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BACKGROUND: The whitefly Bemisia tabaci is an important vector of virus diseases, impacting cassava production in East Africa. To date, breeding efforts in this region have focused on disease resistance. Here we use a spatially-explicit simulation model to explore how breeding strategies for whitefly resistance will influence the population dynamics of whitefly in the context of regional variation in cassava crop management practices. RESULTS: Simulations indicated that regions with a short cropping cycle and two cropping seasons per year were associated with high whitefly abundance. Nymph mortality and antixenosis resistance mechanisms were more effective than mechanisms that lead to longer whitefly development times. When spatial variation was introduced in heterogeneous landscapes, however, negative consequences of the antixenosis effect were observed in fields containing whitefly susceptible varieties, unless the proportion of whitefly resistant variety in the landscape was low (~10%) or the amount of matrix in the landscape was high (~75%). CONCLUSION: We show the importance of considering cropping regime and landscape management context when developing and deploying whitefly-resistant cassava varieties. Recommendations differ significantly between regions. There may also be unintended negative consequences of higher whitefly densities for whitefly susceptible varieties if uptake of the new variety in a landscape is high, depending on the mechanism of resistance and the landscape context. Furthermore, we show that in some cases, such as where there is substantial fallow combined with a short single-season crop, the management characteristics of the existing cropping regime alone may be effective at controlling whitefly populations.

Genotype by environment interaction remains a substantial issue in all breeding programs. Crop genotypes are generally developed in a central breeding location, but always require the evaluation of breeding products in different environments. This is particularly relevant in countries that have a wide range of climates. Eighteen cassava genotypes were evaluated in Cameroon in eight environments—varying in seasonal rainfall and temperature patterns and soil characteristics—over two cropping seasons. Soil nutrient content was analyzed and trials were established in a randomized complete block design in three replications. Response of genotypes to major cassava pests and diseases, yield and carotenoids content was evaluated. It was observed that four genotypes did not show cassava mosaic disease (CMD) symptoms irrespective of the environments. The local check had highest CMD incidence and severity across all environments. Average number of whitefly per plant across all environments was highest on TMS 96/0023. Average cassava green mite (CGM) infestation was low on all the genotypes. Fresh root yield of five genotypes ranged between 25 and 30 tons per ha for both years. Significant and positive correlation was found across locations between fresh root yield and soil K, P and Mg. AMMI analysis revealed highly significant differences among genotypes and environments and significant genotype × environment interaction for most of the estimated traits, indicating variability in genotypes performance with environment.

Cassava represents a staple food source in Cameroon and in many other tropical countries, as it is effectively able to combat hunger. This study was carried out to: (i) determine the growth characteristics of improved and local cassava varieties in Eastern Cameroon under repeated cultivation, (ii) assess the effects of different soils on cassava growth, and (iii) disseminate the improved varieties throughout the region. The experiment was carried out during the 2010–2011 and 2011–2012 seasons. Two improved varieties, TMS-92/0326 and TMS-96/1414, and the local Ntolo variety were cultivated at three sites in a randomized block design with six replications. No fertilizer was applied and the fields were weeded regularly. A twoway ANOVA (variety x site) for each season showed that variety and site were equally significant, and there were no significant interactions between varieties and sites. The improved varieties had higher yields (2.0–5.5-fold higher) than Ntolo, with cassava mosaic disease having seriously affected the Ntolo yield. Soil acidity and organic matter content in the soil surface horizon may be major factors affecting the cassava yield, with TMS-92/0326 and Ntolo being tolerant of higher acidity than TMS-96/1414, which suggests that variety-soil interactions should be considered when improved varieties are introduced.

Cassava mosaic disease (CMD) is reported as the most important constraint on cassava in Sub-Saharan Africa. The understanding of the disease epidemiology, the genetic diversity of the virus and its vector is a key factor for the disease management. In this study, cropping system associating cassava, common beans (Phaseolus vulgaris) and maize, the most frequent cassava cropping system in Eastern DR Congo, was investigated to understand the disease characteristics and existing interactions within this pathosystem: cassava mosaic begomoviruses (CMBs), cassava and non-cassava plants, Bemisia tabaci population and agro-ecosystems in which they are found. Different geographic locations mainly depending on the altitude levels showed a significant influence on CMD incidence and severity, and whitefly population. CMD and its vector pressure were high in low altitude, but significant decrease was observed in intercropped fields. Under high epidemic pressure, respective decreases of 19% and of 33.3% were recorded on disease incidence and whitefly population. Only two CMBs (ACMV and EACMV-UG) were detected both in cassava plants and in whiteflies collected on cassava and other plant species. In contrast, no CMBs were detected in non-cassava associated plants. Bemisia tabaci characterization based on MtCOI sequences analysis revealed the presence of a single haplotype close to Ug1, irrespective of the plant species on which the insects were collected. The low diversity of the CMBs and whitefly population combined with the impact of local intercropping system cassava, common beans and maize are discussed, underlining the interest of the other ways of CMD management besides crop resistance strategies.

