Control of aphid-borne Lily symptomless virus and Lily mottle virus in Lilium in the Netherlands

Genetic diversity and phylogenetic characteristics of viruses in lily plants in Beijing

Lily (Lilium) is an important bulbous perennial herb that is frequently infected by one or more viruses. To investigate the diversity of lily viruses, lilies with virus-like symptoms in Beijing were collected to perform small RNA deep sequencing. Then, the 12 complete and six nearly full-length viral genomes, including six known viruses and two novel viruses were determined. Based on sequence and phylogenetic analyses, two novel viruses were considered to be members of the genera Alphaendornavirus (Endornaviridae) and Polerovirus (Solemoviridae). These two novel viruses were provisionally named lily-associated alphaendornavirus 1 (LaEV-1) and lily-associated polerovirus 1 (LaPV-1). Based on sequence, phylogenetic and recombination analyses, strawberry latent ringspot virus (SLRSV) in the genus Stralarivirus (Secoviridae) was identified for the first time in China, and shown to exhibit the highest nucleotide (nt) diversity among the available full-length SLRSV genome sequences, with the highest identities of 79.5% for RNA1 and 80.9% for RNA2. Interestingly, the protease cofactor region in RNA1 was 752 aa in length, whereas those of the other 27 characterized isolates ranged from 700-719 aa in length. The genome sequences of lily virus A (Potyvirus), lily virus X (Potexvirus), and plantago asiatica mosaic virus (Potexvirus) exhibited varying degrees of sequence diversity at the nucleotide level compared with their corresponding characterized isolates. In addition, plantago asiatica mosaic virus (PlAMV) tended to cluster on a host species-basis. One identified lily mottle virus (Potyvirus) isolate was detected as a recombinant, and which clustered in a different group with four other isolates. Seven identified lily symptomless virus (Carlavirus) isolates, including one recombinant, were clustered into three clades. Our results revealed the genetic diversity of lily-infecting viruses, and sequence insertion, host species and recombination are factors that likely contribute to this diversity. Collectively, our results provide useful information regarding the control of viral disease in lily.

Development of a multiplex RT-PCR assay for simultaneous detection of Lily symptomless virus, Lily mottle virus, Cucumber mosaic virus, and Plantago asiatica mosaic virus in Lilies

BACKGROUND

Viral pathogens causing significant economic losses in lilies (Lilium spp. and hybrids) include Lily symptomless virus (LSV), Lily mottle virus (LMoV), Cucumber mosaic virus (CMV), and Plantago asiatica mosaic virus (PlAMV). Rapid and efficient virus detection methods are pivotal to prevent the spread of these viruses.

RESULTS

In this study, four specific primer pairs designed from conserved regions of genomic sequences of each virus were used to amplify a 116 bp product for LSV, a 247 bp product for LMoV, a 359 bp product for CMV, and a 525 bp product for PlAMV in a multiplex reverse transcription-polymerase chain reaction (multiplex RT-PCR). The amplified products were clearly separated by 2% agarose gel electrophoresis. The optimal reaction annealing temperature and cycle number were 53.8 °C and 35, respectively. The developed multiplex RT-PCR method was then used to test virus infections from lily samples collected from different regions of China.

CONCLUSIONS

An effective multiplex RT-PCR assay was established for the simultaneous detection and differentiation of LSV, LMoV, CMV, and PlAMV in lilies, which offers a useful tool for routine molecular diagnosis and epidemiological studies of these viruses.

Nanopore Metagenomics Sequencing for Rapid Diagnosis and Characterization of Lily Viruses

Lilies (Lilium spp.) are one of the most important ornamental flower crops grown in Korea. Most viral diseases in lilies are transmitted by infected bulbs, which cause serious economic losses due to reduced yields. Various diagnostic techniques and high-throughput sequencing methods have been used to detect lily viruses. According to Oxford Nanopore Technologies (ONT), MinION is a compact and portable sequencing device. In this study, three plant viruses, lily mottle, lily symptomless, and plantago asiatica mosaic virus, were detected in lily samples using the ONT platform. As a result of genome assembly of reads obtained through ONT, 100% coverage and 90.3-93.4% identity were obtained. Thus, we show that the ONT platform is a promising tool for the diagnosis and characterization of viruses that infect crops.

