Cassava leaf harvesting as vegetables; a cause of vulnerability of cassava plant to cassava mosaic disease and eventual yield reduction: Ernte von cassavablättern als gemüse; eine ursache für die anfälligkeit der cassavapflanze für die cassava-mosaikkrankheit und für spätere ertragseinbußen

The consequence of harvesting young leaves of cassava as vegetable on the vulnerability of the crop to cassava mosaic disease (CMD) and on storage root yield was investigated using 30 cassava genotypes planted in IITA fields located in the humid forest (Port Harcourt?:?Onne), forest-savannah transition (Ibadan), southern guinea savannah (Mokwa) and northern guinea savannah (Zaria) agroecologies in Nigeria. Tender apical leaves and shoots of the cassava genotypes were removed from forty plants per cassava genotype with the same number of plants considered as control. Whitefly infestation, disease incidence (DI) and symptom severity (ISS) of the disease were assessed at monthly interval for six months and also at the ninth month after planting (MAP). Yield reduction due to this treatment was calculated as percentage harvest index (HI). Whitefly population fluctuated throughout the period of observation at all locations with higher population obtained generally for treated plants compared to control plants. Sprouting leaves of some treated genotypes were observed with severe mosaic symptoms, while corresponding control showed no mosaic symptoms. Contrarily, no remarkable difference was observed in Zaria between the mean ISS of treated and control cassava genotypes. There was a highly significant difference (P?<?0.01) in DI and ISS among cassava genotypes across all locations. Also, there was a highly significant interaction (P?<?0.01) in symptom severity between location (loc) and genotype, genotype and treatment (trt), loc and trt. Interaction between loc, genotypes and trt with regard to DI was highly significant at 2, 3 and 4 MAP, while with ISS, the interaction was highly significant all through the counting period. There was a positive relationship between DI and ISS on plants of genotypes 96/1039 and ISU. The percentage HI (27.4) of treated plants of genotype 95/0166 in Ibadan was remarkably lower than the value obtained for corresponding control (41.9) plants. Also, sharp distinction in% HI of treated (39.5) and control (43.8) ISU was observed in Onne with their respective ISS values as 3.7 and 3.2. Therefore, harvesting tender apical leaves and shoots of cassava as vegetables should be discouraged as it increases the severity of CMD infection in the regenerating shoots of cassava with attendant storage root yield reduction.     

Screening cassava for resistance to cassava mosaic disease by grafting and whitefly inoculation

Screening for cassava mosaic begomoviruses (CMBs)-resistance using grafting and whitefly inoculation was performed with local and improved cassava. The onset of symptom appearance and the evolution of Cassava mosaic disease (CMD) varied in function of genotypes and virus inoculation techniques used. Grafting position using cassava as scion or rootstock does not affect CMD display and evolution. No relation was established between the number of whiteflies feeding on each genotype and viral inoculation technique tested. Detopping of young leaves induces triggering effect on CMD expression. PCR and ELISA confirmed the EACMV-UG's preferential transmission by whitefly. Hypothesis of virus replication and cultivars's susceptibility were supported by virus increasing particles in infected cassava. Cultivars Mvuazi (TMSI 95/0528) and 96/1089A are suggested field immune to CMBs; Disanka (TMSI 95/0211), Yauma, Timolo, Bangi, Mahungu (TMS 92/297), Mvuama (TMS 83/138), Lueki (TMS 91/377) and Zizila (MV 99/0038) are CMD-resistant; whereas Ponjo, Lofiongi, Ngonga and Mboloko are susceptible. Our results showed that resistant genotypes may express CMD under high inoculum pressure such as grafting.     

