Factors determining recovery and reversion in mosaic-affected African cassava mosaic virus resistant cassava

The severity and persistence of symptoms of mosaic virus disease were monitored during the first six months of two growing seasons in cassava of the African cassava mosaic virus (ACMV)-resistant cv. TMS 30572 either inoculated by grafting with a mild or severe strain or infected from the planted cutting. Symptomless shoots developed between January and March 1995 in two field trials differing in age by c. 6 months; this recovery occurred during particularly hot weather. Recovery was often only temporary in the plants inoculated with the severe strain and occurred later compared with those inoculated with the mild. In 1996, the weather was cooler and recovery that year was delayed until flowering, c. 7 months after planting, when recovered shoots were often produced from buds in the axils of symptomless leaves produced amongst diseased leaves. Most cuttings taken from the upper parts of diseased plants produced symptomless (reverted) progenies whereas most cuttings taken from the base of diseased plants produced diseased progenies. Reversion seemed to be associated with the recovery that had already occurred in the upper stems of the parent plants.     

Risk assessment of gene flow from genetically engineered virus resistant cassava to wild relatives in Africa: an expert panel report

The probability and consequences of gene flow to wild relatives is typically considered in the environmental risk assessment of genetically engineered crops. This is a report from a discussion by a group of experts who used a problem formulation approach to consider existing information for risk assessment of gene flow from cassava (Manihot esculenta) genetically engineered for virus resistance to the ‘wild’ (naturalized) relative M. glaziovii in East Africa. Two environmental harms were considered in this case: (1) loss of genetic diversity in the germplasm pool, and (2) loss of valued species, ecosystem resources, or crop yield and quality due to weediness or invasiveness of wild relatives. Based on existing information, it was concluded that gene flow will occur, but it is not likely that this will reduce the genetic diversity in the germplasm pool. There is little existing information about the impact of the virus in natural populations that could be used to inform a prediction about whether virus resistance would lead to an increase in reproduction or survival, hence abundance of M. glaziovii. However, an increase in the abundance of M. glaziovii should be manageable, and would not necessarily lead to the identified environmental harms.     

Molecular biodiversity of cassava begomoviruses in Tanzania: evolution of cassava geminiviruses in Africa and evidence for East Africa being a center of diversity of cassava geminiviruses

Cassava is infected by numerous geminiviruses in Africa and India that cause devastating losses to poor farmers. We here describe the molecular diversity of seven representative cassava mosaic geminiviruses (CMGs) infecting cassava from multiple locations in Tanzania. We report for the first time the presence of two isolates in East Africa: (EACMCV-[TZ1] and EACMCV-[TZ7]) of the species East African cassava mosaic Cameroon virus, originally described in West Africa. The complete nucleotide sequence of EACMCV-[TZ1] DNA-A and DNA-B components shared a high overall sequence identity to EACMCV-[CM] components (92% and 84%). The EACMCV-[TZ1] and -[TZ7] genomic components have recombinations in the same genome regions reported in EACMCV-[CM], but they also have additional recombinations in both components. Evidence from sequence analysis suggests that the two strains have the same ancient origin and are not recent introductions. EACMCV-[TZ1] occurred widely in the southern part of the country. Four other CMG isolates were identified: two were close to the EACMV-Kenya strain (named EACMV-[KE/TZT] and EACMV-[KE/TZM] with 96% sequence identity); one isolate, TZ10, had 98% homology to EACMV-UG2Svr and was named EACMV-UG2 [TZ10]; and finally one isolate was 95% identical to EACMV-[TZ] and named EACMV-[TZ/YV]. One isolate of African cassava mosaic virus with 97% sequence identity with other isolates of ACMV was named ACMV-[TZ]. It represents the first ACMV isolate from Tanzania to be sequenced. The molecular variability of CMGs was also evaluated using partial B component nucleotide sequences of 13 EACMV isolates from Tanzania. Using the sequences of all CMGs currently available, we have shown the presence of a number of putative recombination fragments that are more prominent in all components of EACMV than in ACMV. This new knowledge about the molecular CMG diversity in East Africa, and in Tanzania in particular, has led us to hypothesize about the probable importance of this part of Africa as a source of diversity and evolutionary change both during the early stages of the relationship between CMGs and cassava and in more recent times. The existence of multiple CMG isolates with high DNA genome diversity in Tanzania and the molecular forces behind this diversity pose a threat to cassava production throughout the African continent.     

