Ecophylogeography of Cassava Mosaic Disease Viruses Reveals a Distribution Linked to the Agro-ecosystems of Yangambi Region in Democratic Republic of Congo

Abstract

A molecular epidemiology study of cassava mosaic begomoviruses was conducted in Yangambi (DRCongo). 120 cassava fields were investigated in 150 Km circle around the Agricultural Research Centre of Yangambi in various agro-ecosystems. Fields incidence and severity of CMD were scored, and whiteflies population counted per plant. 140 Cassava leaves samples, up to 100 weeds (Fabaceae) supposed to be alternate hosts of CMBs and 132 adults whiteflies were collected in cassava fields for molecular analysis. Targeted genes by PCR using specific primers were AC2 and AC4, which are the genes implied in the suppression of cassava mechanism of defence. Phylogeographical analysis of results showed that the virus distribution is linked to the agro-ecosystems. Moderate ACMV and EACMV-UG isolates associated with mild symptoms grouped in separate clusters based on a phylogenetic analysis of genes AC2 & AC4. They were found in spatial discontinuous cassava cropping ecosystem lying in evergreen humid forest, and to a lesser extent, in permanent flood land and the deciduous high forest, where cassava crop is less intensive (disease severity 2.53±0.55). In the more intensively cropped areas around cities and villages and the neighbouring secondary forest, the severe isolates were limited to spatial continuous cropping systems (disease severity 3.78±0.50). Such results were also correlated with the mean number of whitefly per leaf or the incidence of disease. EACMV-UG isolates spread in the different ecosystems suggest multiple introductions of virus. Our study focused on the potential contribution of forest plants species diversity to mitigate the effect of CMD. The results showed also that CMD viruses are widely spread throughout the investigated area. Dual infection was common both in cassava (66%) and in whiteflies vectors (59%). The EACMV-UGSer was predominant, in relationship with highly infested fields.