Sweet Potato Mild Mottle Virus Research Articles

Endemism and Reemergence Potential of the Ipomovirus Sweet Potato Mild Mottle Virus (FamilyPotyviridae) in Eastern Africa: Half a Century of Mystery

Viruses have the ability to frequently colonize new hosts and ecological niches because of their inherently high genetic and evolutionary plasticity. However, a virus may emerge and remain of no or less economic importance until changes in viral or environmental factors dictate its epidemiological status. An example is sweet potato mild mottle virus (SPMMV), which was first reported in the 1970s on sweetpotato in eastern Africa. SPMMV has remained endemic in the region and poorly understood, yet accounting for 60 to 95% of losses, especially in mixed infections. Unlike other sweetpotato viruses which have global incidences, SPMMV has never been confirmed outside eastern Africa. This implicates the region as its center of origin but does not fully account for SPMMV's exclusive geographic delimitation to eastern Africa. Despite its importance, several mysteries and research gaps surround SPMMV, which decelerate efforts for effective virus disease management in sweetpotato. The aim of this review is to articulate research gaps, propose pivotal scientific directions, and stimulate knowledge generation for better management of virus diseases in sweetpotato. Vector-mediated transmission of SPMMV remains enigmatic. Here, we postulate testable hypotheses to explain SPMMV transmission. Comparisons between SPMMV and cassava brown streak ipomoviruses demonstrate epidemiological “hallmarks” for monitoring SPMMV. Evolutionary forces on SPMMV coupled with the virus' broad host range imply a “silent build up” of more fit variants in a changing climate, and this could explode into a worse disease conundrum. These information gaps need urgent filling to ease future management of virus disease emergences in sweetpotato.[Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

CryoEM and stability analysis of virus-like particles of potyvirus and ipomovirus infecting a common host

Sweet potato feathery mottle virus (SPFMV) and Sweet potato mild mottle virus (SPMMV) are members of the genera Potyvirus and Ipomovirus, family Potyviridae, sharing Ipomoea batatas as common host, but transmitted, respectively, by aphids and whiteflies. Virions of family members consist of flexuous rods with multiple copies of a single coat protein (CP) surrounding the RNA genome. Here we report the generation of virus-like particles (VLPs) by transient expression of the CPs of SPFMV and SPMMV in the presence of a replicating RNA in Nicotiana benthamiana. Analysis of the purified VLPs by cryo-electron microscopy, gave structures with resolutions of 2.6 and 3.0 Å, respectively, showing a similar left-handed helical arrangement of 8.8 CP subunits per turn with the C-terminus at the inner surface and a binding pocket for the encapsidated ssRNA. Despite their similar architecture, thermal stability studies reveal that SPMMV VLPs are more stable than those of SPFMV.

Argonaute 2 Controls Antiviral Activity against Sweet Potato Mild Mottle Virus in Nicotiana benthamiana.

RNA silencing is a sequence specific post-transcriptional mechanism regulating important biological processes including antiviral defense in plants. Argonaute (AGO) proteins, the catalytic subunits of the silencing complexes, are loaded with small RNAs to execute the sequence specific RNA cleavage or translational inhibition. Plants encode several AGO proteins and a few of them, especially AGO1 and AGO2, have been shown to be required for antiviral silencing. Previously, we have shown that the P1 protein of the sweet potato mild mottle virus (SPMMV) suppresses the primary RNA silencing response by inhibiting AGO1. To analyze the role of AGO2 in antiviral defense against the SPMMV, we performed a comparative study using a wild type and ago2−/− mutant Nicotiana benthamiana. Here we show that the AGO2 of N. benthamiana attenuates the symptoms of SPMMV infection. Upon SPMMV infection the levels of AGO2 mRNA and protein are greatly increased. Moreover, we found that AGO2 proteins are loaded with SPMMV derived viral small RNAs as well as with miRNAs. Our results indicate that AGO2 protein takes over the place of AGO1 to confer antiviral silencing. Finally, we provide a plausible explanation for the AGO2 mediated recovery of an SPMMV-infected sweet potato.

