Cassava Mosaic Geminiviruses Research Articles

Strip Cropping: A Potential IPM Tool for Reducing Whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) Infestations in Cassava

Insect pests and diseases are a major limiting factor to cassava production in Africa. The cassava mosaic virus disease (CMVD), caused by cassava mosaic geminiviruses (CMGs) (Geminiviridae: Begomovirus) and transmitted by whitefly, Bemisia tabaci Gennadius , threatens the production of the crop in Africa, causing an estimated annual yield loss of over 1.5 billion US dollars. A 6-month field experiments were conducted to explore the potential of using strip cropping to reduce whitefly infestations in cassava, Manihot esculenta Crantz (Euphorbiaceae). Five rows each of cassava, Jatropha and cotton were grown around a 10-row (1 m row width) × 25 m cassava plots in a randomized complete block design. At weekly intervals, the levels of whiteflies infestations (number of eggs, nymphs and adults) per plot were determined. A significantly lower numbers of immature (egg and nymph) and adult Bemisia tabaci were found in cassava plots surrounded on all sides by five rows of both cotton and Jatropha curcas, clearly demonstrating the potential of strip cropping as a management option for the suppression of Bemisia tabaci populations.

Transmission studies with Cassava brown streak Uganda virus ( Potyviridae: Ipomovirus) and its interaction with abiotic and biotic factors in Nicotiana benthamiana

Cassava brown streak disease (CBSD), caused by two distinct species, Cassava brown streak Uganda virus (CBSUV) and Cassava brown streak virus (CBSV), is a major constraint to cassava ( Manihot esculenta Crantz) production in Africa. Absence of infectious clones of CBSUV or CBSV and the lack of efficient means of mechanical transmission of CBSD has hampered laboratory studies of this disease. Mechanical transmission, achieved mainly by plant sap inoculation, is a widely used technique for characterizing plant viruses. Efficient sap transmission of CBSUV/CBSV to the common laboratory host Nicotiana benthamiana is essential for both basic and applied studies of the virus. We report here the development of an efficient protocol for sap transmission of CBSUV to N. benthamiana and N. debneyi. Several factors affecting transmission efficiency were identified such as the effects of buffer composition, antioxidants, inoculum concentration, plant age and temperature. Higher temperatures (30 °C) favored rapid symptom initiation compared to lower temperatures (21 °C) when sap prepared in phosphate buffer of pH 7.0 was applied on the leaves of N. benthamiana dusted with the abrasive (carborundum). We demonstrated the usefulness of the transmission method in transient evaluation of CBSUV[UG:Nam:04]-derived RNA interference constructs for CBSD resistance and also in studying the interaction of CBSUV[UG:Nam:04] with cassava mosaic geminiviruses, another important group of viruses infecting cassava.

Biological parameters of Bemisia tabaci (Gennadius) biotype B (Hemiptera: Aleyrodidae) on Jatropha gossypiifolia, commercial (Manihot esculenta) and wild cassava (Manihot flabellifolia and M. carthaginensis) (Euphorbiaceae)

Bemisia tabaci (Gennadius) is one of the most important pests of cassava in Africa and several countries of Asia due to the damage caused by direct feeding, the excretion of honeydew, and its capacity as a vector of cassava mosaic geminivirus. There is a general consensus that B. tabaci is a complex of morphologically indistinguishable populations with different biotypes. In the Americas, the polyphagous biotype B does not appear to feed on cassava. Recent studies indicate that it is possible, however, for biotype B to gradually adapt to cassava using phylogenetically related hosts. Therefore, the possibility that some wild species of cassava constitute intermediate hosts in the adaptation process may lead to the establishment of biotype B on commercial varieties of Manihot esculenta. In here, we evaluated Jatropha gossypiifolia, two wild species of cassava (Manihot flabellifolia and M. carthaginensis) and a commercial cassava variety (MCol 2063) as hosts of biotype B. The highest oviposition rate (2.7 eggs /two days) occurred on M. esculenta, although the development time (44 d) was the longest when compared to M. carthaginensis and J. gossypiifolia. About 60% of the population could reproduce on the wild cassava species vs. 55% on J. gossypiifolia and 27.5% on the commercial variety. Our data suggest that J. gossypiifolia is a suitable host and the wild species M. carthaginensis can constitute a potential intermediate host in the adaptation of biotype B to commercial varieties of cassava.