The use of African cassava mosaic virus as a vector system for plants

This paper describes the development of a gene-displacement vector based on DNA1, one of two single stranded circular genomic components of a bipartite geminivirus, African cassava mosaic virus (ACMV). The DNA1 molecules of ACMV were cloned as dimers into a plant transformation vector and the constructs have been integrated into tobacco protoplasts by PEG-mediated DNA transfer. In transgenic plants extrachromosomal copies of DNA1 monomers could be detected. Deletion of the coat protein-encoding gene in chimeric constructs resulted in free DNA1 copies of reduced size, and extrachromosomal recombinant molecules were detected after displacement of the coat protein-encoding region by foreign DNA fragments of comparable size. Due to the absence of the second component of ACMV, DNA2, the transgenic plants are free from viral infection symptoms which allows the establishment of healthy transformants that carry a recombinant construct in an extrachromosomal form.     

The cassava green mite in Africa: One or two species? (Acari: Tetranychidae)

Despite a certain amount of confusion that has prevailed for several years with respect to theMononychellus complex (cassava green mite=CGM; Acari: Tetranychidae) which attacks cassava plantations in Africa, strong arguments indicate that a single taxonomic unit corresponding to the speciesMononychellus progresivus Doreste, 1981, was introduced in 1971 from South America.     

Serological studies on cassava latent virus

Particles of cassava latent virus (CLV) were purified by a method that yielded up to 3 mg per 100 g of systemically infected Nicotiana benthamiana leaf. Specific antiserum was prepared and used for enzyme-linked immunosorbent assay (ELISA), which detected purified virus at 5 ng/ml. As estimated by ELISA, CLV antigen reached a greater concentration in leaves of N. benthamiana plants kept at 20–25 °C than in those at 15 °C or 30 °C. CLV was also detected in leaf extracts of naturally infected cassava plants kept at 25 C but its concentration was only 1–7% of that in comparable extracts from N. benthamiana. Staining sections of N. benthamiana leaves with fluorescent antibody indicated that CLV particle antigen accumulates in the nuclei of many phloem cells and of some cells in other tissues. In tests on mosaic-affected cassava plants of Angolan origin, three plants were found in which CLV could not be detected by either ELISA or immunosorbent electron microscopy, or by transmission to indicator plants. This suggests that the mosaic symptoms were caused by a pathogen other than CLV, but no such agent was detected by electron microscopy of leaf extracts. Three kinds of serological test indicated that CLV is related to bean golden mosaic virus. Evidence was also obtained of a distant relationship to beet curly top virus but none was detected to four other geminiviruses.     

Virus content of leaves of cassava infected by African cassava mosaic virus

African cassava mosaic virus (ACMV) was detected in cassava leaves by ELISA. Some normal constituents of cassava leaves interfered with virus detection but leaf extracts of Nicotiana benthamiana did not. The symptom pattern was determined early in the growth of a leaf and subsequently changed little. ACMV was found only in the yellow or yellow green areas of the mosaic pattern. Virus content of the leaves increased with increasing symptom intensity, but decreased with leaf age and ACMV was not detected in mature leaves. Most whiteflies were found on young growing cassava leaves and the number decreased progressively with leaf age. This distribution will aid both the acquisition and inoculation of the virus.     

Non-prey food for subsistence of Amblyseius idaeus (Acari: Phytoseiidae) on cassava in Africa

The nutritional effects of pollen from cassava, oil palm, castor bean, Leucaena leucocephala and a phloem exudate from cassava were tested in the laboratory for their effect on survivorship and oviposition of Amblyseius idaeus (Denmark & Muma). Survival and longevity of even-aged females were enhanced on the aforementioned non-prey diets when compared with the control of no food. Castor bean pollen provided sufficient nutrition to induce an irregular and low ovipositional rate of A. idaeus over a five week interval. Oviposition ceased after 48 h in the presence of the other non-prey foods or free-water only. These studies suggest that A. idaeus is capable of switching or supplementing its diet with non-prey foods during periods when spider mite populations are low.     

Geminate structures of African cassava mosaic virus

Two types of geminate structures were purified from African cassava mosaic geminivirus (ACMV)-infected Nicotiana benthamiana plants and analyzed by electron cryomicroscopy and image reconstruction. After cesium sulfate density gradient centrifugation, they were separated into lighter top (T) and heavier bottom (B) components. T particles comigrated with host proteins, whereas B particles were concentrated in a cesium density typical for complete virions. Both particles were composed of two incomplete icosahedra of 11 capsomers each, but T particles were slightly larger (diameter, 22.5 nm) and less dense in the interior than B particles (diameter, 21.5 nm). T particles were frequently associated with small globules of approximately 14 nm diameter of unknown origin. The overall structure of ACMV, a begomovirus transmitted by whiteflies, was similar to that of Maize streak virus (MSV), a mastrevirus transmitted by leafhoppers, although the vertices of the icosahedra were less pronounced. Models of ACMV coat proteins based on Satellite tobacco necrosis virus support the exposure of parts of the molecule essential for transmission specificity by whiteflies and provide possible structural explanations for the smaller protrusion of the ACMV capsid relative to MSV. The differences of ACMV and MSV virion shapes are discussed with reference to their different animal vectors.     