Plant Virome Analysis by the Deep Sequencing of Small RNAs of Fritillaria thunbergii var. chekiangensis and the Rapid Identification of Viruses

Thunberg fritillary (Fritillaria thunbergii), a perennial used in traditional Chinese herbal medicine, is a members of the family Liliaceae. The degeneration of germplasm is a severe problem in the production of Fritillaria thunbergii var. chekiangensis. However, no information about viral infections of F. thunbergii var. chekiangensis has been reported. In this study, we sequenced the small RNAs of F. thunbergii var. chekiangensis from leaves and bulbs, and viruses were identified using a phylogenetic analysis and BLAST search for sequence. In addition, multiplex reverse transcriptase-polymerase chain reaction (RT-PCR) was used to rapidly detect viruses in this variety. Our study first reported that five viruses infected F. thunbergii var. chekiangensis. Among them, fritillary virus Y (FVY), lily mottle virus (LMoV), Thunberg fritillary mosaic virus (TFMV), and hop yellow virus (HYV) had been reported in F. thunbergii, while apple stem grooving virus was first reported in the genus Fritillaria. A multiplex RT-PCR method was developed to rapidly test the four viruses FVY, LMoV, TFMV, and HYV in F. thunbergii var. chekiangensis. Our results provide a better understanding of the infection of F. thunbergii var. chekiangensis by viruses and a basic reference for the better design of suitable control measures.

The full-length genome sequence of a novel amalgavirus in Lilium spp. in China

We report for the first time the complete genome sequence of a novel amalgavirus, tentatively designated as 'lily amalgavirus 1' (LAV-1), isolated from Lilium spp. in China. LAV-1 is a 3448-nt double-stranded RNA virus that encodes two putative proteins. Open reading frame 1 (ORF1) encodes a 394-aa protein with unknown function. ORF2 encodes a putative RNA-dependent RNA polymerase (RdRp) of 895 aa. The two ORFs putatively encode a '1 + 2' fusion protein generated by a '+1' programmed ribosomal frameshift (PRF). BLASTp analysis revealed that the complete genome sequence of LAV-1 shares 48.23-59.80% sequence identity (query sequence coverage > 77%) with those of members of the genus Amalgavirus, with the highest nucleotide sequence identity of 59.80% with that of Allium cepa amalgavirus 1 (query sequence coverage, 87%). The genome structure, phylogenetic relationships, and sequence similarities to other plant amalgaviruses suggest that LAV-1 is a new member of the genus Amalgavirus.

Comprehensive Flavor Analysis of Volatile Components During the Vase Period of Cut Lily (Lilium spp. 'Manissa') Flowers by HS-SPME/GC-MS Combined With E-Nose Technology

Volatile compounds could affect the flavor and ornamental quality of cut flowers, but the flavor change occurring during the vase period of the cut flower is unclear. To clarify the dynamic changes during the vase period of cut lily (Lilium spp. 'Manissa') flowers, comprehensive flavor profiles were characterized by the electronic nose (E-nose) and headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME/GC-MS). The response value of sensor W2W was significantly higher than other sensors, and its response value reached the highest on day 4. A total of 59 volatiles were detected in cut lilies by HS-SPME/GC-MS, mainly including aldehydes, alcohols, and esters. There were 19 volatiles with odor activity values (OAVs) greater than 1. Floral and fruity aromas were stronger, followed by a pungent scent. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) could effectively discriminate lily samples derived from different vase times on the basis of E-nose and HS-SPME-GC-MS. In summary, our study investigates the flavor change profile and the diversity of volatile compounds during the vase period of cut lilies, and lilies on day 4 after harvest exhibited excellent aroma and flavor taking into consideration of the flavor intensity and diversity. This provided theoretical guidance for the assessment of scent volatiles and flavor quality during the vase period of cut lily flowers and will be helpful for the application of cut lilies during the postharvest process.