Differential interaction between cassava mosaic geminiviruses and geminivirus satellites

Geminiviruses are often associated with subviral agents called DNA satellites that require proteins encoded by the helper virus for their replication, movement and encapsidation. Hitherto, most of the single-stranded DNA satellites reported to be associated with members of the family Geminiviridae have been associated with monopartite begomoviruses. Cassava mosaic disease is known to be caused by viruses belonging to nine different begomovirus species in the African continent and the Indian subcontinent. In addition to these species, several strains have been recognized that exhibit contrasting phenotypes and infection dynamics. It is established that Sri Lankan cassava mosaic virus can trans-replicate betasatellites and can cross host barriers. To extend these studies further, we carried out an exhaustive investigation of the ability of geminiviruses, selected to represent all cassava-infecting geminivirus species, to trans-replicate betasatellites (DNA-beta) and to interact with alphasatellites (nanovirus-like components; previously called DNA-1). Each of the cassava-infecting geminiviruses showed a contrasting and differential interaction with the DNA satellites, not only in the capacity to interact with these molecules but also in the modulation of symptom phenotypes by the satellites. These observations could be extrapolated to field situations in order to hypothesize about the possibility of acquisition of such DNA satellites currently associated with other begomoviruses. These results call for more detailed analyses of these subviral components and an investigation of their possible interaction with the cassava mosaic disease complex.

Cassava mosaic geminiviruses: actual knowledge and perspectives.

SUMMARY Cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses (CMGs) is one of the most devastating crop diseases and a major constraint for cassava cultivation. CMD has been reported only from the African continent and Indian subcontinent despite the large-scale cultivation of cassava in Latin America and several South-East Asian countries. Seven CMG species have been reported from Africa and two from the Indian subcontinent and, in addition, several strains have been recognized. Recombination and pseudo-recombination between CMGs give rise not only to different strains, but also to members of novel virus species with increased virulence and a new source of biodiversity, causing severe disease epidemics. CMGs are known to trigger gene silencing in plants and, in order to counteract this natural host defence, geminiviruses have evolved suppressor proteins. Temperature and other environmental factors can affect silencing and suppression, and thus modulate the symptoms. In the case of mixed infections of two or more CMGs, there is a possibility for a synergistic interaction as a result of the presence of differential and combinatorial suppressor proteins. In this article, we provide the status of recent research findings with regard to the CMD complex, present the molecular biology knowledge of CMGs with reference to other geminiviruses, and highlight the mechanisms by which CMGs have exploited nature to their advantage.

Occurrence of East African cassava mosaic virus‐Uganda (EACMV‐UG) in Burkina Faso

