Epidémiologie, diversité génétique et phylogéographie des virus de la mosaïque africaine du manioc dans la région de Yangambi en République Démocratique du Congo

(eng) 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 cultivation is less intensified, the disease severity was 2.370.32, the incidence varied between 60-72% and whiteflies population vector is 6.520.94. Severe isolates are limited in the secondary mixed forest where cassava cultivation is intense with a disease severity is 3.930.64 associated with whiteflies population of 8.891.15 and a high incidence (85 - 95%). The disease has limited impact in the deciduous forest (severity 3.470.43) with a whiteflies population is 8.511.22 and an incidence ranging from 70-90%. This reinforces then the hypothesis that high biodiversity in the dense rainforest Yangambi has mitigated CMD. Disease severity shows a direct relationship with the population of whiteflies. Grafting was a pragmatic approach used in cassava resistance screening. The chosen cassava genotypes behaved differently to infection. Grafting techniques have not influence on cassava sensitivity and mosaic disease evolution. However, whiteflies tend to infect preferentially EACMV-UG to cassava inoculated. The harvesting of apical young leaves stimulates the expression of mosaic disease in cassava. Three local cassava cultivars such as cultivars Yauma, Timolo, Bangi have a resistant behavior and deserve to be better valued; in contrast cultivars Ponjo, Lofiongi, Ngonga and Mboloko are susceptibles. Improved varieties from IITA such as Mvuazi (TMS I 95/0528), Mahungu (TMS 92/297), 96/1089A and Disanka (TMS I 95/0211) seem generally more productive than cassava local varieties which showed a susceptibility to CMD. Impact of CMD on the yield was generally greater in local cassava varieties (13-47%), it was less significant on IITA resistant varieties (12-27%).

(fre) 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 cultivars locaux (Yauma, Timolo, Bangi) sont résistants. Les maniocs de l’IITA sont plus productifs que les variétés locales, de même la mosaïque a plus d’impact sur les cultivars locaux (13-47%) et moins sur les variétés de l’IITA (12-27%).