Viruses Infecting Sweet Potato (Ipomoea batatas (L.) Lam.) in Nigeria

Sweet potato is the second most important root crop after cassava in Nigeria. Due to decline in Nigeria‘s economy in recent years, the production of sweet potato has significantly increased from 2.4 million metric tonnes in 2000 to 4.1 million metric tons in 2017 to meet up with its demand for local consumption. However, the profitable production of the crop is being threatened by virus diseases such as Sweet potato feathery mottle virus (SPFMV), Sweet potato mild mottle virus (SPMMV), Sweet potato chlorotic stunt virus (SPCSV), Sweet potato leaf curl virus (SPLCV), and Cucumber mosaic virus (CMV). Useful information about these viruses is instrumental in the effective management of the crop in Nigeria. This paper reviewed the major viruses that affect sweet potato production in Nigeria.

Factors influencing reversion from virus infection in sweetpotato.

Viruses limit sweetpotato (Ipomoea batatas) production worldwide. Many sweetpotato landraces in East Africa are, however, largely virus-free. Moreover, some plants infected by the prevalent Sweet potato feathery mottle virus (SPFMV) may be able to revert to virus-free status. In this study, we analysed reversion from SPFMV, Sweet potato virus C, Sweet potato mild mottle virus, Sweet potato chlorotic stunt virus (SPCSV) and Sweet potato leaf curl Uganda virus using the indicator plant I. setosa and PCR/reverse-transcriptase PCR. We also investigated environmental factors (temperature and soil nutrients) that may influence reversion from virus infection. We tested reversion in the East African cultivars New Kawogo, NASPOT 1 and NASPOT 11, and the United States cultivars Resisto and Beauregard. Reverted plants were asymptomatic and virus was undetectable in assayed parts of the plant. After graft inoculation, only the East African cultivars mostly reverted at a high rate and from most viruses though cultivar Beauregard fully reverted following sap inoculation with Sweet potato virus C. None of the tested cultivars fully reverted from single or double infections involving SPCSV, and reversion was only observed in co-infections involving potyviruses. Root sprouts derived from SPFMV-reverted plants were also virus free. Reversion generally increased with increasing temperature and by improved soil nutrition. Overall, these results indicate variation in reversion by cultivar and that the natural ability of sweetpotato plants to revert from viruses is malleable, which has implications for both breeding and virus control.

Prevalence of sweetpotato viruses in Acholi sub-region, northern Uganda

The purpose of the study was to identify different viruses infecting sweetpotato and the level of co-infection and spatial distribution of the viruses within the Acholi sub-region of northern Uganda. Multiplex PCR was used to screen and determine level of co-infection in 380 sweetpotato plants. The PCR scores were computed to give overall frequency of occurrence of different viruses. The spatial distribution of viruses was represented on an ArcGIS map. Of all screened samples, 24% (92/380) were infected with at least one virus. Sweetpotato feathery mottle virus (65/92), sweetpotato chlorotic fleck virus (17/92) and sweetpotato mild mottle virus (8/92) were the most frequent viruses detected. Of sampled fields, 74% (28/38) had at least one virus-infected sweetpotato plant. The four viruses detected are the major viruses causing significant yield losses in major sweetpotato growing regions of Uganda and East Africa. The findings of limited distribution and low prevalence of the viruses in the region indicate it causes less burden to sweetpotato production in the sub-region compared with other parts of Uganda.

Simultaneous detection and differentiation of three Potyviridae viruses in sweet potato by a multiplex TaqMan real time RT-PCR assay

A multiplex TaqMan real time RT-PCR was developed for detection and differentiation of Sweet potato virus G, Sweet potato latent virus and Sweet potato mild mottle virus in one tube. Amplification and detection of a fluorogenic cytochrome oxidase gene was included as an internal control. The assay was compared with a multiplex RT-PCR developed in the initial study for the detection and differentiation of the three viruses and host 18S rRNA. Primers and/or probes of the two assays were designed from conserved regions of each virus. The two assays were optimized for primers/probes and primer concentrations and thermal cycling conditions. Sensitivity and specificity of the assays were compared each other and with other assay. Both assays were evaluated by 74 field samples original from five different provinces of China. Resultsshowed that the TaqMan real time RT-PCR offered rapid, sensitive, effective and reliable for the simultaneous detection and differentiation of the three viruses in sweet potato plants. The assay will be useful to quarantine and certification programs and virus surveys when large numbers of samples are tested.