Reaction of Cassava Genotypes to the Cassava Mosaic Disease in Three Distinct Agroecologies in Nigeria

Nine cassava genotypes were grown at six representative sites in Nigeria for 3 years to study their response to cassava mosaic disease (CMD), investigate the influence of genotype × environment (G × E) interactions on their reactions to the disease, and identify genotypes with stability to the disease, using the Additive Main Effects and Multiplicative Interaction statistical model. Environments, genotypes and G × E interactions were highly significant (P < 0.01) for the disease. The G × E interactions accounted for 19.5% of the treatment sums of squares for CMD and influenced the relative ranking of genotypes across environments. The magnitude of the G × E interaction effect for CMD was larger than that of genotypes. Examination of the G × E interaction structure revealed specific areas where screening of cassava genotypes for resistance to CMD could be performed best. The study identified genotypes such as TMS 30001 and 63397 with resistance to CMD and CMD‐stable clone U/41044, which could be distributed to growers, and sites such as Ibadan and Ubiaja with high CMD severity for screening genotypes for reaction to CMD.     

Occurrence and distribution of cassava begomoviruses in Kenya

A survey for cassava mosaic disease (CMD) was conducted in Kenya, to investigate the factors contributing to the generally increased incidence and severity of CMD in the cassava growing regions and to study the distribution of the disease's causal begomoviruses, African cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV) and their strains. Special emphasis was given to the occurrence of the destructive recombinant Uganda variant strain of EACMV (EACMV-UG2). Samples from 91 farmers' fields in the main cassava-growing areas of coastal and western Kenya were collected and subjected to ELISA and PCR for detection and typing of the begomoviruses present. CMD incidence was highest in western Kenya (80–100%) and lowest in the Coast province (25–50%). In Western and Nyanza provinces, 52% of the samples tested contained EACMV-UG2, 22% ACMV and 17% contained both ACMV and EACMV-UG2. EACMV was found in four cases at different sites. In cassava samples from the coast province, only EACMV with DNA-A sequences similar to EACMV strains present in Kenya and Tanzania was found. East African cassava mosaic Zanzibar virus (EACMZV) was present in several farms in the Kilifi district. In 15% of all cassava samples with CMD symptoms, flexuous, filamentous virus-like particles were also found, providing evidence for a more complex virus situation in cassava grown at the Kenyan coast. In western Kenya, where intense cassava cultivation takes place, CMD is rampant and EACMV-UG2 was found in mixed virus infections with ACMV driving the epidemics. In coastal areas, where farms are scattered and in isolation, EACMV is endemic, however, with a lower disease incidence and with a limited impact to cassava production.     

Loss of CMD2‐mediated resistance to cassava mosaic disease in plants regenerated through somatic embryogenesis

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are the two most important viral diseases affecting cassava production in Africa. Three sources of resistance are employed to combat CMD: polygenic recessive resistance, termed CMD1, the dominant monogenic type, named CMD2, and the recently characterized CMD3. The farmer‐preferred cultivar TME 204 carries inherent resistance to CMD mediated by CMD2, but is highly susceptible to CBSD. Selected plants of TME 204 produced for RNA interference (RNAi)‐mediated resistance to CBSD were regenerated via somatic embryogenesis and tested in confined field trials in East Africa. Although micropropagated, wild‐type TME 204 plants exhibited the expected levels of resistance, all plants regenerated via somatic embryogenesis were found to be highly susceptible to CMD. Glasshouse studies using infectious clones of East African cassava mosaic virus conclusively demonstrated that the process of somatic embryogenesis used to regenerate cassava caused the resulting plants to become susceptible to CMD. This phenomenon could be replicated in the two additional CMD2‐type varieties TME 3 and TME 7, but the CMD1‐type cultivar TMS 30572 and the CMD3‐type cultivar TMS 98/0505 maintained resistance to CMD after passage through somatic embryogenesis. Data are presented to define the specific tissue culture step at which the loss of CMD resistance occurs and to show that the loss of CMD2‐mediated resistance is maintained across vegetative generations. These findings reveal new aspects of the widely used technique of somatic embryogenesis, and the stability of field‐level resistance in CMD2‐type cultivars presently grown by farmers in East Africa, where CMD pressure is high.     