) is a major staple food crop cultivatedthroughout Africa, and is affected most severely by cassava mosaic disease(CMD) occurring at high incidence throughout the continent. CMD iscaused by cassava mosaic geminiviruses (CMGs) belonging to the genusBegomovirus (family Geminiviridae). The complex of CMGs includeseven African species; the three main species are: African cassava mosaicvirus (ACMV), East African cassava mosaic virus (EACMV) and SouthAfrican cassava mosaic virus (SACMV) (Bull et al., 2006). In Uganda, anextremely severe epidemic of CMD has developed since the 1990s and hasprogressed since then in Uganda’s neighbouring countries and CentralAfrica. The severe CMD phenotype is caused by the synergistic interactionbetween ACMV and a distinct recombinant strain EACMV-UG, commonlyknown as the Uganda variant. CMD is reported to be widespread inBurkina Faso (Konate et al., 1995), but the molecular features of the causalagent have never been investigated.In August/September 2008, leaves were collected from cultivars of localcassava presenting moderate to severe CMD symptoms from the centralregion around Ouagadougou. Ten leaf samples were tested and found tobe positive for the presence of begomoviruses by PCR, using degenerateprimers VD360 and CD1266 (Delatte et al., 2005). For two of thesesamples, complete DNA-A genomes were cloned using the Phi29 DNApolymerase-based rolling circle amplification strategy (Inoue-Nagataet al., 2004) and sequenced by Macrogen Inc. (Korea). One sequence(EMBL-GenBank-DDBJ Accession No. FM877473) showed 97% highestnucleotide identity with ACMV-[Ivory Coast:1999] (AF259894) andACMV-[Nigeria:Ogoroco:1990] (AJ427910). The second sequence(FM877474) showed 98% nucleotide identity with EACMV-UG[Uganda:Severe2:1997] (AF126806).This is the first report of the Uganda variant in West Africa. These resultsshow that both species ACMV and EACMV-UG affect cassava plantingsin Burkina Faso, and suggest a potential risk of the occurrence of a severeepidemic of CMD as in East Africa.AcknowledgementsThis research was funded by the following institutions: CRSBAN/UFR-SVT(University of Ouagadougou), INERA (CNRST, Burkina Faso), ConseilRegional de La Reunion, CIRAD, European Union (FEDER) and GISCentre de recherche et de veille sanitaire sur les maladies emergentes dansl’Ocean Indien (N°PRAO/AIRD/CRVOI/08/03).ReferencesBull SE, Briddon RW, Sserubombwe WS, Ngugi K, Markham PG, Stanley J, 2006. Genetic diversity and phylogeography of cassava mosaic viruses in Kenya.

English

Field-infected cassava stems whose leaves were identified with differential symptoms ofAfrican cassava mosaic geminivirus (ACMV) disease were assessed for index of severity of symptoms (ISS) to determine their infection status by scoring young resultant plants in the laboratory. Extracts of young stem tissues and leaves of emergent shoots from topped stems were also assayed for occurrence of ACMV by the enzyme-linked immunosorbent assay (ELISA). Geminivirus disease incidence and severity of symptoms were most abundant on leaves of plants from cuttings of field-symptomatic (I) stems of the moderately-resistant TMS 4(2)1425 (63.2%) and the susceptible TMS 60506 (51.1%) genotypes while those from apparently symptom-free (H) stems of the resistant genotype TMS 30001 recorded significantly the least disease (0%). Similarly, the index of severity of symptoms on all plants (ISSAP) and diseased plants only (ISSDP) were significantly highest on leaves of TMS 4(2)1425 (2.42 and 2.83), and were significantly the least (1) on leaves of TMS 30001, respectively. ACMV was not detected in extracts of all sections of lignified cassava stems by ELISA as all absorbance values were below threshold (0.0890). However, the virus was detected at greater concentrations in leaves of emergent axillary shoots regenerating on topped plants of all node types of TMS 60506 and TMS 4(2)1425 as well as the base (node 1) of TMS 30001 stems but not on those from middle and uppermost nodes (10 and 20, respectively) of the latter genotype. The highest absorbance values were recorded on shoots on node 1 of TMS 60506 (0.1720 ± 0.096), TMS 4(2)1425 (0.1640 ± 0.115) and TMS 30001 (0.1580 ± 0.080) in that order, while the least values were on nodes 10 (0.0298 ± 0.020) and 20 (0.0289 ± 0.019) of TMS 30001.   Key words: African cassava mosaic geminivirus, infection status, cassava genotypes, detection limitations, enzyme-linked-immunosorbent assay.