Rapid Detection of Lily mottle virus and Arabis mosaic virus Infecting Lily (Lilium spp.) Using Reverse Transcription Loop-Mediated Isothermal Amplification

The Lily mottle virus (LMoV) impedes the growth and quality of lily crops in Lanzhou, China. Recently Arabis mosaic virus (ArMV) has been detected in LMoV-infected plants in this region, causing plant stunting as well as severe foliar symptoms, and likely posing a threat to lily production. Consequently, there is a need to develop simple, sensitive, and reliable detection methods for these two viruses to prevent them from spreading. Reverse transcription (RT) loop-mediated isothermal amplification (LAMP) assays have been developed to detect LMoV and ArMV using two primer pairs that match six conserved sequences of LMoV and ArMV coat proteins, respectively. RT-LAMP assay results were visually assessed in reaction tubes using green fluorescence and gel electrophoresis. Our assays successfully detected both LMoV and ArMV in lily plants without the occurrence of viral cross-reactivity from other lily viruses. Optimal conditions for LAMP reactions were 65°C and 60°C for 60 min for LMoV and ArMV, respectively. Detection sensitivity for both RT-LAMP assays was a hundredfold greater than that of our comparative RT-polymerase chain reaction assays. We have also found this relatively rapid, target specific and sensitive method can also be used for samples collected in the field and may be especially useful in regions with limited or no laboratory facilities.

RNA viromes of the oriental hybrid lily cultivar "Sorbonne"

BACKGROUND

The lily is a perennial flowering plant belonging to the genus Lilium in the family Liliaceae. Most cultivated lily plants are propagated by bulbs. Therefore, numerous lily bulbs are frequently infected by diverse viruses causing viral diseases. To date, no study has examined the viromes of plants of one type with identical genetic backgrounds collected from different geographical regions.

RESULTS

Here, we examined different viromes of the lily cultivar "Sorbonne" using 172 gigabytes of transcriptome data composed of 23 libraries from four different projects for the cultivar "Sorbonne." We identified 396 virus-associated contigs from all but one library. We identified six different viruses, including Plantago asiatica mosaic virus (PlAMV), Cucumber mosaic virus (CMV), Lily symptomless virus (LSV), Tulip virus X (TVX), Lily mottle virus (LMoV), and Tobacco rattle virus (TRV). Of them, PlAMV was the most common virus infecting the lily. Scale and flower samples possessed a high number of virus-associated reads. We assembled 32 nearly complete genomes for the six identified viruses possessing the polyadenylate tails. Genomes of all six viruses were highly conserved in the lily cultivar "Sorbonne" based on mutation analysis. We identified defective RNAs from LSV, TVX, and PlAMV localized in the triple gene block region. Phylogenetic analyses showed that virus genomes are highly correlated with geographical regions and host plants.

CONCLUSIONS

We conducted comprehensive virome analyses of a single lily cultivar, "Sorbonne," using transcriptome data. Our results shed light on an array of lily virome-associated topics, including virus identification, the dominant virus, virus accumulation in different plant tissues, virus genome assembly, virus mutation, identification of defective RNAs, and phylogenetic relationships of identified viruses. Taken together, we provide very useful methods and valuable results that can be applied in other virome-associated studies.

Virome analysis of lily plants reveals a new potyvirus

Lily plants exhibiting virus-like symptoms of leaf yellowing, twisting and brownish necrotic spots were collected, and next-generation sequencing of small RNAs was conducted to identify the associated viruses. Cucumber mosaic virus, lily symptomless virus and a hitherto unrecorded potyvirus, tentatively named "lily yellow mosaic virus" (LYMV), were detected. The genomic RNA of LYMV was 9811 nt in length, encoding a large polyprotein of 3,124 amino acids with a predicted M_r of 353.3 kDa. BLAST analysis showed that LYMV shared a high degree of amino acid sequence identity with Thunberg fritillary mosaic virus (55%), bean yellow mosaic virus (52%), clover yellow vein virus (51%), leek yellow stripe virus (51%), and lily mottle virus (52%), and these viruses clustered together in a phylogenetic tree.

