Cassava mosaic geminiviruses_Thesis copy.pdf
Creators: name: Fondong, Vincent
Publisher: name: figshare
Year: 2020
Subjects: subject: 60506 Virology
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: 60704 Plant Pathology

description: The cassava mosaic disease (CMD) occurs in all cassava growing areas of Africa and the Indian sub-continent. CMD is caused by at least three different geminiviruses in Africa, <i>African cassava mosaic virus</i>(ACMV), <i>East African cassava mosaic virus</i> (ACMV) and <i>South African cassava mosaic virus</i> (SACMV) and by <i>Indian cassava mosaic virus</i> (ICMV) in the Indian sub-continent. These viruses are spread naturally by their whitefly (<i>Bemisia tabaci</i> Gennadius) vectors and through infected cuttings which are routinely used for cassava propagation. The viruses contain two genomic components, DNA-A and DNA-B and belong to the genus <i>Begomovirus</i> of the family <i>Geminiviridae</i>. <i>Manihot spp.</i> are the only known natural hosts of the cassava mosaic geminiviruses (CMGs). Yield losses due to infection by CMGs vary considerably and depend on cultivar, time of infection, and environmental conditions. The yield losses range from 5% to a total loss of the crop. The spread of CMD under differing conditions in Cameroon was studied by planting CMD-free cuttings of six cassava cultivars at Ekona (450 metre above sea level, masl), Njombe (80 masl) and Dschang (1300 masl). The spread of the disease was assessed by recording incidence per month while the whitefly populations were recorded weekly. Spread was most rapid in all cultivars at Ekona followed by Njombe, both in the lowland forest zone, whereas little spread occurred at Dschang in the savannah area. Improved (with resistance to CMGs) cultivars showed a comparatively higher level of resistance to CMD than the local cultivars at all the locations tested. However, the severity of the symptoms on infected plants was greater at Dschang compared with Ekona and Njombe. Spread was less in cassava intercropped with maize than when intercropped with cowpea or in monoculture due probably to a lower number of whiteflies in maize containing plots. However, intercropping did not influence symptom severity in infected cassava plants.Field experiments were conducted in contrasting ecological conditions at two sites in southern Cameroon to study the reversion phenomenon - the extent to which the virus causing CMD was absent from cuttings collected from CMD-affected plants. The proportions of disease-free plants developing from the different batches of cuttings were recorded at one month after planting to indicate the extent of reversion. This was significantly less frequent in susceptible cultivars than in improved cultivars. Reversion was also significantly higher in cuttings collected from the youngest portions of the stem compared with older portions and with short cuttings compared with long ones. There was an inverse relationship between symptom severity in different cultivars and the extent of reversion. In order to identify viruses causing CMD, cuttings were collected from plants displaying CMD symptoms in Cameroon and tested using the polymerase chain reaction (PCR) with primers specific for the coat protein (CP) gene of ACMV and EACMV. Out of a total of 314 samples tested 280 were infected by ACMV/CM alone, 37 samples were co-infected by ACMV/CM and EACMV/CM and five samples were infected by EACMV/CM alone. All samples infected by EACMV/CM were collected in the rainforest part of the country and none of the samples collected in the savannah region was infected by EACMV/CM. The intergenic region of 13 isolates, the coat protein (CP) gene of 22 isolates and 11 full-length sequences of both viruses were determined. Sequence comparisons with published CMGs sequences showed that only ACMV/CM and EACMV/CM occur in Cameroon. Comparison of the full-length components of ACMV/CM isolates showed that ACMV/CM was highly similar to ACMV from Kenya (ACMV/KE) and Nigeria (ACMV/NG). However, evidence for a unique double nucleotide recombination in EACMV was noted; recombination in DNA-A was within the region containing AC2-AC3 open reading frames (ORFs), and in DNA-B also evidence for recombination was in the BC1 ORF. Cloned components of both ACMV/CM and EACMV/CM were infectious to <i>Nicotiana benthamiana</i>although EACMV/CM produced apparently asymptomatic phenotypes. Southern blot analysis of viral DNAs from <i>N. benthamiana</i> plants infected with ACMV/CM and/or EACMV/CM showed that there were significantly higher levels of accumulation of both ACMV/CM components, and to a lesser extent of EACMV/CM components, in mixed infected plants than in singly infected plants. This observation confirmed evidence of a synergistic interaction between the two viruses as displayed in mixed infected <i>N. benthamiana</i> and cassava plants.Pseudo-recombinants made between the two components of ACMV/CM and EACMV/CM were infectious to <i>N. benthamiana</i> suggesting trans-complementation between these two viruses, and therefore may at least partly, explain the synergism occurring between the two viruses. Infection of cassava by ACMV/CM and EACMV/CM was demonstrated by inoculating clones of both viruses independently to cassava plantlets (cultivar TMS 60444) using the biolistic gun procedure. The symptoms displayed were typical of CMD symptoms showing therefore that both viruses are causal agents of the CMD in Cameroon.