Current characteristics of cassava mosaic disease in postepidemic areas increase the range of possible management options

As a case study to document the current characteristics of cassava mosaic virus disease (CMD) in postepidemic areas, surveys were carried out, in 2003 and 2004, in Siaya District of western Kenya. This was an area affected by a severe CMD pandemic in the late 1990s. Data recorded on cassava varieties were CMD incidence, severity index and number of adult whiteflies. Farmers (174) were interviewed on their understanding of the disease and their knowledge and practice of management interventions. Cassava cultivation was being re‐established, but local landraces predominated. Resistant varieties were present 13% in 2003, and 4% in 2004, of the surveyed fields. Adhiambolera was the most common variety, occurring in 35% and 40% of fields in 2003 and 2004, respectively, and had an average CMD incidence of 82% in 2003 and 73% in 2004. By contrast, the CMD‐resistant variety Migyera had a low mean incidence (28% in 2003). The overall incidence for both years was 71%, consisting of 61% as a result of infection through planting diseased cuttings and 10% as a result of whitefly infection. In 2003, the total incidence was 72% and the average severity 2.7 (severity index), while in 2004 the incidence was 78% and the severity 2.6. There were significant severity variations in each division of the Siaya District during the 2 years except for Karemo and Ukwala. The abundance of whiteflies on the top five leaves of plants was low in 2003 but high in 2004, with means of 1 and 16, respectively, over the same seven divisions in both years, although this variation was thought to be because of seasonal factors. East African cassava mosaic virus‐Uganda was the predominant geminivirus present in every division. Phytosanitation by farmers was minimal, as evidenced by 29% of farmers using a selection of CMD‐free stems for planting and 15% using hand‐roguing for CMD management. Occurrence of more than 25% CMD‐free plants in 2004, moderate CMD severity and limited spread provide a conducive environment for the use of phytosanitation as a CMD control measure that can be immediately used by farmers growing their own cassava varieties.     

Whitefly Bemisia tabaci (Homoptera: Aleyrodidae) infestation on cassava genotypes grown at different ecozones in Nigeria

Large-scale screening of cassava, Manihot esculenta Crantz, genotypes for resistance to infestation by whitefly Bemisia tabaci Gennadius, the vector of cassava mosaic geminiviruses, is limited. A range of new cassava elite clones were therefore assessed for the whitefly infestation in the 1999/2000 and 2000/2001 cropping seasons in experimental fields of International Institute of Tropical Agriculture, Ibadan, Nigeria. On each scoring day, between 0600 and 0800 hours when the whiteflies were relatively immobile, adult whitefly populations on the five topmost expanded leaves of cassava cultivars were counted. All through the 6-mo scoring period, there was a highly significant difference in whitefly infestation among the new cassava elite clones. Vector population buildup was observed in Ibadan (forest-savanna transition zone) and Onne (humid forest), 2 mo after planting (MAP). Mean infestation across cassava genotypes was significantly highest (16.6 whiteflies per plant) in Ibadan and lowest in Zaria (0.2). Generally, whitefly infestation was very low in all locations at 5 and 6 MAP. During this period, cassava genotypes 96/1439 and 91/02324 significantly supported higher infestations than other genotypes. Plants of 96/1089A and TMS 30572 supported the lowest whitefly infestation across cassava genotypes in all locations. The preferential whitefly visitation, the differences between locations in relation to whitefly population, cassava mosaic disease, and the fresh root yield of cassava genotypes are discussed.     

Rapid screenhouse assessment of bacterial blight in cassava genotypes in Nigeria

Bacterial blight disease caused by Xanthomonas axonopodis pv. manihotis (Berthet-Bondar) Dye was assessed in 11 artificially inoculated cassava genotypes in a screenhouse. Disease progress was estimated at intervals of 3 days by measuring the length of necrotic lesions on stems and leaves, as well as estimating the average disease score and area under disease progress curve (AUDPC). Based on the average disease scores, cassava genotypes 30572, TME 1, TME 7 and TME 9 were classified as resistant to bacterial blight, genotypes 4(2)1425, TME 2, TME 4 and TME 12 were tolerant while cassava genotypes 30001, TME 3, and TME 28 were susceptible. Direct correlations, statistically significant at p < 0.05, were obtained between stem necrosis, leaf necrosis, average disease scores and AUDPC in the 11 cassava genotypes. Screenhouse experiments afford rapid assessment of resistance status of cassava genotypes to bacterial blight in Nigeria.     