Rapid visual detection of lily mottle virus using a loop-mediated isothermal amplification method

Lily mottle virus (LMoV; genus Potyvirus, family Potyviridae) infects plants of the genus Lilium, causing a reduction in flower and bulb quality. A rapid and sensitive loop-mediated isothermal amplification (LAMP) assay was developed to detect the coat protein gene of LMoV. This LAMP method was highly specific for LMoV, with no cross-reaction with other lily viruses. The sensitivity of LMoV using the LAMP assay was 100 times more sensitive than that using conventional polymerase chain reaction. A reverse transcription LAMP (RT-LAMP) was then successfully applied to detect LMoV RNA. The newly established LAMP and one-step RT-LAMP provide an alternative method for detecting LMoV in lily plants.

Simultaneous detection of three lily viruses using Triplex IC-RT-PCR

Viruses commonly infecting lily (Lilium spp.) include: Lily symptomless virus (LSV), Cucumber mosaic virus (CMV) and Lily mottle virus (LMoV). These viruses usually co-infect lilies causing severe economic losses in terms of quantity and quality of flower and bulb production around the world. Reliable and precise detection systems need to be developed for virus identification. We describe the development of a triplex immunocapture (IC) reverse transcription (RT) polymerase chain reaction (PCR) assay for the simultaneous detection of LSV, CMV and LMoV. The triplex IC-RT-PCR was compared with a quadruplex RT-PCR assay. Relative to the quadruplex RT-PCR, the specificity of the triplex IC-RT-PCR system for LSV, CMV and LMoV was 100% for field samples. The sensitivity of the triplex IC-RT-PCR system was 99.4%, 81.4% and 98.7% for LSV, CMV and LMoV, respectively. Agreement (κ) between the results obtained from the two tests was 0.968, 0.844 and 0.984 for LSV, CMV and LMoV, respectively. This is the first report of the simultaneous detection of LSV, CMV and LMoV in a triplex IC-RT-PCR assay. In particular we believe this convenient and reliable triplex IC-RT-PCR method could be used routinely for large-scale field surveys or crop health monitoring of lily.

Rapid and sensitive detection of Lily symptomless virus by reverse transcription loop-mediated isothermal amplification

Lily symptomless virus (LSV) is one of the most prevalent viruses that infect lily plants worldwide. A rapid and sensitive reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed for detection of LSV, using two primer pairs that specifically amplified the conserved sequence of LSV coat protein. The optimum reaction conditions were as follows: 4mM MgCl₂ and 0.8M betaine with incubation at 64°C for 30min. The limit of detection of LSV from infected lily leaves was 10-fold higher for RT-LAMP than for conventional RT-PCR. Moreover, RT-LAMP detected LSV in not only symptomatic, but also in symptomless tissues of infected plants. These findings indicate that our RT-LAMP method for LSV can serve as a low-cost, simple, and rapid alternative to conventional detection assays.

Purification and immuno-gold labeling of lily mottle virus from lily leaves

Lily mottle virus (LMoV) is prevalent in Lilium species worldwide causing dwarfing, flower breaking, and reduced bulb yield. In this paper, an easy to use and efficient procedure is described for purification of LMoV from lily leaves. The resulting sample is characterized by a 260/280 nm absorbance ratio of 1.20 at a concentration of 1.27 mg/ml. The procedure results in high protein purity and particle integrity as shown by UV-spectrophotometry, polyacrylamide gel electrophoresis (PAGE), Western blotting, reverse transcriptase (RT)-PCR and transmission electron microscopy (TEM) in combination with immuno-gold labeling. This is the first time that an immuno-gold labeling (IGL) assay was performed to identify a virus of lily. Purified products can be used as a source of antigen in the preparation of antibodies against LMoV and may assist in the development of a diagnostic test for LMoV and in epidemiological surveys.