Molecular Ecology and Emergence of Tropical Plant Viruses

An appreciation of the risks caused by emergent plant viruses is critical in tropical areas that rely heavily on agriculture for subsistence and rural livelihood. Molecular ecology, within 10 years, has unraveled the factors responsible for the emergence of several of the economically most important tropical plant viruses: Rice yellow mottle virus (RYMV), Cassava mosaic geminiviruses (CMGs), Maize streak virus (MSV), and Banana streak virus (BSV). A large range of mechanisms--most unsuspected until recently--were involved: recombination and synergism between virus species, new vector biotypes, genome integration of the virus, host adaptation, and long-distance dispersal. A complex chain of molecular and ecological events resulted in novel virus-vector-plant-environment interactions that led to virus emergence. It invariably involved a major agricultural change: crop introduction, cultural intensification, germplasm movement, and new genotypes. A current challenge is now to complement the analysis of the causes by an assessment of the risks of emergence. Recent attempts to assess the risks of emergence of virulent virus strains are described.

The use of biolistic inoculation of cassava mosaic begomoviruses in screening cassava for resistance to cassava mosaic disease

Inoculation of cassava with infectious clones of cassava mosaic geminiviruses ( Geminiviridae: Begomovirus) and total DNA extracts from plants infected with well-characterised viruses was evaluated using the Bio-Rad Helios Gene Gun System. Total DNA extracts from infected plants and cloned viruses were produced for coating gold particles and bombardment onto new cassava genotypes, 96/1089A, 96/1039, 96/0160, 96/0304 and three local landraces TME 117, TME 3 and TME 4. Cloned DNA of a Kenyan isolate of the recombinant variant of East African cassava mosaic virus (EACMV-UG2-[Ka]), was only infectious to TME 117 (7/10 plants), 3 weeks post-inoculation with mild infection symptoms in the newly developing leaves. Biolistic inoculation with a chimeric pseudorecombinant virus between DNA A and B components from EACMV-[Ke-Kilifi] and EACMV-UG2-[Ka], respectively, was infectious to TME 117, 96/1039 and 96/0304 and developed very severe and persistent symptoms. TME 3 and TME 4 also developed symptoms, 12 days post-inoculation (d.p.i.). Total DNA extracts of ACMV and EACMV-[Ke-Kilifi] resulted in serious infections with symptoms already evident, 10 d.p.i. In general, biolistic inoculation trials with total DNA extracts resulted in a higher number of infected plants expressing symptoms at a much earlier stage (10–12 d.p.i.) compared with trials inoculated with virus clones.

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

Abstract 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.

RNAi dependent epigenetic marks on a geminivirus promoter

Nicotiana benthamiana plants were stably transformed with an intron-spliced dsRNA producing construct cognate to bidirectional promoter of African cassava mosaic geminivirus (ACMV) DNA A. Transgenic lines expressed multiple siRNAs species upon ACMV infection. The de novo DNA methylation and an increased proportion of histone H3 Lysine-9 methylation (H3K9) at intergenic region (IGR) of ACMV DNA A were observed.

Identification of a defective molecule derived from DNA‐A of the bipartite begomovirus of East African cassava mosaic virus

Geminivirus defective interfering DNAs arise spontaneously in mechanically inoculated test plants, and have previously been found with DNA‐B of the bipartite cassava mosaic geminiviruses, but not DNA‐A. Reported here for the first time is the cloning and characterization of a naturally occurring truncated form of cassava mosaic geminivirus DNA‐A, which at 1525 nt is around half the expected full size. Sequence analysis has shown it to be a defective (df) form of East African cassava mosaic virus (EACMV) DNA‐A that has retained its cis elements essential for replication by the helper virus, and it has been termed df DNA‐A 15. Phylogenetic comparisons placed the df DNA‐A 15 molecule close to mild and severe isolates of EACMV‐UG2. Biolistic inoculation of Nicotiana benthamiana with infectious df DNA‐A 15 clone and East African cassava mosaic Cameroon virus (EACMCV) resulted in symptom amelioration as compared with EACMCV singly inoculated plants, and there was an accumulation of df DNA‐A 15 in systemically infected leaves. In addition, the level of EACMV DNA‐B accumulation was reduced in the coinoculated plants compared with those inoculated with EACMCV alone. PCR and sequence analysis confirmed the helper virus as EACMV.

