometimes higher than at 22 ^oC. The concentration of CMV and AlfMV was considerably less in cultured tissues than in infected leaf material, but that of TMV was similar. Cultures infected with CMV or TMV grew more slowly than healthy cultures. Growth of AlfMV infected and healthy cultures was similar.     

A Novel Satellite RNA of Cucumber Mosaic Virus Induces Unique Line-pattern Mosaic Symptoms in Tobacco

The CMV(YW) isolate of cucumber mosaic virus (CMV) induced unique line‐pattern mosaic symptoms in systemically infected leaves of tobacco (Nicotiana tabacum cv. Ky57). By northern hybridization analysis using cDNA to CMV(Y) satellite RNA as a probe, it was confirmed that CMV(YW) contained a satellite RNA. which was designated sat‐YW RNA; this was 388 nucleotides in length and did not have either a conserved domain that induces necrosis in tomato or chlorosis in tobacco. CMV(YW) free of sat‐YW RNA. which was isolated by the single lesion isolation method using Chenopodium amaranticolor, did not induce the unique line‐pattern mosaic symptom. Furthermore, the sat‐YW RNA‐mediated line‐pattern mosaic symptom was also induced by in vitro transcribed infectious sat‐YW RNA in tobaccos infected with either CMV(YW) or CMV(Y) genomic RNA. These results clearly demonstrated that sat‐YW RNA induces the unique line‐pattern mosaic symptom on CMV‐infected tobaccos.     

Viral twig necrosis of sweet cherry. Modes of transmission and spread of petunia asteroid mosaic virus (PeAMV)

Grafting symptomless scions, derived from petunia asteroid mosaic virus (PeAMV)-infected trees, to healthy rootstocks resulted in only 3.3% infection in the resulting trees. Up to 90% of seeds from infected sweet cherries contained high quantities of PeAMV, but the virus was not transmitted to the seedlings apparently because of low virus content in the embryo and loss of infectivity during seed maturation and storage. Replanting healthy cherry trees cv. Sam, grafted to different rootstocks, into contaminated soils resulted in new infections. Eight of 13 trees on rootstocks derived from Prunus avium (F 12/1 and cv. Sam on its own roots) were infected with PeAMV within a period of four years but only one of 16 trees on Weiroot-rootstocks (selections from Prunus cerasus) became infected. The detection of PeAMV in naturally contaminated soil samples by the bait plant procedure, using Nicotiana clevelandii, was superior to testing soil eluates by enzyme-linked immunosorbent assay (ELISA) and immuno electron microscopy (IEM). Wild plants may contribute to virus propagation and maintenance of virus contamination of the soil as 25 of 310 samples from 712 herbaceous plants growing in the vicinity of infected trees contained PeAMV; the contaminated samples represented 12 species. The perpetuation of PeAMV by infected scion wood is probably of minor significance, and infection via the soil probably represents the most important means of spread of viral twig necrosis in northern Bavaria.     

Seed transmission of turnip yellow mosaic virus in winter turnip and winter oilseed rapes

This is the first record of seed transmission of turnip yellow mosaic virus (TYMV) in oilseed and turnip rapes. The seed transmission of TYMV in a naturally infected winter turnip rape (Brassica napus var. silvestris) cultivar Perko PVH was investigated. By ELISA 1.6%, 3.2% and 8.3% seed transmission of the virus was found in seed of plants from three localities. The proportion of infected seeds produced by artificially infected plants of winter oilseed rape (Brassica napus ssp. oleifera) and winter turnip rape cultivars was determined. The virus transmission rate, expressed as the proportion of virus-infected plants which germinated from the seed was for the oilseed rape cvs Jet Neuf 0.1%, Solida 0.4%, Silesia 0.8%, Darmor 1.2%, SL-507 0.2%, SL-509 0.0% and for the winter turnip rape cv. Perko 1.5%. ELISA cannot be used in direct tests on bulk seed lots to estimate proportion of infected seed, but must be used on germinated seedlings.