Application of a Simplified Molecular Protocol to Reveal Mixed Infections with Begomoviruses in Cassava

Samples of cassava leaves exhibiting severe symptoms of cassava mosaic disease (CMD) were collected with the PhytoPASS kit in fields surrounding the city of Bujumbura (Burundi). These materials were then sent to Belgium for polymerase chain reaction determination of the CMD begomoviruses inducing the observed symptoms. Different pairs of specific primers were used to amplify DNA sequences specific to African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Malawi virus (EACMMV), East African cassava mosaic Zanzibar virus (EACMZV), the Uganda variant of East African cassava mosaic virus (EACMV-UG) and South African cassava mosaic virus (SACMV). It was revealed that mixed infections were prevailing in the analyzed materials. Most of the samples submitted to this analysis were found to be co-infected by three different begomoviruses (ACMV + EACMV + EACMV-UG). The so revealed mixed infections could explain the high severity of CMD symptoms noticed on cassava in the region of Bujumbura while the diversity within the CMD causal agents illustrates the importance to take this parameter into consideration for a successful use of plant genetic resistance to control the disease.     

Abundance,diversity and geographic distribution of cassava mosaic disease pandemic‐associated Bemisia tabaci in Tanzania

Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), one of the most economically important agricultural pests worldwide, is the vector of cassava mosaic geminiviruses that cause cassava mosaic disease (CMD). In East and Central Africa, a severe CMD pandemic that spread from Uganda in the late 1980s still continues to devastate cassava crops. To assess the association of distinct B. tabaci genetic groups with the CMD pandemic, mitochondrial cytochrome oxidase I gene sequences were analysed from whiteflies collected during surveys conducted from 2010 to 2013 in Tanzania. Four genetic groups – Sub‐Saharan Africa 1 (SSA1), Mediterranean, Indian Ocean and East Africa 1, and a group of unknown whitefly species were identified. SSA1 comprised four subgroups: SSA1‐SG1, SSA1‐SG2, SSA1‐SG1/2 and SSA1‐SG3. SSA1‐SG1 was confined to the pandemic‐affected north‐western parts of Tanzania whilst SSA1‐SG2 and SSA1‐SG3 were found in the central and eastern parts not yet affected by the pandemic. The CMD pandemic front was estimated to lie in Geita Region, north‐western Tanzania, and to be spreading south‐east at a rate of ca 26 km/year. The pandemic‐associated B. tabaci SSA1‐SG1 predominated up to 180 km ahead of the CMD front indicating that changes in whitefly population characteristics precede changes in disease characteristics.     

The cassava mosaic geminiviruses occurring in Uganda following the 1990s epidemic of severe cassava mosaic disease

The cassava mosaic geminiviruses (CMGs) isolated from cassava plants expressing mild and severe symptoms of cassava mosaic disease (CMD) in 2002 in Uganda were investigated using the polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) molecular techniques and DNA sequencing. Two previously described cassava mosaic geminiviruses: African cassava mosaic virus (ACMV) said East African cassava mosaic virus - Uganda variant (EACMV-UG2) were detected in Uganda. The RFLP technique distinguished two polymorphic variants of ACMV (ACMV-UG1 and ACMV-UG2) and three of EACMV-UG2 (EACMV-UG2[1], EACMV-UG2[2] and EACMV-UG2[3]). ACMV-UG1 produced the fragments predicted for the published sequences of ACMV-[KE]/UGMld/ UGSvr, while ACMV-UG2, which produced the RFLP fragments predicted for the West African ACMV isolates ACMV-[NG], ACMV-[CM], ACMV-[CM/DO2] and ACMV-[CI], was shown to be ACMV-UGMld/UGSvr after DNA sequencing. EACMV-UG2[1] produced the RFLP fragments predicted for the published sequences of EACMV-UG2/UG2Mld/UG2Svr. However, both EACMV-UG2[2] and EACMV-UG2[3], which produced East African cassava mosaic vzras-[Tanzania]-like polymorphic fragments with RFLP analysis, were confirmed to be isolates of EACMV-UG2 after DNA sequencing. Thus, this study emphasises the importance of DNA sequence analysis for the identification of CMG isolates. EACMV-UG2 was the predominant virus and occurred in all the surveyed regions. It was detected in 73% of the severely and 53% of the mildly diseased plants, while ACMV was less widespread and occurred most frequently in the mildly diseased plants (in 27% of these plants). Mixed infections of ACMV and EACMV-UG2 were detected in only 18% of the field samples. Unlike previously reported results the mixed infection occurred almost equally in plants exhibiting mild or severe disease symptoms (21% and 16%, respectively). The increasing frequency of mild forms of EACMV-UG2 together with the continued occurrence of severe forms in the field warrants further studies of virus-virus and virus-host interactions.     