A viral suppressor of RNA silencing inhibits ARGONAUTE 1 function by precluding target RNA binding to pre-assembled RISC.

In most eukaryotes, RNA silencing is an adaptive immune system regulating key biological processes including antiviral defense. To evade this response, viruses of plants, worms and insects have evolved viral suppressors of RNA silencing proteins (VSRs). Various VSRs, such as P1 from Sweet potato mild mottle virus (SPMMV), inhibit the activity of RNA-induced silencing complexes (RISCs) including an ARGONAUTE (AGO) protein loaded with a small RNA. However, the specific mechanisms explaining this class of inhibition are unknown. Here, we show that SPMMV P1 interacts with AGO1 and AGO2 from Arabidopsis thaliana, but solely interferes with AGO1 function. Moreover, a mutational analysis of a newly identified zinc finger domain in P1 revealed that this domain could represent an effector domain as it is required for P1 suppressor activity but not for AGO1 binding. Finally, a comparative analysis of the target RNA binding capacity of AGO1 in the presence of wild-type or suppressor-defective P1 forms revealed that P1 blocks target RNA binding to AGO1. Our results describe the negative regulation of RISC, the small RNA containing molecular machine.

Mixed Infections of Four Viruses, the Incidence and Phylogenetic Relationships of Sweet Potato Chlorotic Fleck Virus (Betaflexiviridae) Isolates in Wild Species and Sweetpotatoes in Uganda and Evidence of Distinct Isolates in East Africa.

Viruses infecting wild flora may have a significant negative impact on nearby crops, and vice-versa. Only limited information is available on wild species able to host economically important viruses that infect sweetpotatoes (Ipomoea batatas). In this study, Sweet potato chlorotic fleck virus (SPCFV; Carlavirus, Betaflexiviridae) and Sweet potato chlorotic stunt virus (SPCSV; Crinivirus, Closteroviridae) were surveyed in wild plants of family Convolvulaceae (genera Astripomoea, Ipomoea, Hewittia and Lepistemon) in Uganda. Plants belonging to 26 wild species, including annuals, biannuals and perennials from four agro-ecological zones, were observed for virus-like symptoms in 2004 and 2007 and sampled for virus testing. SPCFV was detected in 84 (2.9%) of 2864 plants tested from 17 species. SPCSV was detected in 66 (5.4%) of the 1224 plants from 12 species sampled in 2007. Some SPCSV-infected plants were also infected with Sweet potato feathery mottle virus (SPFMV; Potyvirus, Potyviridae; 1.3%), Sweet potato mild mottle virus (SPMMV; Ipomovirus, Potyviridae; 0.5%) or both (0.4%), but none of these three viruses were detected in SPCFV-infected plants. Co-infection of SPFMV with SPMMV was detected in 1.2% of plants sampled. Virus-like symptoms were observed in 367 wild plants (12.8%), of which 42 plants (11.4%) were negative for the viruses tested. Almost all (92.4%) the 419 sweetpotato plants sampled from fields close to the tested wild plants displayed virus-like symptoms, and 87.1% were infected with one or more of the four viruses. Phylogenetic and evolutionary analyses of the 3′-proximal genomic region of SPCFV, including the silencing suppressor (NaBP)- and coat protein (CP)-coding regions implicated strong purifying selection on the CP and NaBP, and that the SPCFV strains from East Africa are distinguishable from those from other continents. However, the strains from wild species and sweetpotato were indistinguishable, suggesting reciprocal movement of SPCFV between wild and cultivated Convolvulaceae plants in the field.