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.

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.

Cassava mosaic geminiviruses associated with cassava mosaic disease in Rwanda

Cassava mosaic disease (CMD), and cassava mosaic geminiviruses (CMGs), were investigated in a country-wide survey conducted in six cassava-producing prefectures (administrative regions) of Rwanda in 2001. CMD occurred throughout the country at an average incidence of 30%. Incidence was highest in Kigali (44%) and lowest in Butare (11%). Cutting infection resulting from the use of CMD-affected planting material was the main type of infection. Disease symptoms were generally severe, with little difference between cultivars or locations. Most affected plants lacked the ‘candle-stick’ symptoms characteristic of severe CMD associated with infection by the ‘Uganda Variant’ of East African cassava mosaic virus (EACMV-UG2). PCR analysis detected two viruses in CMD-affected plants: African cassava mosaic virus (ACMV) and EACMV-UG2. ACMV predominated and occurred throughout the surveyed areas, whereas EACMV-UG2 was restricted to the prefectures of Byumba, which border south-western Uganda and Kibungo in south-eastern Rwanda. No dual infections were detected.

Adaptation of Bemisia tabaci biotype B (Gennadius) to cassava, Manihot esculenta (Crantz)

Bemisia tabaci is a recognized pest in cassava ( Manihot esculenta) crops in Asia and Africa, where it transmits the cassava mosaic geminiviruses (CMGs) (family: Geminiviridae, genus: Begomovirus). A general consensus exists that B. tabaci is a complex of morphologically indistinguishable populations with different biological biotypes. In the Americas, though the polyphagous B. tabaci biotype B appears to feed on cassava, it is postulated that the absence of CMGs is related to the inability of this biotype to colonize this crop effectively. However, its potential adaptation is considered a threat for cassava production in the Neotropics. This study was initiated to verify whether B. tabaci can become gradually adapted to M. esculenta. Trials in rearing chambers were carried out measuring population development of whitefly individuals passed through a series of intermediate hosts, previously selected and based on phylogenetic closeness to Manihot. The capacity of biotype B to adapt gradually to cassava, started on a legume ( Phaseolus vulgaris), followed on two Euphorbiaceae ( Euphorbia pulcherrima and Jatropha gossypiifolia) until, finally reaching a commercial cassava variety. B. tabaci female mean longevity on cassava, coming from P. vulgaris, E. pulcherrima and J. gossypiifolia was 3.1, 5.6 and 3.3 days, respectively. The highest oviposition rate (2.6 eggs/female/2 days), the shortest development time (44.4 days) and the highest value of r m (0.48 day −1) were for populations coming from J. gossypiifolia, where 27.5% of the individuals coming from this host survived and reproduced on cassava. The importance and potential impact of phylogenetically close plants as intermediate hosts faciliting the adaptation of B. tabaci biotype B to cassava is discussed.

Occurrence and activity of Bemisia tabaci 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.

The effect of cassava mosaic geminiviruses on symptom severity, growth and root yield of a cassava mosaic virus disease‐susceptible cultivar in Uganda