Occurrence and activity of <Emphasis Type="Italic">Bemisia tabaci</Emphasis> parasitoids on cassava in different agro-ecologies in Uganda

Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) is the vector of cassava mosaic geminiviruses that cause cassava mosaic disease (CMD), which in turn causes devastating yield losses. Surveys were conducted from October 2000 to November 2001 in four agro-ecologies in Uganda to enhance the understanding of parasitoid fauna and parasitism of B. tabaci in cassava fields. Such an understanding is an essential prerequisite for the development of biological control methods of B. tabaci to complement current CMD control practices. Parasitoid abundance and parasitism efficiency varied between locations and sampling dates within the locations; highest parasitoid densities were observed at Namulonge in the Lake Victoria crescent while the lowest was at Kalangala. In all locations, parasitism was mainly due to Encarsia sophia Dodd and Girault and Eretmocerus mundus Mercet (all Hymenoptera: Aphelinidae). Two occasionally observed species included Encarsia mineoi Viggiani (Hymenoptera: Aphelinidae), only observed at Namulonge, and blackhead Encarsia (Hymenoptera: Aphelinidae) observed at Bulisa, Namulonge and Lyantonde. Parasitism efficiency was highest at Bulisa (57.9%), but ranged from 40.2 to 46.9% at the other three sites. This paper discusses the possible causes of variations in parasitoid abundance and parasitism efficiency, and proposes further studies that might be carried out to assess the potential for augmentation of parasitoids to control B. tabacipopulations and CMD.     

Molecular Variability and Distribution of Cassava Mosaic Begomoviruses in Nigeria

Several begomovirus species and strains causing Cassava mosaic disease (CMD) have been reported from cassava in Africa. In Nigeria, African cassava mosaic virus (ACMV) was the predominant virus in this important crop, and East African cassava mosaic virus (EACMV), first reported from eastern Nigeria in 1999, was also found occasionally. A survey was conducted in 2002 to resolve the diversity of the virus types present in cassava in Nigeria and to further understand the increasing complexity of the viruses contributing to CMD. A total of 234 leaf samples from cassava with conspicuous CMD symptoms were collected in farmers’ fields across different agroecological zones of Nigeria and subjected to polymerase chain reaction (PCR) with type‐specific primers. In addition and, to provide a full characterization of the viruses present, DNA‐A genome components of several viruses and informative genome fragments were sequenced. In Nigeria, ACMV proved to be the dominant virus with 80% of all samples being positive for ACMV. The East African cassava mosaic Cameroon virus (EACMCV) prevalent in Cameroon and Ivory Coast was detected in single infections (2%) and in mixed infections (18%) with ACMV. There was no indication for other virus strains of EACMV present in the country. The EACMCV samples collected showed a high nucleotide sequence identity >98% and resembled the described sequence of a Cameroon isolate (EACMCV‐CM) more than an Ivory Coast isolate, EACMCV‐CM[CI]. Evidence is provided that the EACMCV has reached epidemiological significance in Nigeria.     