Incidence of sweet potato viruses in the coastal Savannah agro-ecological zone of Ghana

An experiment was conducted under field conditions to assess the incidence of viral diseases in 20 accessions of sweet potato [Ipomoea batatas (L.) Lam.]. Using symptomatology, biological indexing, nitrocellulose membrane ELISA (NCM-ELISA) and PCR, the accessions were assessed for disease incidence and symptom severity. Symptom expression varied among the accessions with severity index ranging from 2.24 in UK/BNARI at eight weeks after planting (wap) to 3.63 in US029 at 12 wap. Mean disease incidence varied significantly among the accessions, ranging from 43.2% to 96.3%. Using NCM-ELISA, 85% of the sweet potato plants tested positive to Sweet potato feathery mottle virus (SPFMV), Sweet potato mild speckling virus (SPMSV, 55%), Sweet potato collusive virus (SPCV, 45%), Sweet potato chlorotic fleck virus (SPCFV, 30%), Sweet potato virus G (SPVG, 20%), Sweet potato mild mottle virus (SPMMV, 5%), Sweet potato chlorotic stunt virus (SPCSV %?) and Cucumber mosaic virus (CMV %?). PCR detected Sweet potato leaf curl virus (SPLCV) in accessions UK/BNARI and CRI054. Biological indexing resulted in characteristic symptoms, commonly vein-clearing with eventual death of plants grafted with scions mix-infected with SPCSV and SPFMV. This work provides the first detailed report on sweet potato virus diseases in Ghana correlating symptoms, grafting, serology as well as PCR. It emphasizes the importance of selecting resistant cultivars as planting materials and the enforcement of quarantine measures to minimise the introduction of viruses into Ghana and their subsequent spread.

Development of transgenic sweet potato with multiple virus resistance in South Africa (SA)

Multiple infections of Sweet potato feathery mottle virus (SPFMV), Sweet potato chlorotic stunt virus (SPCSV), Sweet potato virus G (SPVG) and Sweet potato mild mottle virus (SPMMV) cause a devastating synergistic disease complex of sweet potato (Ipomoea batatas Lam.) in KwaZulu-Natal, South Africa. In order to address the problem of multiple virus infections and synergism, this study aimed to develop transgenic sweet potato (cv. Blesbok) plants with broad virus resistance. Coat protein gene segments of SPFMV, SPCSV, SPVG and SPMMV were used to induce gene silencing in transgenic sweet potato. Transformation of apical tips of sweet potato cv. Blesbok was achieved by using Agrobacterium tumefaciens strain LBA4404 harboring the expression cassette. Polymerase chain reaction and Southern blot analyses showed integration of the transgenes occurred in six of the 24 putative transgenic plants and that all plants seemed to correspond to the same transformation event. The six transgenic plants were challenged by graft inoculation with SPFMV, SPCSV, SPVG and SPMMV-infected Ipomoea setosa Ker. Although virus presence was detected using nitrocellulose enzyme-linked immunosorbent assay, all transgenic plants displayed delayed and milder symptoms of chlorosis and mottling of lower leaves when compared to the untransformed control plants. These results warrant further investigation on resistance to virus infection under field conditions.

Identification and distribution of viruses infecting sweet potato (Ipomoea batatas L.) in KwaZulu-Natal province, South Africa

A survey to determine occurrence and distribution of viruses infecting sweet potato (Ipomoea batatas (L.) Lam.) was conducted in major sweet potato growing areas in KwaZulu-Natal (KZN), South Africa. Eighty-four symptomatic vine samples were collected and graft-inoculated onto universal indicator plants, Ipomoea setosa Ker. Gawl. and Ipomoea nil (L.) Roth. Six weeks post-inoculation, typical sweet potato virus-like symptoms of chlorotic flecking, leaf deformation, stunting, chlorotic mosaic, and distinct interveinal chlorotic patterns were observed on indicator plants. Under a transmission electron microscope, negatively stained preparations of crude leaf sap and ultra-thin sections from symptomatic grafted I. setosa plants revealed the presence of elongated flexuous particles and pinwheel-type inclusion bodies that are characteristic to the cytopathology of potyviruses. A majority of leaf samples (52%) from graft-inoculated indicator plants tested positive for virus and showed occurrence of Sweet potato feathery mottle virus (SPFMV), Sweet potato mild mottle virus (SPMMV), Sweet potato chlorotic stunt virus (SPCSV), Sweet potato chlorotic fleck virus (SPCFV), Sweet potato virus G (SPVG), Sweet potato mild speckling virus (SPMSV) and Sweet potato caulimo-like virus (SPCaLV) using nitrocellulose membrane enzymelinked immunosorbent assays. The most frequently detected viruses were SPFMV (39%), SPVG (30%), followed by SPCSV (13%) and SPMMV (12%). SPCaLV and SPCFV at 10% and SPMSV at 7% were found exclusively in samples collected from one area. Reverse-transcription polymerase chain reaction of total plant RNA isolations confirmed the presence of SPFMV, SPVG, SPCSV and SPMMV as the most prevalent viruses infecting sweet potato in KZN.