SummaryA study was carried out to assess the effect of different cassava mosaic geminiviruses (CMGs) occurring in Uganda on the growth and yield of the susceptible local cultivar ‘Ebwanateraka’. Plants infected with African cassava mosaic virus (ACMV), ‘mild’ and ‘severe’ strains of East African cassava mosaic virus‐Uganda (EACMV‐UG2) and both ACMV and EACMV‐UG2 were grown in two experiments in Kabula, Lyantonde in western Uganda. The most severe disease developed in plants co‐infected with ACMV and EACMV‐UG2 and in those infected with the ‘severe’ form of EACMV‐UG2 alone; disease was least severe in plants infected with the ‘mild’ strain of EACMV‐UG2. ACMV‐infected plants and those infected with the ‘mild’ strain of EACMV‐UG2 were tallest in the 1999–2000 and 2000–2001 trials, respectively; plants dually infected with ACMV and EACMV‐UG2 were shortest in both trials. Plants infected with ‘mild’ EACMV‐UG2 yielded the largest number and the heaviest tuberous roots followed by ACMV and EACMV‐UG2 ‘severe’, respectively, whilst plants dually infected with ACMV and EACMV‐UG2 yielded the least considering the two trials together. Reduction in tuberous root weight was greatest in plants dually infected with ACMV and EACMV‐UG2, averaging 82%. Losses attributed to ACMV alone, EACMV‐UG2 ‘mild’ and EACMV‐UG2 ‘severe’ were 42%, 12% and 68%, respectively. Fifty percent and 48% of the plants infected with both ACMV and EACMV‐UG2 gave no root yield in 1999–2000 and 2000–2001, respectively. These results indicate that CMGs, whether in single or mixed infections, reduce root yield and numbers of tuberous roots produced and that losses are substantially increased following mixed infection.

Cassava mosaic geminiviruses in Africa

Cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses (CMGs) (Geminiviridae:Begomovirus) is undoubtedly the most important constraint to the production of cassava in Africa at the outset of the 21st century. Although the disease was recorded for the first time in the latter part of the 19th century, for much of the intervening period it has been relatively benign in most of the areas where it occurs and has generally been considered to be of minor economic significance. Towards the end of the 20th century, however, the inherent dynamism of the causal viruses was demonstrated, as a recombinant hybrid of the two principal species was identified, initially from Uganda, and shown to be associated with an unusually severe and rapidly spreading epidemic of CMD. Subsequent spread throughout East and Central Africa, the consequent devastation of production of the cassava crop, a key staple in much of this region, and the observation of similar recombination events elsewhere, has once again demonstrated the inherent danger posed to man by the capacity of these viruses to adapt to their environment and optimally exploit their relationships with the whitefly vector, plant host and human cultivator. In this review of cassava mosaic geminiviruses in Africa, we examine each of these relationships, and highlight the ways in which the CMGs have exploited them to their own advantage.

East African cassava mosaic Zanzibar virus - a recombinant begomovirus species with a mild phenotype.

Cassava plants exhibiting mild symptoms of cassava mosaic disease (CMD) were collected from Unguja island, Zanzibar. Cuttings grown from these plants in the glasshouse produced similar symptoms, which were milder than those caused by other known cassava mosaic geminiviruses (CMGs). The whitefly vector, Bemisia tabaci (Gennadius), transmitted the putative virus to 27.7% (n = 18) of target plants. Total DNA extracted from diseased leaves did not yield diagnostic PCR-bands using virus-specific primers to known CMGs. Degenerate primers, however, produced a diagnostic band indicating the presence of a begomovirus. Full-length DNA-A (2785 nucleotides) and DNA-B (2763 nucleotides) components were subsequently PCR-amplified, cloned and sequenced. Phylogenetic analyses of DNA-A and -B sequences showed that they were most similar to strains of East African cassava mosaic virus from Tanzania and Uganda at 83% and 86% nucleotide identities, respectively. The number and arrangement of open reading frames were similar to those of bipartite begomoviruses from the Old World. DNA-A was predicted to have recombined in the intergenic region (IR), AC1 and AC4 genes, and DNA-B in the IR. A maximum nucleotide identity of 83% in the DNA-A component with other sequenced begomoviruses, together with different biological properties allows this virus to be recognised as belonging to a new species named East African cassava mosaic Zanzibar virus (EACMZV).