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. © 2012 Copyright Taylor and Francis Group, LLC.

Molecular epidemiology on genetic diversity and ecophylogeography of cassava mosaic disease (CMD) viruses focused on genes AC2 & AC4 was conducted in Yangambi in Democratic Republic of Congo (DRCongo). Cassava leaves, Fabaceae and whiteflies (Bemisia tabaci) were collected and analyzed by PCR and ELISA. The molecular data were plotted on a Yangambi map published by Geomatics Unity, UCL. CMD viruses ecophylogeographical study has considered the different agroecosystems in relation to cassava cropping systems practiced. The resistance of 14 cultivars of cassava adopted by farmers and the real impact of CMD on these cultivars in the area environmental conditions were surveyed. The results showed that mix infections by ACMV & EACMV-UG are the most spread as well as in cassava, in Fabaceae species and in whiteflies vector. This suggests possible virus transmission by whitefly to non-cassava under high pressure epidemic. The prevalence of EACMV-UG associated with severe symptoms and the frequency of double infections of ACMV & EACMV-UG (70-80%) were more remarkable. Study has permitted to conclude that CMD virus have a low interspecific diversity, only ACMV and EACMV-UG were identified in the Yangambi region. Futhermore, a high diversity whithin isolates of CMD viruses was revealed with a distribution closely related to the agroecosystems in Yangambi. Molecular and phylogenetic analysis indicates that ACMV and EACMV-UG isolates are clustered respectively into two major types. They indicate that others isolates were consistently in double infection. The distribution of EACMV-UG suggests multiple introductions in the Yangambi region. Restriction of some ACMV isolates in the dense forest suggests a slow evolution in a closed environment. Phylogeography indicates that moderate strains of virus are spread in dense primary forest, while aggressive strains are in secondary forest where cassava is grown intensively. In locaties characteriszed by dense forest, where cassava cultiv, L’épidémiologie moléculaire des virus de la mosaïque africaine du manioc focalisée sur les gènes AC2 & AC4 à Yangambi (RDC) a révélé une forte prévalence d’infections mixtes par ACMV & EACMV-UG dans le manioc, les Fabaceae et les mouches blanches (Bemisia tabaci) vecteur du virus. Cela suggère une possible transmission du virus par B. tabaci à d’autres plantes sous forte pression épidémique. La prévalence de EACMV-UG a été associé aux symptômes sévères et à une double infection. Les isolats de ACMV & EACMV-UG identifiés à Yangambi forment respectivement deux souches dont certaines sont systématiquement en double infection. Le mode de distribution de EACMV-UG suggère une multiple introduction dans la région. La restriction de certains isolats de ACMV dans la forêt dense suggère une évolution lente dans un environnement fermé. L’étude révèle que la diversité des souches présente une distribution éco-phylogéographique liée aux agro-écosystèmes. Les souches modérées sont répandues dans la forêt dense; les souches agressives dans la forêt secondaire. Dans la forêt dense, où la culture du manioc est moins intensifiée, la sévérité de la maladie est de 2.370.32, l’incidence varie entre 60-72% et la population de B. tabaci est de 6.520.94. Dans la forêt secondaire à culture du manioc intense, la sévérité est plus élevée de l’ordre de 3.930.64 associée à une population de B. tabaci de 8.891.15 et à une forte incidence (85-95%). La sévérité de la mosaïque a montré un rapport direct avec la population de mouches blanches. Ces résultats renforcent l’hypothèse du contrôle de la maladie de la mosaïque dans un milieu diversifié. Quatorze génotypes de manioc infectés par greffage ont réagi différemment à l’infection. Les techniques de greffage n’ont pas influencé leur sensibilité et l’évolution de la mosaïque. Les mouches blanches ont transmis préférentiellement EACMV-UG aux maniocs inoculés. La récolte de feuilles apicales de manioc stimule l’expression de la mosaïque. Trois cu, (AGRO 3) -- UCL, 2010