Meristem culture of selected sweet potato (Ipomoea batatas L. Lam.) cultivars to produce virus-free planting material

SummarySweet potato (Ipomoea batatas L. Lam.) is a staple root crop, consumed as a carbohydrate-rich food source in many tropical countries. The production and productivity of sweet potato is limited by several factors, mainly virus infections. This study was conducted to produce virus-free sweet potato planting material using meristem culture. Meristems without leaf primordia were excised from four virus-positive field-grown local sweet potato cultivars ‘Bellela’, ‘Temesgen’, ‘LO-323’, and ‘Zapallo’. Meristems were cultured in Murashige and Skoog (MS) medium supplemented with different combinations and concentrations of 6-benzylaminopurine (BAP), α-naphthaleneacetic acid (NAA), and gibberellic acid (GA3) for shoot initiation. ‘Bellela’ and ‘Temesgen’ exhibited 100% shoot initiation on 5 mg l–1 BAP, 0.01 mg l–1 NAA, and 1 mg l–1 GA3, although the shoots were bushy. ‘LO-323’ showed 90% shoot initiation with the best shoot quality on 2 mg l–1 BAP, 0.01 mg l–1 NAA, and 1 mg l–1 GA3. Shoot tips and nodes from in vitro-grown plantlets were cultured on BAP (0, 0.5, 1.0, or 2.0 mg l–1) or on BAP (0, 0.5, 1.0, or 2.0 mg l–1) + kinetin (Kin; 0, 0.1, 0.5 or 1.0 mg l–1) for multiple shoot formation. The best shoot proliferation was obtained on MS + 0.5 mg l–1 BAP + 0.5 mg l–1 Kin. In vitro-grown shoots were cultured on MS medium containing 0.0, 0.01, 0.1, 0.5, or 1.0 mgl–1 indole-3-butyric acid (IBA). The best root formation was obtained on IBA-free medium. The plantlets thus produced were tested for the presence of ten sweet potato viruses [Sweet potato chlorotic stunt virus (SPCSV), Sweet potato feathery mottle virus (SPFMV), Sweet potato mild mottle virus (SPMMV), Sweet potato chlorotic fleck virus (SPCFV), Sweet potato caulimo-like virus (SPCaLV), Sweet potato mild speckling virus (SPMSV), C-6 (a flexuous rod virus), Sweet potato latent virus (SwPLV), Sweet potato virus G (SPVG), and Cucumber mosaic virus (CMV)] using nitrocellulose membrane enzyme-linked immunosorbent assay (NCM-ELISA). Ninety-nine percent of plantlets were free from all viruses.

Low genetic diversity of Squash vein yellowing virus in wild and cultivated cucurbits in the U.S. suggests a recent introduction