Serological and biological variations of African cassava mosaic virus, in Nigeria

To determine the occurrence of variants of African cassava mosaic virus, 316 cassava leaf samples were collected from mosaic‐affected cassava plants in 254 farmers. fields in 1997 and 1998, covering the humid forest, coastal/derived, southern Guinea and northern Guinea savannas and arid and semi‐arid agroecologies of Nigeria. The samples were tested in triple antibody sandwich enzyme‐linked immunosorbent assay using a panel of 10 monoclonal antibodies (MAbs) against the virus in which 29 reaction patterns were observed. In cluster analysis, nine serotypes were obtained at 0.80 Jaccard similarity coefficient index in which at least 50% of isolates of each serotype reacted alike. The serotypes ranged between two extremes: serotype 1 with 90% isolates reacting with the 10 MAbs and serotype 8 in which 90% of its isolates failed to react with the antibodies. Isolates of serotypes 1, 2, 4 and 8 were widely distributed while those of the other serotypes were estricted to certain agroecologies. Four representative isolates 227 (serotype 1), 231 (serotype 2), 235 and 283 (serotype 8) elicited different responses in Nicotiana, benthamiana, with isolate 283 not able to infect this and other test plants used. The serological variations did not necessarily reflect the biological variations. In polymerase chain reaction tests, one out of the five pairs of ACMV primers tested distinguished only isolate 283. The humid forest, derived/coastal and southern Guinea savannas where most of the crop is grown in Nigeria had a high number of variants, which makes the agroecologies suitable for the selection of resistant cassava clones against ACMV.     

Viruses Associated with Cassava Mosaic Disease in Senegal and Guinea Conakry

A survey in Senegal and Guinea Conakry established the presence and incidence of cassava mosaic virus disease (CMD) in both countries. CMD occurred in all the fields surveyed, although its incidence was higher in Senegal (83%) than in Guinea (64%). Populations of the whitefly vector, Bemisia tabaci, were low in both countries averaging 1.7 adults per shoot in Guinea and 3.2 in Senegal. Most infections were attributed to the use of infected cuttings, 86 and 83% in Senegal and Guinea, respectively, and there was no evidence of rapid current‐season, whitefly‐borne infection at any of the sampled locations. Disease severity was generally low in the two countries and averaged 2.5 in Guinea and 2.3 in Senegal. No plants with unusually severe CMD symptoms characteristic of the CMD pandemic in East and Central Africa were observed. Restriction fragment length polymorphism (RFLP)‐based diagnostics revealed that African cassava mosaic virus (ACMV) is exclusively associated with CMD in both the countries. Neither East African cassava mosaic virus (EACMV), nor the recombinant Uganda variant (EACMV‐UG2) was detected in any sample. These survey data indicate that CMD could be effectively controlled in both countries by phytosanitation, involving the use of CMD‐free planting material and the removal of diseased plants.     

Colonization of non-cassava plant species by cassava whiteflies (Bemisia tabaci) in Uganda

Bemisia tabaci (Genn.) (Homoptera: Aleyrodidae) is the vector of cassava mosaic geminiviruses (CMGs), which are the main production constraint to cassava [Manihot esculenta Crantz (Euphorbiaceae)], both in Uganda and elsewhere in Africa. Two B. tabaci genotype clusters, Ug1 and Ug2, differentiated at 8% nucleotide (nt) divergence within the mitochondrial cytochrome oxidase I (mtCOI) gene, have been shown to occur on cassava in Uganda. However, the role of alternative hosts in the ecology of cassava B. tabaci genotypes and their possible involvement in the epidemiology of cassava mosaic disease (CMD) in Uganda remain unknown. In this study, we investigated the restriction of cassava B. tabaci genotypes to cassava and the colonization of alternative host species in select cassava‐growing areas of the country in 2003 and 2004. Bemisia tabaci adults and 4th instar nymphs were collected from cassava and 11 other cultivated and uncultivated species occurring adjacent to the sampled cassava fields. Phylogenetic analysis of mtCOI sequences revealed that only a single genotype cluster, Ug1, was present on both cassava and non‐cassava plant species sampled in this study. The Ug1 genotypes (n = 49) shared 97–99% nt identity with the previously described cassava‐associated B. tabaci populations in southern Africa, and were ～8% and ～13% divergent from Ug2 and the ‘Ivory Coast cassava’ genotypes in Uganda and Ivory Coast, respectively. The Ug1 genotypes occurred (as adults) on all 12 source‐plant species sampled. However, based on the presence of B. tabaci 4th instar nymphs, the Ug1 genotypes (n = 13) colonized cassava and five other non‐cassava plant species: Manihot glaziovii, Jatropha gossypifolia, Euphorbia heterophylla, Aspilia africana, and Abelmoschus esculentus, suggesting that cassava B. tabaci (Ug1 genotypes) are not restricted to cassava in Uganda. No Ug2‐like genotypes were detected on any of the plant species sampled, including cassava, in this study. The identification of additional hosts for at least one genotype cluster, Ug1, known also to colonize cassava, and which was hitherto thought to be ‘cassava‐restricted’ may have important epidemiological significance for the spread of CMGs in Uganda.     

