Symptom severity of cassava mosaic disease in relation to concentration of African cassava mosaic virus in different cassava genotypes

The concentration of African cassava mosaic virus (ACMV) was assessed by enzyme-linked immunosorbent assay in relation to symptom severity among resistant, moderately resistant and susceptible cassava genotypes. Resistant genotype NR 8083 had significantly lower symptom severity scores ( P  < 0·05) than the susceptible genotype TMS 91934, but the two genotypes contained similar levels of virus concentration. The moderately resistant genotypes TMS 30572 and NR 8082 expressed significantly lower symptom severities ( P  < 0·05) than the susceptible genotypes TMS 91934 and TME 117, but they contained significantly higher virus concentrations ( P  < 0·05) than TMS 91934 and similar virus concentration as in TME 117. However, two other resistant genotypes, TME 1 and TME 8, had low symptom severity scores and virus concentrations. There was significant interaction ( P  ≤ 0·05) between cropping season and virus concentration in all the genotypes except TMS 30572. The resistant and moderately resistant genotypes that had high virus concentrations sustained storage root yield losses. The severity of symptoms expressed was not necessarily a reflection of the virus concentration in some of the genotypes. In addition to the use of symptom severity scores to group genotypes into resistant classes, it is recommended that virus concentration should also be considered. Genotypes displaying mild symptoms, but with high levels of virus accumulation, could be an important source of inoculum in the spread of ACMV by the whitefly vectors. This suggests that each genotype should be tested for virus accumulation prior to its release to the farmers.     

Characterization and partial sequence of a new furovirus of wheat in China

A soil-borne wheat virus causing severe mosaic and stunting symptoms on wheat in China has been characterized. It had been considered to be soil-borne wheat mosaic virus (SBWMV) because of its rod-shaped virions and similarities to epidemiology and host range. In this study, the virions purified from infected wheat tissue were approximately 20 nm in diameter and of two lengths (140–160 nm and 280–300 nm), with a coat protein of 19 kDa and two RNA components of approximately 7 and 3.5 kb. A rabbit antiserum was produced against the virus and a serological relationship to SBWMV from the USA (Oklahoma) was demonstrated. However, the coat protein was not recognized by most monoclonal antibodies against Oklahoma SBWMV in either ELISA, ISEM or Western blot analysis, indicating epitope differences. In RT-PCR experiments the viral nucleotide sequences were significantly different from those of SBWMV, and this was confirmed by partial sequencing of the cloned PCR fragments generated from RNA1 ( c . 1100 nt) and RNA2 ( c . 1400 nt), which showed homologies of about 79 and 63%, respectively, to corresponding regions of SBWMV. Because of these significant differences in serology and nucleotide sequence it is suggested that it is a new furovirus for which the name Chinese wheat mosaic virus (CWMV) is proposed.     

Partial characterization of a bipartite begomovirus infecting yellow passion flower in Brazil

During the spring of 2001, approximately 10 000 yellow passion flower plants, from two orchards in the county of Livramento de Nossa Senhora, Bahia State, Brazil, exhibited intense yellow mosaic symptoms and drastic reduction of the leaf lamina and plant development. A large population of whiteflies ( Bemisia tabaci ) was also found colonizing the plants. All field samples collected tested positive for Passion fruit woodiness virus in DAS-ELISA. Five out of 20 passion flower plants inoculated with adult whiteflies collected from diseased plants in the field developed symptoms 20–30 days after inoculation. Two of these plants gave a positive reaction in TAS-ELISA using antiserum against a begomovirus. Degenerated PCR primers amplified viral DNA fragments from the DNA-A and DNA-B components of a begomovirus infecting these plants. The fragment corresponding to the core region of the coat protein (DNA-A) was cloned and sequenced. A phylogenetic analysis placed this begomovirus isolated from passion flower in the same clade of the New World begomoviruses as several other species from Brazil. Based on the symptoms induced by this virus alone, the disease was tentatively named passion flower little leaf mosaic.     

Effects of crop management practices on Echinochloa crus-galli and Chenopodium album seed production in a maize/soyabean rotation

Seed production of residual weed populations needs to be taken into account when estimating the long-term impact of low-input agronomic practices. The objective of this study was to measure the effects and interactions of crop, weed control, tillage practice and nutrient source on the seed production of the dominant residual weed species in a maize/soyabean rotation at two sites: Echinochloa crus-galli (L.) Beauv. on a Sainte-Rosalie clay and Chenopodium album L. on a Duravin clay loam. Seed production per unit area was estimated in each experimental unit. Weed seed production was greater under mechanical weed control compared with chemical weed control. In 1997, E. crus-galli seed production reached over 326 000 seeds m^–2 in mechanical weed control treatments, but averaged less than 500 seeds m^–2 in the chemical weed control treatments. Chenopodium album produced in the range of 766 000 and 73 000 seeds m^–2 in mechanical and chemical weed control treatments respectively. Very few or no weed seeds were produced in soyabean under chemical control. Tillage intensity and nutrient source did not affect seed production of either weed species, with the exception that E. crus-galli produced more seeds in chisel than in mouldboard plough tillage in soyabean. Weed control method had more impact on seed production than tillage intensity and nutrient source in a maize/soyabean rotation.     