Simultaneous detection of three lily-infecting viruses using a multiplex Luminex bead array

A Luminex bead array was applied to detect multiple-virus coinfection in lily plants exhibiting typical symptoms, and the efficiency of this detection system was assessed. Specific primer sets for the simultaneous detection of 4 targets in virus-infected lily plants were constructed and used for reverse transcription (RT)-polymerase chain reaction (PCR), and specific probes were used for Luminex-based assay. Each of the 4 targets was amplified, and the amplicons were used for Luminex bead array experiments. A Luminex bead array analysis of lily-infecting viruses was performed using the quadruplex RT-PCR products followed by hybridization between the biotinylated targets and anti-tagged microsphere beads. The hybridization products produced fluorescence signals that were detected by the Luminex system. Signal strengths were analyzed by their median fluorescence intensity (MFI) values. Detection of the different target elements was found to be very specific to the corresponding viruses in lilies, and coinfection with multiple viruses was specifically detected via the MFI signals. Therefore, the use of a Luminex bead array for the detection of co-infected multiple viruses in lily plants can be an improved system for screening and analyzing multiple-virus infection.

Sequence and structure prediction of RNA-dependent RNA polymerase of lily symptomless virus isolated from L. × 'Casablanca'

The DNA sequence of the RNA-dependent RNA polymerase (RdRp) gene of lily symptomless virus (LSV), a lily-infecting member of the genus Carlavirus, was determined from nine overlapping cDNA fragments of different sizes. The complete sequence of this RdRp gene (HM070294) consisted of 5,847 nucleotides coding for a protein of 220 kDa. It had 97-98% sequence identity with RdRps of other known isolates at both the DNA and the amino acid level. Phylogenetic analysis indicated that this RdRp (designated as RdRp-DL) was closely related to the RdRp of the Korean isolate (AM516059), as well as to the RdRps from Passiflora latent virus (PLV) and Kalanchoe latent virus (KLV) of the genus Carlavirus. Hydrophobic analysis of RdRp-DL revealed a hydrophobic N-terminus and a hydrophilic C-terminus. Helices and Loops were the major secondary structures of RdRp-DL. In addition, RdRp-DL also had three coil structures. Four conserved domains were identified: typoviral methyltransferase, RNA-dependent RNA polymerase, P-loop-containing nucleoside triphosphate hydrolases and carlavirus endopeptidase. A model of the tertiary structure predicted by I-TASSER was obtained for each of these conserved domains. This is the first report of a detailed phylogenetic analysis of LSV RdRp with those of other members of the genus Carlavirus, and the first to predict the domain structures of LSV RdRp.

Detection and Frequency of Lily Viruses in Argentina

In a survey of lily growing fields in various regions of Argentina, three viruses, Lily symptomless virus (LSV), Lily mottle virus (LMoV), and Cucumber mosaic virus (CMV), were found in Longiflorum, Asiatic, Oriental, Longiflorum × Asiatic (LA), and Oriental × Trumpet (OT) hybrids. The areas surveyed were between latitude 26° 56' S and 43° 03' S, and longitude 65° 21' W and 71° 29' W. Virus detection was performed by double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) using polyclonal antiserum. In infected samples, viruses detected in decreasing order were LSV (60.5%), LMoV (51.0%), and CMV (28.7%) present in single or mixed infections. Virus infection varied among tested hybrids from 36.0% (Oriental Montecristo) to 94.7% (Lilium longiflorum Avita) in 2006 and from 38.9% (OT Yelloween) to 82.1% (LO Triumphator) in 2007, with an overall incidence of 64.1 and 70.7% in 2006 and 2007, respectively. A variation in virus incidence among localities was also observed. The highest virus incidence (89.6 and 87.6% in 2006 and 2007, respectively) was observed in Bahía Blanca (38° 44' S, 62° 16' W). The lowest virus incidences, detected in Trevellin (43° 03' S, 71° 29' W) and in Malargüe (35° 28' S, 69° 35' W), were 47.4 and 48.6% in 2006 and 2007, respectively. Moreover, a different distribution of each virus was observed between localities. The high occurrence of viruses infecting lily crops in Argentina could be due to both the use of infected bulbs for propagation and the lack of preventive virus vector control measures.