Squash vein yellowing virus (SqVYV) isolates were collected from cultivated and weedy cucurbits representing major hosts and locations in the U.S. and analyzed to better understand the diversity and population structure. No differences in symptoms were observed in field-collected isolate source plants or subsequently inoculated greenhouse plants, and the complete genome of an SqVYV isolate from a wild cucurbit host (smellmelon, Cucumis melo var. dudaim) was highly similar (99.4% nucleotide identity, 99.3% amino acid identity) to the previously published type isolate from squash. Although analysis of the coat protein (CP) and two serine proteases (P1a and P1b) sequences for 41 isolates showed little diversity across seven years of sampling, it revealed two distinct groups of SqVYV isolates with low intra-group diversity. Our analyses also suggested that recombination had occurred between SqVYV isolates, similar to other ipomoviruses. Selection pressures on the genome regions analyzed were negative indicating purifying selection was occurring. The magnitude of negative selection in SqVYV was consistent with what has been reported for other ipomoviruses, and was greatest for the CP and least for the P1b. The observed genetic diversity was similar to that reported for Cucumber vein yellowing virus but less than that reported for Sweet potato mild mottle virus, Cassava brown streak virus and Ugandan cassava brown streak virus. Collectively, these results indicate that the current U.S. population of SqVYV has undergone a recent genetic bottleneck and was introduced from elsewhere.

Eggplant mild leaf mottle virus (EMLMV), a new putative member of the genus Ipomovirus that harbors an HC-Pro gene

Since 2003, a new viral disease of eggplant (Solanum melongena L.) has been spreading in fields in the Jordan and Arava Valleys, Israel. The symptoms of this disease include mild leaf mottling and varying degrees of fruit distortion. This disease can be transmitted by mechanical sap inoculation, as well as by the whitefly Bemisia tabaci (Homoptera, Aleyrodidae) and has been tentatively named eggplant mild leaf mottle virus (EMLMV). Our study aimed to determine the complete sequence and genome organization of EMLMV. The extracted viral RNA was subjected to SOLiD next-generation sequence analysis and used as a template for reverse transcription synthesis, which was followed by ds-cDNA synthesis or PCR amplification. The ssRNA genome of EMLMV includes 9,280 nucleotides, excluding a 3' terminal poly-adenylated tail. The genome includes a putative single, large open reading frame (ORF) that encodes a polyprotein of 3,011 amino acids, a short overlapping ORF of PIPO protein comprised of 71 amino acids and 5' and 3' non-coding regions of 108 and 136 nucleotides, respectively. The deduced amino acid sequence of the EMLMV polyprotein is relatively close to that of sweet potato mild mottle virus (SPMMV), with 37% shared sequence identity. Among the four ipomoviruses, only SPMMV and the putative genus member EMLMV contain a helper component-proteinase (HC-Pro) gene. Like SPMMV-HC-Pro, EMLMV-HC-Pro also contains the highly conserved PTK domain that is thought to be involved in the aphid-assisted transmission of potyviruses.

Analysis of the tomato mild mottle virus genome indicates that it is the most divergent member of the genus Ipomovirus (family Potyviridae)

The complete genome of a tomato mild mottle virus (ToMMV) isolate was analysed, and some biological features were characterized. The ssRNA genome of ToMMV from Ethiopia encompasses 9283 nucleotides (excluding the 3' poly(A) tail) and encodes a polyprotein of 3011 amino acids. Phylogenetic and pairwise comparisons with other members of the family Potyviridae revealed that ToMMV is the most divergent member of the genus Ipomovirus, with a genome organization similar to that of members of the species Sweet potato mild mottle virus, the type species of the genus. In contrast to earlier reports, ToMMV isolates from Yemen and Ethiopia were not transmitted by the aphid Myzus persicae, but they were transmitted very erratically by the whitefly Bemisia tabaci. A comparison of the 3'-proximal sequences of different isolates provided evidence for geographically associated genetic variation.

Diagnostics of viruses infecting local farmer preferred sweetpotato cultivars in Kenya

Sweetpotato is an important starchy tuberous root crop grown in many tropical and subtropical regions of the world, with about 75% of African sweetpotato production occurring in East Africa. Its production is however, constrained by viral diseases particularly sweet potato feathery mottle virus (SPFMV), sweet potato chlorotic stunt virus (SPCSV), sweet potato mild mottle virus (SPMMV), sweet potato chlorotic fleck virus (SPCFV), and sweet potato caulimo-like virus (SPCaLV), prevalent in Kenya. This work involved the designing of primers and their validation by screening of various sweetpotato materials from various sources including field and tissue culture material. The sensitivity of the diagnostic technique was evaluated and compared with the ELISA-based techniques. It was apparent that the tool was more sensitive in diagnosing the sweetpotato viruses than the available ELISA-based tool. This tool is recommended for the virus indexing for the plant materials farmers’ fields, plant quarantine and laboratories involved in sweetpotato research within the region. Key words: Sweet potato viruses, RT-PCR, diagnostics.