Identification and transmission of Piper yellow mottle virus and Cucumber mosaic virus infecting black pepper (Piper nigrum) in Sri Lanka

Sri Lankan black pepper with symptoms of yellow mottle disease contained a mixture of viruses: Piper yellow mottle virus (PYMV) particles (30 × 130 nm), Cucumber mosaic virus (CMV, 30 nm diameter isometric particles), and unidentified, isometric virus-like particles (30 nm diameter). An effective purification procedure is described for PYMV. Immunosorbent and conventional electron microscopy successfully detected badnavirus particles only when at least partially purified extracts were used. PYMV was confirmed as the cause of the disease, with the other two viruses apparently playing no part in producing symptoms. PYMV was transmitted by grafting, by the insect vectors citrus mealy bug ( Planococcus citri ) and black pepper lace bug ( Diconocoris distanti ), but not by mechanical inoculation or through seeds. The CMV isolate was transmitted to indicator plants by mechanical inoculation and by the vector Aphis gossypii , but not by Myzus persicae ; but neither mechanical nor insect transmission of CMV to black pepper was successful. A sensitive polymerase chain reaction assay was developed to detect PYMV in black pepper.     

Potato latent virus: a proposed new species in the genus Carlavirus

A previously undescribed carlavirus, potato latent virus (PotLV), was found infecting the potato cultivar Red La Soda imported from the USA. The particles were filamentous and slightly curved, with modal lengths of 530 and 670 nm. The 11 kDa protein encoded downstream from the coat protein contained a 'zinc-finger' motif characteristic of carlaviruses, and RT–PCR using a carlavirus-specific primer gave a PCR product of 857 bp. Antibodies produced to PotLV did not detect other carlaviruses when used in ELISA and the coat-protein nucleic acid sequence of PotLV showed < 67% similarity with the other carlaviruses tested. The closest similarity was with the Andean strain of potato virus S. Unusually for a carlavirus, PotLV systemically infected Nicotiana bigelovii , N. glutinosa , N. rustica , N. tabacum and Physalis floridana .     

Pfaffia mosaic virus: a new potyvirus found infecting Pfaffia glomerata in Brazil

In Brazil plants of Pfaffia glomerata with mosaic symptoms were found to be infected with a previously undescribed potyvirus, Pfaffia mosaic virus (PfMV). Virus particles were long and flexuous, c.  10 × 700–800 nm, and cylindrical inclusions typical of potyviruses were present in cells of infected tissue. Partial host-range studies revealed that in addition to P. glomerata , PfMV infected only Chenopodium amaranticolor and Chenopodium quinoa . It was efficiently transmitted by the aphids Aphis gossypii and Myzus persicae . Polyclonal antiserum produced against the PfMV coat protein (CP) reacted with Potato virus Y (PVY), but not with four other potyviruses in PTA-ELISA. The similarity of the nucleotide sequence of the PfMV coat-protein gene ( CP ) varied from 7 to 76% when compared with other members of the family Potyviridae . Similarity of the 3' NTR sequence varied from 4 to 23%. In both cases the highest similarity was with PVY. These data indicate that PfMV is a new species in the genus Potyvirus .     

Plum pox virus and the estimated costs associated with sharka disease

Since first being recorded in 1917–18 in Bulgaria, sharka (plum pox) disease has progressively spread via infected plant material to be present in most Prunus -growing nations today. The disease has serious agronomic and political consequences because it causes enormous economic losses. In countries in which sharka is endemic, a high percentage of apricot and European plum production is unmarketable because of the disease. To these figures should be added the costs of sanitary controls, surveys and eradication programmes against sharka virus. Estimated costs associated with sharka management worldwide in the last 30 years exceed 10 000 million euros. However, improvements in knowledge of the disease and in techniques used to identify the disease are significantly aiding disease control and management.     