Expression, purification and characterization of the Lily symptomless virus coat protein from Lanzhou Isolate

BACKGROUND

Lily symptomless virus (LSV) is widespread in many countries where lily are grown or planted, and causes severe economic losses in terms of quantity and quality of flower and bulb production. To study the structure-function relationship of coat protein (CP) of LSV, to investigate antigenic relationships between coat protein subunits or intact virons, and to prepare specific antibodies against LSV, substantial amounts of CP protein are needed.

RESULTS

Thus, full-length cDNA of LSV coat protein was synthesized and amplified by RT-PCR from RNA isolated from LSV Lanzhou isolate. The extended 33.6 kDa CP was cloned and expressed prokaryoticly and then purified by Ni-ion affinity chromatography. Its identity and antigenicity of recombinant CP were identified on Western-blotting by using the prepared anti-LSV antibodies.

CONCLUSIONS

The results indicate that fusion CP maintains its native antigenicity and specificity, providing a good source of antigen in preparation of LSV related antibodies. Detailed structural analysis of a pure recombinant CP should allow a better understanding of its role in cell attachment and LSV tropism. This investigation to LSV should provide some specific antibodies and aid to development a detection system for LSV diagnostics and epidemiologic surveys.

Recombination and phylogeographical analysis of Lily symptomless virus

The complete genomic nucleotide sequence of an Indian isolate of Lily symptomless virus (LSV) was determined by sequencing 11 overlapping cDNA fragments of different sizes. The genome consisted of 8,394 nucleotides, excluding the poly (A) tail and contained six open reading frames (ORFs) coding protein for ORF1 220 kDa [1,948 amino acid (aa)], ORF2 25 kDa (228 aa), ORF3 12 kDa (106 aa), ORF4 7 kDa (64 aa), ORF5 32 kDa (291 aa) and ORF6 16 kDa (140 aa) from 5' to 3' end. Sequence was analyzed with other previously characterized full genomes of LSV. Phylogenetic analysis on the basis of RNA-dependent RNA polymerase (RdRp), Triple gene block proteins (TGB's), Coat protein (CP), and ORF6 (16 kDa protein) amino acid sequence revealed that Indian isolate is closely related to The Netherlands Isolate (AJ564638). The overall genome of the present LSV isolate shares 97-98% nucleotide sequence homology with the previously characterized isolates. The phylogenetic analysis, sequence alignment studies, and recombination detection program (RDP3) analysis provided evidence for the occurrence of recombination between the present isolate (AM422452) as major parent and The Netherlands Isolate (AJ564638) and Chinese isolate (AM263208) as minor parents in two different independent recombination events. Based on the recombination analysis, it is suggested that the 3' end of the present isolate is involved in recombination with Chinese isolate (AM263208) and gave rise to the Korean isolate. To the best of our knowledge, this is the first report of complete nucleotide sequence from India and also the first evidence of homologous recombination in LSV.

Comparison of two gel filtration chromatographic methods for the purification of Lily symptomless virus

Lily symptomless virus (LSV) occurs frequently in many Lilium species worldwide and often causes developmental abnormalities such as a smaller flower and lower bulb yield. In this study, two moderate and efficient gel filtration chromatography (GFC) methods was compared, these two techniques were, respectively, based on Superdex-200 HR and Sephacryl S-1000 SF. The products purified by the two methods were then characterized by measurements with UV-spectrophotometer, reverse transcriptase (RT)-PCR, transmission electron microscope (TEM), polyacrylamide gel electrophoresis (PAGE), Western blotting and matrix assisted laser desorption-ionisation/time-of-flight mass spectrometry (MALDI-TOF-MS). The final yield of purified LSV by the Superdex-200 HR GFC was 9.4 mg from 50 g of fresh infected tissues of Lanzhou lily. However, from the same amount tissues, only 5.6 mg of LSV were obtained by using Sephacryl S-1000 SF GFC. The Superdex-200 HR method was thus shown to be more suitable for the purification of LSV than the Sephacryl S-1000 SF GFC. The Superdex-200 HR method does not require costly equipment for density centrifugation and ultracentrifugation. Furthermore, it can provide an economical and efficient way to obtain purified products for the preparation of antibodies for serological diagnosis or LSV infection and related investigations.