The effects of cassava mosaic virus disease on yield and compensation in mixed stands of healthy and infected cassava

The effects of cassava mosaic virus disease (CMD) on yield in fully and partly infected stands of cassava were investigated in field trials in Uganda in 1990-91 and 1991-92. Three cultivars (Ebwanateraka, Bao and Bukalasa 1 l), each at three levels of cutting infection (O%, 50% and 100%) and harvested 510 and 15 months after planting (MAP) were used in a randomised block design with split-split plots and four replicates. Moreover, yield and growth data for individual infected and uninfected plants were considered in relation to the health status of their nearest neighbours. In each experiment, fresh tuberous root yields of plants from 100% infected plots gave sigdicantly lower yields than those from 0% or 50% infected plots at each harvest date and the losses were greatest in cv. Bao. Yields of plants from 0% and 50% plots for each of the three cultivars were not significantly different, 10 and 15 MAP. The loss in yield differed between cultivars and harvest dates. Fresh stem, leaf and root yields and the number of tuberous roots were influenced by the health status of the plants harvested and that of their nearest neighbours. Uninfected plants surrounded by infected ones had more roots and heavier total fresh root, stem and leaf weights than those surrounded by uninfected ones. Overall, 26% and 42% compensation was recorded in 1990-91 and 1991-92, respectively. The effects of CMD on cassava production and of compensation in mixed stands of infected and uninfected plants are discussed, especially in relation to control strategies such as roguing.     

AFLP analysis of African cassava (Manihot esculenta Crantz) germplasm resistant to the cassava mosaic disease (CMD)

Amplified fragment length polymophism was assessed in 20 land races and nine elite lines of cassava from Africa, resistant and susceptible to the cassava mosaic disease (CMD). Eleven accessions from a representative core collection from Latin America, previously studied by AFLPs, were included as a reference. AFLP data from all accessions was analyzed by both the unweighted pair group mean average (UPGMA) and multiple cluster analysis (MCA) methods of analysis. Genetic differentiation between clusters and the coefficient of genetic differentiation was also calculated. Results reveal a genetic divergence between African and Latin American accessions, although some overlap was found between them. African land races resistant to CMD, were also found to be genetically differentiated from susceptible land races and from resistant elite lines. AFLP analysis identified a considerable number of duplicates in the African accessions, suggesting a sizeable percentage of redundancy. A unique AFLP fragment, found in a relatively high frequency in African accessions, but absent in the Latin American accessions, was found to be associated with branching pattern by QTL mapping in an F1 progeny derived from African and Latin American parents. The likely source and the utility of the unique AFLP fragment in understanding the processes of genetic divergence in Africa is discussed. Received: 15 May 1999 / Accepted: 28 July 1999     

The natural history of Ebola virus in Africa

Several countries spanning the equatorial forest regions of Africa have had outbreaks of Ebola hemorrhagic fever over the last three decades. This article is an overview of the many published investigations of how Ebola virus circulates in its natural environment, focusing on the viral reservoir, susceptible animal species, environmental conditions favoring inter-species transmission, and how the infection is transmitted to humans. Major breakthroughs have been made in recent years but many outstanding questions must be dealt with if we are to prevent human outbreaks by interfering with the viral life cycle.     

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.     

The epidemiology of human immunodeficiency virus in South Africa

We review the epidemiology of human immunodeficiency virus (HIV) in South Africa where the prevalence of HIV infection is among the highest in the world. The epidemic reached South Africa relatively recently but the prevalence of infection has increased rapidly and there are significant differences among provinces. Although few 15-year-old people are infected the prevalence increases rapidly with age thereafter, especially among women. The prevalence of herpes simplex virus type 2 exceeds that of HIV and curable sexually transmitted infections are common. 'Circular migration' may help to explain the high rates and rapid spread of HIV in the region. The incidence of tuberculosis has increased dramatically as a result of the HIV epidemic. Antiretroviral therapy for the prevention of vertical transmission has been shown to be effective in local conditions but transmission through breast-feeding remains problematical. While some epidemiological models have been developed, much more needs to be done in this regard in order to plan, coordinate and evaluate an effective response to the epidemic. We conclude by discussing some of the research that is needed and steps that could be taken to reduce the continued spread of the infection.