Molecular identification of a begomovirus causing yellow vein disease on Calendula officinalis in India

A whitefly transmitted begomovirus was detected by PCR using begomovirus-specific primers from naturally infected Calendula officinalis plants showing yellow vein disease symptoms. An approximately 800 bp PCR amplicon was cloned and sequenced to identify the species of the virus isolate. Analysis of nucleotide sequence data resulted in its identification as the complete coat protein gene open reading frame (CP ORF) of 771 bp, which encoded 256 amino acid residues. The coat protein of the virus isolate shared maximum identities of 96–97% with four strains of Tobacco curly shoot virus (ToCSV) and an Ageratum enation virus (AgEV) during BLAST analysis of sequence data. Nucleotide- and amino acid-based phylogenetic analysis revealed the close relationship of the isolate with ToCSV strains, therefore it has been identified as an isolate of ToCSV and C. officinalis is considered to be a new host of ToCSV begomovirus. Association of a DNA-β molecule with the virus isolate was also detected by PCR and Southern hybridization tests using DNA-β specific primers and probe.     

Systemic spread of Plum pox virus (PPV) in Mariana plum GF 8-1 in relation to shoot growth

Infection of Prunus spp. by Plum pox virus (PPV) is characterized by an uneven distribution of the virus within the tree and branches. In order to gain a better understanding of this distribution, a method for modelling tree growth was used. PPV spread was followed within susceptible Mariana plum clone GF 8-1 shoots for 4 months after inoculation. Shoot growth was unaffected by the presence of the virus. Symptoms appeared on leaves produced in the most actively growing parts of the shoots, i.e. at the beginning of the season. PPV was detected in leaves other than those showing symptoms. The proportion of leaves with detectable virus decreased from the zone showing symptoms, with 100% ELISA-positive responses, to the shoot tip with no detectable virus in leaves produced between 111 and 127 days after inoculation. Furthermore, a higher proportion of positive ELISA results was obtained below the zone showing symptoms (77%) compared with 50% above. PPV was detected in 95% of the most vigorous shoots 71 days after inoculation compared with 37% of slower-growing, later-produced shoots.     

Pumpkin yellow vein mosaic disease is caused by two distinct begomoviruses: complete viral sequences and comparative transmission by an indigenous Bemisia tabaci and the introduced B-biotype

Pumpkin yellow vein mosaic disease (PYVMD) causes significant damage to pumpkin production throughout India. A begomovirus causing PYVMD in South India was characterized recently but the nature of virus causing the disease in North India was not known. Samples of PYVMD were obtained from North India and two putative begomoviruses were PCR‐amplified and sequenced. Comparison of complete DNA‐A sequences indicated that PYVMD in North and South India were caused by two distinct begomoviruses and shared only approximately 88% DNA‐A nucleotide identity. The South Indian isolate was most closely related to Squash leaf curl China virus between 91 and 96% identities, and the two North Indian isolates to Tomato leaf curl New Delhi virus between 94 and 96% identities. The South Indian isolate was previously shown to be transmitted by the indigenous biotype of Bemisia tabaci, however, the situation has since changed with the introduction of the B‐biotype to South India in 1999. Comparative transmission experiments between the indigenous biotype v/s the introduced B‐biotype for the time required for virus acquisition (30 min v/s 15 min), inoculation (15 min v/s 10 min) and incubation (30 min v/s 4 h) have indicated that the B‐biotype transmits the virus quickly and more efficiently than the indigenous biotype. An epidemic of PYVMD was recorded for the first time in South India in 2004 with disease incidences of up to 100% and significant yield losses. This may be due to a combination of several factors including the large numbers of B‐biotype populations, the ability of the B‐biotype to transmit the virus efficiently and the cultivation of susceptible varieties. These possibilities and the threat to pumpkin cultivation associated with the spread of the B‐biotype in India are discussed.     

Biology of a new virus isolated from Lupinus nootkatensis plants in Alaska

A new virus named Nootka lupine vein-clearing virus (NLVCV) was isolated from Lupinus nootkatensis plants that were confined to a relatively small area in the Talkeetna mountains of south-central Alaska. Annual surveys (2000–03) consistently found leaf symptoms of pronounced vein clearing and mosaic on 3- to 4-week-old plants in late June. Spherical particles ≈30 nm in diameter were isolated from these leaves. Virions contained a single-stranded RNA of ≈4·0–4·2 kb and one species of capsid protein estimated to be ≈40 kDa. The double-stranded RNA profile from naturally infected leaves consisted of three major bands ≈4·2, 1·9 and 1·5 kbp. Protein extractions from either sap or virions of diseased plants reacted to polyclonal antiserum made against the virions in Western blot assays. A predicted PCR product ≈500 bp was synthesized from virion RNA using primers specific to the carmovirus RNA-dependent RNA polymerase (RDRP) gene. The nucleotide sequence of the amplified DNA did not match any known virus, but contained short regions of identity to several carmoviruses. Only species belonging to the Fabaceae were susceptible to NLVCV by mechanical inoculation. Based on dsRNA profile, size of virion RNA genome and capsid protein, and similarity of the RDRP gene to that of other carmoviruses, it is suggested that NLVCV is a member of the family Tombusviridae , and tentatively of the genus Carmovirus . As the host range, RDRP gene and dsRNA profile of NLVCV are different from those of known viruses, this is a newly described plant virus.     