Protecting crops from non-persistently aphid-transmitted viruses: A review on the use of barrier plants as a management tool

Barrier plants are a management tool based on secondary plants used within or bordering a primary crop for the purpose of disease control. Aphid-transmitted viruses account for approximately 50% of the 600 known viruses with an invertebrate vector. Barrier plants may act as real natural sinks for non-persistent aphid-transmitted viruses and have proved in the past to be an effective crop management strategy to protect against virus infection. Increasing the knowledge on aphid host seeking and flying behaviour, and on how barrier plants may affect the behaviour of aphids and their natural enemies will allow further development of this environmentally-friendly habitat manipulation strategy. An ideal plant barrier should be a non-host for the virus and the vector, but appealing to aphid landing and attractive to their natural enemies and should allow sufficient residence time to allow aphid probing before taking-off occurs. In this review, we have addressed why aphids are manageable by barrier cropping, the mechanisms by which barrier plants affect the occurrence of non-persistently aphid-transmitted viruses and the limitations of using barrier plants as a virus control strategy. Finally, we have pointed out future directions of research that should be conducted to integrate barrier cropping with other disease management strategies, and optimise and extend the use of barrier plants as a strategy for managing aphid-transmitted virus diseases.

A newly described plant disease complex involving two distinct viruses in a native Alaskan lily,Streptopus amplexifolius

The etiology of a previously unknown disease of Streptopus amplexifolius (L.) DC. (twisted stalk) that occurred in south central Alaska was elucidated. Symptomatic plants with yellow-green dashes and streaks on their leaves contained distinct viruses belonging to the family Potyviridae and (or) tentatively to the family Flexiviridae genus Carlavirus . Each virus’ identity was confirmed by a combination of morphology, serology (ELISA and (or) Western analysis), and sequences obtained from reverse transcriptase – polymerase chain reaction products previously generated by potyvirus- or carlavirus-specific primers. The mechanical transmission of particles (purified from twisted stalk as a mixture of both viruses or as a single preparation of each virus) to S. amplexifolius, Chenopodium amaranticolor Coste et Reyn, and Chenopodium quinoa Willd., fulfilled Koch’s postulates. The relatively high incidence of infected plants and associated severe symptoms in populations from several sites in south central Alaska gave evidence that native plants are susceptible to, and are adversely affected by viral diseases. Even though Streptopus species grow throughout the temperate regions of the world, this is the first report of a pathogen(s) infecting plants in the genus Streptopus.

Barrier crops as a cultural control measure of non-persistently transmitted aphid-borne viruses

Barrier crops have been used since the early 1950s as a cultural control strategy for reducing the spread of non-persistently transmitted aphid-borne viruses. Since then, this strategy has been investigated by several authors, resulting in a wide range of divergent conclusions on its effectiveness. In a series of field and laboratory experiments carried out in central Spain during 1995-1998, we investigated the efficacy and mode of action of various barrier crops for reducing the spread of Potato virus Y and Cucumber mosaic virus in pepper. The barriers acted as natural 'sinks' for non-persistent viruses and did not reduce the number of aphids landing in the protected crop, as suggested by some authors. There was a significant reduction in virus spread and an increase in yield in two of the four years of trials. Such differences in the effectiveness of the barrier crop strategy show that its efficacy depends on a series of factors such as the kind of virus spread pattern (monocyclic or polycyclic), the height of the barrier crop at the time of maximum risk of infection and the extent of competition between the barrier and the protected crop. Moreover, the barrier crop should not host any potential insect pest or pathogen able to damage the protected crop. It is concluded that use of barrier crops can be an effective crop management strategy to protect against virus infection, but only under specific circumstances.