Particle purification and properties of cassava Ivorian bacilliform virus

A virus found in cassava from the north-west of the Ivory Coast was transmitted by inoculation with sap extracts to herbaceous species in six plant families. Chenopodium quinoa was used as a propagation host and C. murale was used for local lesion assays. The virus particles are bacilliform, c. 18 nm in diameter, with predominant lengths of 42,49 and 76 nm and a structure apparently similar to that found in alfalfa mosaic virus. Purified preparations of virus particles had A₂₆₀/A₂₈₀ of 1.7 ±0.05, contained one protein of M_rc. 22 000, and yielded three species of RNA with M_r (× 10^-6) of c. 0.7, 0.8 and 1.2. Although the virus particles were poorly immunogenic, an antiserum was produced and the virus was detected by enzyme-linked immunosorbent assay (DAS-ELISA) in leaf extracts at concentrations down to c. 6 ng/ml. Four other field isolates were also detected, including a strain which caused only mild systemic symptoms in C. quinoa instead of necrosis. The naturally infected cassava source plants were also infected with African cassava mosaic virus (ACMV) but when the new virus was cultured in Nicotiana benthamiana, either separately or together with ACMV, its concentration was the same. The new virus did not react with antisera to several plant viruses with small bacilliform or quasi-bacilliform particles, and alfalfa mosaic virus reacted only weakly and inconsistently with antiserum to the cassava virus. The new virus, for which the name cassava Ivorian bacilliform virus is proposed, is tentatively classified as the second member of the alfalfa mosaic virus group.     

Efficacy of chemotherapy and thermotherapy in elimination of East African cassava mosaic virus from Tanzanian cassava landrace

Cassava mosaic disease is caused by cassava mosaic begomoviruses (CMBs) and can result in crop losses up to 100% in cassava (Manihot esculenta) in Tanzania. We investigated the efficacy of chemotherapy and thermotherapy for elimination of East African cassava mosaic virus (EACMV) of Tanzanian cassava. In vitro plantlets from EACMV‐infected plants obtained from coastal Tanzania were established in the greenhouse. Leaves were sampled from the plants and tested to confirm the presence of EACMV. Plantlets of plants positive for EACMV were initiated in Murashige and Skoog (MS) medium. On the second subculture, they were subjected into chemical treatment in the medium containing salicylic acid (0, 10, 20, 30 and 40 mg/L) and ribavirin (0, 5, 10, 15 and 20 mg/L). In the second experiment, EACMV‐infected plantlets were subjected to temperatures between 35 and 40°C with 28°C as the control. After 42 days of growth, DNA was extracted from plant leaves and PCR amplification was performed using EACMV specific primers. It was found that plant survival decreased with increasing levels of both salicylic acid and ribavirin concentrations. In general, plants treated with salicylic acid exhibited a lower plant survival % than those treated with ribavirin. However, the percentage of virus‐free plants increased with an increase in the concentration of both ribavirin and salicylic acid. The most effective concentrations were 20 mg/L of ribavirin and 30 mg/L of salicylic acid; these resulted in 85.0% and 88.9% virus‐free plantlets, respectively. With regard to thermotherapy, 35°C resulted in 79.5% virus‐free plantlets compared to 69.5% at 40°C. Based on virus elimination, ribavirin at 20 mg/L, salicylic acid 30 mg/L and thermotherapy at 35°C are recommended for production of EACMV free cassava plantlets from infected cassava landraces.