Effect of temperature on symptom expression and accumulation of tomato spotted wilt virus in different host species

The effect of two temperature regimes (daytime, 29 ± 2°C, night-time, 24 ± 3°C; and daytime, 23 ± 1°C, night-time, 18 ± 2°C) on the symptoms caused by tomato spotted wilt virus (TSWV), and the accumulation of TSWV virions, was compared in Datura stramonium , Nicotiana tabacum cv. White Burley and Physalis ixocarpa . Tobacco plants were more severely affected by TSWV at the high temperature regime, but the incidence (percent of plants with symptoms) was 100% for both regimes. In P. ixocarpa and D. stramonium the higher temperature caused an increase in both incidence and rate of development of symptoms. At high temperature, all three species showed both local and systemic symptoms; however, at low temperature only P. ixocarpa consistently developed systemic symptoms. In general, virus accumulation in the inoculated leaves (presumably the combined effect of virus replication and local movement) of all plants was higher at the lower temperature. Long distance movement in tobacco, leading to virion accumulation in other plant organs, was favoured by high temperature; but there was relatively little effect in P. ixocarpa and D. stramonium .     

Propanil activity, uptake and metabolism in resistant Echinochloa spp. biotypes

Field resistance of Echinochloa spp. to propanil has been previously reported in Costa Rica, Colombia and Arkansas (USA). In this study, the mechanism of resistance was investigated in three resistant (R) and three susceptible (S) biotypes. The shoot fresh weight reduction in pot-grown plants from a post-emergence spray of propanil at 2.44 kg a.i. ha⁻¹ on biotypes R/S from Costa Rica, Colombia and Arkansas was 35/98%, 25/79% and 20/82% respectively. In vitro chlorophyll fluorescence data from leaf tissue incubated in propanil showed that photosynthesis was inhibited in all biotypes, indicating that the propanil-binding site and enzyme were not altered. After transfer to herbicide-free solution, photosynthesis recovered only in resistant biotypes, indicating that the mechanism of resistance was caused by enhanced metabolism of the herbicide. Simultaneous treatment with fenitrothion, an aryl acylamidase inhibitor, prevented the recovery of photosynthesis in leaf tissue in two resistant biotypes. In contrast, the cytochrome P450 mono-oxygenase inhibitor, 1-aminobenzotriazole, did not prevent recovery from propanil in leaf tissue. Application of ¹⁴C-propanil to the second leaf of intact Echinochloa plants showed that c . 90% of the radioactivity remained in the treated leaf for up to 72 h after application. No major differences in translocation between R and S biotype plants were found. TLC analysis of tissue extracts from the treated leaves showed substantially less radioactivity associated with propanil, present after 72 h in rice or in the three R biotypes, compared with S biotypes.     

Wind-mediated spread of Rice yellow mottle virus (RYMV) in irrigated rice crops

A study was carried out to demonstrate that Rice yellow mottle virus (RYMV), a virus known to be transmitted by beetles, can spread between rice plants by direct leaf contact caused by wind. Almost all healthy plants surrounding an infected plant became infected when exposed to a fan blowing for 15 min at a distance of 50 cm. Spread of RYMV by plant contact, mediated by wind, was also demonstrated in field experiments, the extent of spread depending on plant density. Infection was almost 10 times higher in plots with a density of 33 plants m⁻² than in plots with 16 plants m⁻². Less spread was observed in plots protected by 1·5 m high windscreens. It is suggested that wind-mediated spread of RYMV may result from abrasive contact between leaves of plants.     