Detection and elimination of sweetpotato viruses

In sub-Saharan Africa, sweetpotato (Impomoea batatas L.) production is greatly constrained by sweetpotato virus disease (SPVD) complex. This study was conducted to assess the incidence of viruses in healthy-looking sweetpotato in Uganda and to optimise modern technologies for virus diagnosis. A collection of healthy-looking sweetpotato vines from central Uganda were serologically assayed for sweetpotato viruses and the positive samples were confirmed by RT-PCR. A multiplex RT-PCR assay was optimised for simultaneous detection of Sweet potato chlorotic stunt virus (SPCSV), Sweet potato feathery mottle virus (SPFMV) and Sweet potato mild mottle virus (SPMMV). The use of in vitro thermotherapy was also investigated as a means of eliminating sweetpotato viruses. Four viruses namely SPCSV, SPFMV, SPMMV and SPCFV were detected mostly as single infections in the healthy looking plants. SPCSV (70. 6%) recorded highest incidence followed by co-infection of SPFMV and SPCSV (8.3%). Based on shoot survival and effectiveness of virus elimination, the best results were obtained by exposing plantlets to daily temperature regime of 32 oC for 8 hr of darkness and 36 oC for 16 hr of light for four weeks. Meristem-tip culture combined with thermotherapy allowed elimination of SPFMV and SPMMV in 77% of plants that were previously infected with the respective viruses. However, elimination of SPCSV was unsuccessful.

SURVEY OF SWEET POTATO VIRUSES IN WESTERN KENYA AND DETECTION OF CUCUMBER MOSAIC VIRUS

Sweet potato is an important food crop worldwide, but several pests and diseases limit its production. In eastern Africa, virus-induced diseases rank second to weevils in causing yield reduction. Symptomatic sweet potato cuttings (327) were collected from Nyanza and Western Provinces in western Kenya in 2009. The samples were tested for Sweet potato feathery mottle virus (SPFMV), Sweet potato chlorotic stunt virus (SPCSV), Sweet potato mild mottle virus (SPMMV), Sweet potato chlorotic fleck virus (SPCFV), Sweet potato latent virus (SPLV), Sweet potato caulimo-like virus (SPCa-LV), Cucumber mosaic virus (CMV), C-6, Sweet potato virus G (SPVG) and Sweet potato mild speckling virus (SPMSV) using nitrocellulose membrane enzyme-linked immunosorbent assay (NCM-ELISA). SPFMV, SPCSV, SPCFV, SPMMV and CMV were detected and 89% of the samples as a whole were found to be infected. SPFMV was detected in all infected samples followed by SPCSV (55%). Multiple infections were detected in the majority of the samples (80%) and the most common dual infection was with SPFMV and SPCSV (52%). The occurrence of CMV was low (5%) but was confirmed by RT-PCR with amplification of a 670 bp coat protein gene fragment from total RNA. This is the first record of CMV in sweet potato in Kenya.

Analysis of complete genomic sequences of isolates of the Sweet potato feathery mottle virus strains C and EA: molecular evidence for two distinct potyvirus species and two P1 protein domains

The complete nucleotide sequence of the isolate C1 of Sweet potato feathery mottle virus (SPFMV) strain C and the 5' region of several other strains were determined and analyzed together with the sequences of isolates representing the EA, RC and O strains. This provided molecular evidence for the reclassification of SPFMV strains into two species and the occurrence of a complex recombinant isolate. Analysis also revealed a hypervariable domain in the P1 protein, which separates an N-terminal region unique to SPFMV and members of the ipomovirus species Sweet potato mild mottle virus from the C-terminal protease domain, which is conserved among all potyviruses.