Relationships between seed germination, fumonisin content, and Fusarium verticillioides infection in selected maize samples from different regions of Costa Rica

Thirty-five maize seed samples from three geographic regions of Costa Rica were analysed for fumonisin content, germinability, and Fusarium verticillioides infection with and without surface disinfection. The concentration of fumonisins in the maize samples varied from 4 ng g⁻¹ to 16 000 ng g⁻¹ with an all over-all average of 2500 ng g⁻¹. There was a significant difference in fumonisin content between samples from Alajuela and Guanacaste. Germination of the seed samples ranged from 12% to 98% with significant differences between regions. F. verticillioides infection was 12–98% and 13–97% for surface-disinfected and nondisinfected seeds, respectively. There was a significant negative correlation ( r  = -0.52) between fumonisin content and seed germination, but no significant correlation was found between fumonisin content and F. verticillioides infection, or between F. verticillioides infection and seed germination. Most of the high fumonisin seed samples had some mechanical or insect damage. Whether or not the fumonisins had a direct effect on germination was not further established. It is concluded that the large differences in fumonisin content of maize seeds within and between regions are primarily caused by differences in seed quality, genetic diversity of F. verticillioides strains in natural populations, climatic differences between regions, and varietal differences. Some of the fumonisin levels found in this study coincide with levels associated with risks to humans and animals in other countries.     

Identification and characterization of peanut stunt cucumovirus from naturally infected alfalfa in Iran

A mechanically transmissible virus was isolated from a naturally infected alfalfa plant ( Medicago sativa ) in Karaj, Iran. It induced fern-leaf symptoms in Lycopersicon esculentum , general chlorosis and stunting in Arachis hypogoea , local lesions and systemic infection in Chenopodium quinoa , Phaseolus vulgaris and Vigna unguiculata but only local lesions in C. amaranticolor . In gel-immunodiffusion tests, it reacted strongly with an antiserum to peanut stunt cucumovirus (PSV), moderately with two of six antisera to cucumber mosaic cucumovirus (CMV) and with two of five antisera to tomato aspermy cucumovirus. Its spherical virions (28 nm in diameter) contained a coat protein of approximately 29 kDa and encapsidated four species of RNAs with similar electrophoretic mobilities to RNAs 1–4 of CMV strains Fny and LS and those of PSV strains J and W. Its encapsidated RNAs hybridized in slot-blot hybridization assay with the complementary DNA probe of PSV-W RNAs but not with those of CMV strains. Therefore, on the basis of biological, serological and physico-biochemical properties, the virus was identified as PSV. No satellite RNA was associated with the virus. This is the first report of PSV in Iran.     

A controlled environment test for resistance to Soil-borne cereal mosaic virus (SBCMV) and its use to determine the mode of inheritance of resistance in wheat cv. Cadenza and for screening Triticum monococcum genotypes for sources of SBCMV resistance

Several wheat genotypes, including eight with known field responses, were evaluated for their reaction to Soil-borne cereal mosaic virus (SBCMV, genus Furovirus) by growing in naturally infested soil under controlled environment conditions. Virus antigen titres in the foliage 8–9 weeks after sowing mostly reflected the field responses, showing that growth chamber-based tests can be used to improve the speed and reliability of germplasm screening. Such tests were used to determine the mode of inheritance of the SBCMV resistance in cv. Cadenza, commonly used in UK wheat-breeding programmes. One hundred and eleven doubled haploid (DH) lines derived from an F ₁ of a cross between cvs Cadenza (resistant) and Avalon (susceptible) were evaluated. This DH population segregated for the reaction to SBCMV in a ratio of 1 : 1 (resistant : susceptible). This suggests that the SBCMV resistance is controlled by a single gene locus. As a first step towards identification of new sources of improved SBCMV resistance (e.g. immunity) as well as sources of the resistance to the virus vector, Polymyxa graminis , a set of 26 Triticum monococcum lines of diverse geographical origin was also screened. Most lines were susceptible to SBCMV, but one line of Bulgarian origin was resistant to the virus and possibly partially resistant to the virus vector.     

Identification of potyviruses infecting vanilla by direct sequencing of a short RT-PCR amplicon

A simple one-tube one-step RT-PCR assay with degenerate primers followed by direct sequencing of a 327 bp coat protein gene fragment was used to identify the potyviruses infecting vanilla. With this technique, unambiguous species allocation was achieved for 34 potyvirus-infected vanilla samples collected in the Indian Ocean and the Pacific areas between 1997 and 2005. Virus identification relied on blast homology and nucleotide identities of 162 to 327 nt fragments with known potyvirus sequences. Species allocation was confirmed by neighbour-joining of the 149 nt common to 32 vanilla sequences and 51 known potyviruses. Subject to further identification, these data revealed four additional Potyvirus species that may infect vanilla: Bean yellow mosaic virus , Cowpea aphid-borne mosaic virus , Ornithogalum mosaic virus and Wisteria vein mosaic virus . The procedure was rapid, cost-effective, easy to use and showed a good taxonomic discriminating value. It also enabled the identification of potyviruses in adjacent weeds and should thus aid the understanding of outbreaks of potyviruses occurring in varied epidemiological circumstances.